Overarching question: Why do some chameleons eat plants?
Related Questions:
While even addressing—no less answering—all of the above questions is beyond the scope of this blog, I would like to discuss some, and I want it to be as clear as possible what I am saying, what I think the evidence suggests, and where I think the evidence is inconclusive.
------------------
[1]This is Necas’ point…more to follow.
------------------
Data point:
Some captive chameleons have been observed consuming the plants in their cages.
While captive Chamaeleo calyptratus (plural calyptratii?) provide the most famous example, this behaviour has also been noted in captive specimens of C. chamaeleon(Ibrahim, 2013), (Herrel, 2007). Moreover, this behaviour does not appear to be restricted to captivity. Wild examples of C. calyptratus calyptratus (Necas, 1999/2004), C. calyptratus calcarifer (Necas, 1999/2004), C. chamaeleon (Karen-Rotem, Bouskila, & Geffen, 2006) (Herrel, 2007) (Ibrahim, 2013), C. namaquensis (Burrage, 1973)and Furcifer oustaleti (Takahashi, 2008) have either been observed eating plant material, or else significant plant material was found in their faecal samples. Takahashi, Burrage and Ibrahim have noted not only vegetation, but fruit in the faeces of their respective species. Given this, it is reasonable to amend our original data point:
Some chameleons eat plant material, including fruits.
Several theories have been proffered to explain this (Measey, Raselimanana, & Herrel, 2014). Among these are:
A) accidental ingestion (Cooper & Vitt, 2002);
B) as a dietary tool to move soft prey (e.g. caterpillars) through the digestive tract (Necas, 2018a);
C) as a means of supplementing hydration in dry habitats (Burrage, 1973), and
D)to supplement some nutritional element missing from the diet.
Theory a) may explain some instances. Hornworms, for instance, are notorious for their grip. A delicate leaf may give way before the worm’s grip, in which case both worm and leaf fragment are ingested. In fact, I have observed this very situation, but the chameleon actually spit out the leaf. Likewise, I can imagine certain species of Brookesia or Rhampholeon ingesting the occasional fallen leaf while hunting tiny bugs on the forest floor; but such cases seem implausible as a comprehensive explanation here. For one thing, the species in question—namely C. calyptratus, calcalifer, chameleon, namaquensis and Furcifer oustaleti—are all large chameleons. And with the exception of namaquensis—whose hunting grounds are often not covered by leaf litter—all are arboreal feeders, so the Brookesia/Rhampholeon example does not apply. More importantly, chameleons are precision hunters—highly adapted to make accurate strikes on their prey. It strains credulity to think that they are accidentally ingesting enough plant material to account for the data. A fortiori, it has been observed that in at least some examples of herbivory, the chameleon uses a non-lingual feeding behaviour (Takahashi, 2008)(and pretty much everyone who has watched their chameleon eating a hibiscus flower), which seems inconsistent with accidental ingestion.
Theory B) has been suggested by Petr Necas (2018a),(2019)from his first hand experience with C. calyptratus both in Yemen, and in his decades of keeping, breeding, and researching this and many other species. The idea here is thus: Calyptratus has a relatively short intestinal tract (small intestine and colon) that lacks the curves and bends characteristic of other animals (Necas, 2018a). Consequently, loose or liquid gut contents are difficult to pass. During certain times of year, the most prolific prey items for C. calyptratus are soft-bodied caterpillars, and the nymphs of various orthopterans (Necas, 2019). Such prey items are low in chitin, and relatively soft bodied—the very kinds of prey items that one would think would create the kind of loose homogenous gut content in question. In the absence of spiny, firm body parts to help the intestines get a “grip” on these looser contents, calyptratus will ingests plant material to fill this role. Basically, plant matter replaces chitin as dietary fibre. I must admit that I have had a certain intuitive draw to this position. First, it posits a novel and creative explanation of the data. Second, it pulls together facts about chameleon biology, ecology and morphology in a cogent and intuitive way. Necas goes on to explain that the consumption of plant material as a means of hydration is inconsistent with the data: the majority of the herbivorous activity takes place during the wet season (Necas, 2019). This jives with our observations of captive calyptratus, which consume plants even when they are well hydrated.
Again, I have been sympathetic to this account, but there are some shortcomings here. First, the theory appears to be light on the peer-reviewed support, and my admittedly brief survey of the relevant literature suggests the same. Although nothing I have read directly addresses Necas’ hypothesis, I have also found no other researcher actively pursuing this line. That being said, Galileo didn’t enjoy a lot of public support in his time either. Second, the theory and its companion assertion (the part about hydration) fail to account for the other data about F. oustaleti, C. chamaeleon and C. namaquensis. It’s not entirely clear that any of these latter species experience a season wherein the insect prey consists mostly of soft-bodied insects thus necessitating the consumption of plants as fibre. Ibrahim (2013) noted equal amounts of plant matter in the faeces of C. chamaeleon throughout the year. Likewise, tying together data from (Ibrahim, 2013) (Pleguezuelos, Poveda,, Monterrubio, & Ontiveros, 1999)(Burrage, 1973)(Karen-Rotem, Bouskila, & Geffen, 2006), we see a rather interesting correlation between the aridity of the environment and the level of plant material in the faeces of C. chamaeleon. Plant matter was found in the faeces of specimens from the relatively arid climates of the Northern Sinai (Egypt), Israel and Lybia, but conspicuously absent from specimens taken from a significantly less arid area of Spain (Measey, Raselimanana, & Herrel, 2014, p. 108). For clarity’s sake: there seems to be a correlation between the ingestion of plant matter by C. chamaeleon and the aridity of its habitat. Intuitively, this seems to partially undermine Necas’ hypothesis, and his discounting of the hydration view (theory c), above). Finally, Necas’ account assumes not only a particular view of the digestive capacity of C. calyptratus—something that stands in contrast to the scant treatment digestion gets in a volume dedicated to chameleon biology (see (Tolley & Herrel, 2014)), but would also require that we generalize to all the abovementioned species. That being said, the evidence in not conclusive either way.
As I mentioned in the previous section, there is some empirical support for c)—the theory that the ingestion of plant matter by chameleons is an adaptation to supplement hydration. Chameleons of the same species present levels of plant material in their faeces that appear to vary with the aridity of their environments (see above). However, this data is equally challenged by the findings Takahashi (2008) and Necas (2019). We will discuss Takahashi’s data below, but he has observed (and re-created) instances of Furcifer oustaleti seemingly seeking out fruit regardless of hydration. And anecdotal though the following evidence is, we have all witnessed well-hydrated chameleons consuming plant matter. So, while it is not surprising that, e.g., C. namaquensis regularly consumes fleshy plant material (Burrage, 1973), dehydration seems an unlikely immediate cause of all instances of herbivory. Although, herbivory could turn out to be a well-entrenched vestigial genetic adaptation—still present, but no longer necessary…More on this later.
Theory d)—the view that chameleons are supplementing their nutrient intake via the ingestion of plants—is perhaps the most contentious and demanding of the four. To be clear, this account is not merely arguing that plant material is important for digestion, but that the body is actually able to extract certain nutrients from the plant material—nutrients that form an important or necessary part of the diet. It is both contentious and demanding for the following reasons. First, there is so much community pushback against it (and I sympathize with this intuition, but admit it is merely an inductive leap that admits of many inconsistent explanations). Second, the view either involves a number of contentious assumptions about chameleon digestion, or else demands a massive research project into the same. The problems are myriad. From what I can tell, given the cursory treatment of digestion in multiple sources e.g. (Anderson & Higham, 2014), we just don’t know enough about digestion in chameleons to know whether nutrient extraction from plant material is even possible. Indeed, the small intestine and colon are given a whopping two paragraphs in an entire volume about chameleon biology see (Anderson & Higham, 2014, p. 52)—the most cited source therein coming from over forty years ago (Parson and Cameron, 1977)! That alone testifies to the state of our knowledge. There are probably some compounds that any digestive system can sequester from plant matter (even celery adds something), but the question is whether anything important/necessary is plant-derived. To this, I am less than optimistic about something forthcoming in the near future. However, thoroughly unqualified though I undoubtedly am, I can (at the very least) bring conceptual analysis to bear here…
From other vertebrates, we can glean something about herbivory and omnivory. Obligate herbivores, as well as frequently herbivorous omnivores, tend to have longer, more complex digestive systems. These systems employ various means of breaking down plant matter. Having several stomachs, chewing cud, having a long small intestine and a colon capable of fermentation, excessive, sometimes day-long eating, and symbiotic relationships with certain microorganisms are all adaptations for breaking down plant material. More relevantly, herbivorous reptiles such as iguanas often display teeth capable of efficient chewing/shredding of leaves, a long intestinal tract and/or special intestinal valves that slow the movement of food through the gut, as well as various digestive aids including enzymes and micro organism symbiosis (Cooper & Vitt, 2002); (Baer, Oftedal, Rumpler, & Ullrey, 1997) (Iverson, 1988) (Mcbee & McBee, 1988). While these adaptations are present in herbivorous (and some omnivorous reptiles), reptiles that consume only small amounts of plant matter tend to lack these more complex adaptations, and consequently consume more of the easily digested plant parts such as fruit, nectar, flowers and pollen (Cooper & Vitt, 2002).
So, how does this information help us? Well, for starters, it is commonly held that chameleons have short digestive tracts (Necas, 2018a) (Necas, 2019) (Anderson & Higham, 2014). Photos available online at Chameleons! Online E-Zine—courtesy of Bill Strand and Tom Greek DVM—bear this out: The esophagus and stomach appear to be as long as the entire gastro-intestinal tract. Likewise, there does not appear to be any textual support for chameleons being capable of the kind of gastro-intestinal fermentation that occurs in herbivorous reptiles like iguanas, or even omnivores such as the bearded dragon[2]. Chameleon dentition and chewing also seem inconsistent with an ability to break down plant matter in a way conducive to digestion (Anderson & Higham, 2014)—something for which their herbivorous reptilian relatives are well adapted (Cooper and Vitt, 2002). That being said, the photo evidence from Strand and Greek comes from dissected Trioceros deremensis, and Calumma parsonii—neither of which species are commonly thought to have herbivorous tendencies. It might very well turn out that veiled chameleons are anatomically different—displaying specific adaptations for plant digestion. However, I cannot find any literature that supports this view (nor any that tells against it)[3]. For the most part, the literature appears to assume digestive anatomical homogeneity across Chamaeleonidae. However, one would expect that if something novel were discovered in the digestive tract of C. calyptratus it would have made the journals.
------------------------
[2] Is this why chameleon poop doesn't stink?
[3]I am indebted to Kinyonga for pointing this out. In personal correspondences, she has repeatedly asked me whether veiled chameleons, in particular, have short digestive tracts. Her point is particularly topical here, as it really is veileds that we want to know about. Unfortunately, I have no answer here. The literature appears to lump all chameleons into one digestive-tract phenotype.
------------------------
Tying together the previous two paragraphs, there seems to be some prima facie reasons for doubting theory d)—the view that chameleons are able to extract certain nutrients from plant material—nutrients that form an important or necessary part of their diet. Again, this is hardly conclusive. Indeed, it conflates a distinction cut by Cooper and Vitt (2002)above; namely, that different plant parts are more or less easily digested. Folivorous reptiles generally have special adaptations to process the relatively hard to digest foliage of plants (Cooper & Vitt, 2002). However, reptiles that are not so endowed tend to feed on the more easily digestible parts of plants such as fruit, nectar and pollen. So, even if theory d) seems dubious in light of the previous two paragraphs, that does not tell against the possibility that chameleons are able to extract nutrients from e.g. fruit.
In a highly relevant paper, Takahashi (2008) examines the fruit eating habits of Furcifer oustaleti. Having observed a wild specimen feed on the fruit of Grangeria porosa, Takahashi devised a study wherein 24 wild-caught F. oustaleti were each presented with a horsefly in one instance, and the fruit of Chassalia princei in another. Takahashi found that tongue projection was employed in the case of the horsefly, but jaw prehension was used for the fruit. According to Takahashi (2008) and Measey, Raselimanana and Herbel (2014, p. 108), this suggests that the subjects distinguished between the two food items and used prey-appropriate methods of feeding. More clearly, it looks like the chameleons were able to distinguish the fruit as such and yet proceeded to ingest it. Takahashi is not alone in his findings. Burrage (1973) also observed C. namaquensis employing different feeding techniques for fruit. Moreover, Takahashi’s observations and subsequent experiments were conducted over several years between November and April (the wet season)—the implication being that hydration was not a factor. This makes theory d) less dubious, at least for fruit. If chameleons are purposefully consuming fruit, the mechanisms of evolution would seem to support their doing so for a reason. If that reason isn’t hydration, then it could be nutrition.
Prima facie, the scant evidence seems to support the view that (some/most?) chameleons do not have the hardware to extract nutrients from the hard to digest foliage of plants[4]. On the other hand, certain chameleons appear to be able to distinguish the more easily digestible fruit as a different kind of food item—appropriate to elicit a different kind of feeding behaviour. And the equally scant evidence here might support the view that these chameleons are doing it for nutritional reasons.
--------------------
[4] Let me stress that this last point is tenuous at best. Indeed, all I did was piece together some superficial anatomical facts about herbivores, compare them to some equally flimsy data about chameleon anatomy in a cursory sort of way, and adduce a half-formed hypothesis that is supported by as much intuition as it is empirical evidence. But the research just isn’t there.
--------------------
While we obviously need to amend theory d) to reflect the fruit/foliage distinction, there is a more pressing concern: What often prompts us to ask why chameleons sometimes eat plants is the observation that our captive veiled chameleons are often observed devouring the plants in their cages. And the plants in their cages are rarely covered in fruit or flowers—the more easily digested parts. So, even if wild specimens of F. oustaleti sometimes purposefully consume fruit, this does not help us understand why captive veiled chameleons eat leaves.
Earlier, during the discussion of theory c), I said that I would come back to a point about evolution. The point was that herbivory in chameleons might be an evolutionary adaptation that imbued certain members of a species with a slight survival advantage—thereby allowing them to pass on whatever gene was responsible for the behaviour. Just to spit ball some ideas here: Perhaps sometime in their evolutionary history, chameleons from arid climates did consume leaves to supplement their hydration needs. Similarly, perhaps famine did prompt certain individuals to consume plant matter. And, inefficient though the digestive system of a chameleon is for extracting nutrients from plants, the small energetic gain might have tipped the balance in favour of survival for those intrepid few. As I said, even celery probably has some energetic benefit given a particular metabolic rate. Perhaps during certain times of year, the winds carry small amounts of calcium carbonate and deposit it on the foliage, making herbivory incidental to the supplemental source of calcium[5]. Alternatively, it could be all of the above, or some other evolutionary quirk. The point is that herbivory in the evolutionary history of some chameleons might well have imbued certain individuals with a slight survival advantage over their conspecifics, and this was enough to ensure the passing on of whatever gene/s were responsible for the behaviour.
---------------------
[5] I credit the genesis of this idea to Kinyonga, who has observed more prevalent herbivory in female veileds. Perhaps sometime in their evolutionary history, gravid females went on to produce larger and healthier clutches if they ate leaves dusted in calcium rich sediment..
---------------------
What exactly does this mean for us? First, if herbivory turns out to be some adaptation that helped chameleons survive during drought or famine or else was incidental to the increase uptake of calcium, then it does not appear to apply to captive specimens. In other words, it does not appear to be a requirement for captive individuals, as we don’t typically subject them to these extreme conditions. So, according to this line, the adapted behaviour of herbivory is a vestigial relic of times past. If this is right, then the answer it delivers to our over-arching question is thus: Chameleons eat plants because sometime in their evolutionary history such behaviour helped them to eek-out a living during tough times. Both the behaviour and the gene responsible for it have been encoded in the genetic make up of certain species; and forty years of captivity are not enough to undo tens of thousands of evolution. In short, the view is that chameleons do not require plants in their diet, but continue to consume plant material because of a no longer necessary genetic adaptation.
Is this a satisfying account of every instance of herbivory in chameleons? I certainly find it a plausible account for many instances. Indeed, it actually ties together several of the theories we considered: Chameleons have used herbivory to supplement their hydration requirements, to gain just enough energetic benefit to get through a famine, even to supplement their calcium requirements. Indeed, nothing about this last theory/hypothesis (guess?) discounts Necas’ dietary fibre view (see above). Necas has said that in the absence of something to help move soft items through the gut, veiled chameleons have been observed vomiting (Necas, 2018a). Herbivory, in this context, might have helped them retain the precious contents of their guts, rather than loosing those potential nutrients by vomiting. So I suppose we need to amend our list of theories to include this fifth, evolutionary account. Luckily the next letter in our theory list is e)…fortuitous.
To be clear, this is not a novel account. Necas(2018a), Tolley and Herrel (2014), Reilly et. al (2007)and a host of others have suggested this view. However, it is interesting how the evolutionary account is compatible with many of the theories proffered to explain the phenomenon of chameleon herbivory. Indeed, theory e) stands as a partial vindication for those who speculate that chameleons consume plants for hydration, for nutrition, for fibre, etc. Chameleons probably did, in fact, use plants for all these reasons. For those still convinced that we ought to be offering fruits and veggies to our modern-day captive pets, feel free to stop reading now: Some species have adapted to be able to consume small amounts of these items, and it probably doesn’t hurt. For everyone else, it might be worthwhile considering that although herbivory might be a genetically entrenched behaviour, the purpose it used to serve (at least according to this line of thought) is no longer applicable, since our captive charges don’t face the kind of survival challenges that—in combination with the mechanisms of natural selection—resulted in their ancestors’ passing on the genes responsible for herbivorous behaviour[6].
------------------------
[6] Please note that I am not conflating the onto/phylogenetic distinction. Ontogenetic behavioral oddities that imbue an individual with a survival advantage often end up becoming phylogenetic adaptations.
-----------------------
Of course, a lot of this germinal theory depends on some pretty long logical leaps, quite a bit of speculation, and no small amount of plain old guesswork. Perhaps the most glaring of these shortcomings is the following inference:
What of our list of questions? Well, I was thinking about going through them one by one, and noting where a possible answer occurs in this longwinded ramble. But I just broke 4000 words, and it’s New Year’s. I’ll leave the minutia to anyone patient enough to read this...
Works Cited
Anderson, C. V., & Higham, T. E. (2014). Chameleon Anatomy. In K. Tolley, & A. Herrel, The biology of chameleons(pp. 7-55). Berkeley/Los Angeles, California, US: University of California Press.
Baer, D. J., Oftedal, O. T., Rumpler, W. V., & Ullrey, D. E. (1997). Dietary fibre influences nutrient utilization, growth and dry matter intake of green iguanas. Journal of nutrition, 127(8), 1501-1507.
Burrage, B. R. (1973). Comparative ecology of Chamaeleo pumilis pumilis and C. namaquensis. Annals of the South African Museum, 61, 1-158.
Cooper, W. E., & Vitt, L. J. (2002). Distribution, extent and evolution of plant consumption by lizards. Journal of Zoology, 257, 487-517.
Herrel, A. (2007). Herbivory and oraging mode in lizards. In S. M. Reilly, L. D. Mcbrayer, D. B. Miles, & (eds.), Lizard Ecology: The evolutionary consequences of the foraging mode(pp. 209-236). Cambridge, UK: Cambridge University Press.
Ibrahim, A. A. (2013). Some Aspects of Ecology of the Common Chameleom, Chamaeleo chamaeleon musae (Squamata: Chamaeleonidae) in Northern Sinai, Egypt. Russian Journal of Herpetology, 20(3), 203-212.
Iverson, J. (1988). Adaptations to herbivory in iguanine lizards. In G. M. Burghardt, A. S. Rand, & (eds), Iguanas of the world: their behavior, ecology and conservation.Saddle River, NJ, US: Noyes Publications.
Karen-Rotem, T., Bouskila, A., & Geffen, E. (2006). Ontogenetic habitat shift and risk of canabalism in the common chameleon. Behavioral Ecology and Sociobiology, 59, 723-731.
Mcbee, R. H., & McBee, V. H. (1988). the hindgut fermentation in the green iguana, iguana iguana. In G. M. Burghardt, & A. S. Rand, Iguanas of the world: their behavior, ecology and conservation.Saddle river, NJ, US.
Measey, G. J., Raselimanana, A., & Herrel, A. (2014). Ecology and life history of chameleons. In K. A. Tolley, & A. Herrel, The biology of chameleons(pp. 85-109). Berkeley/Los Angeles, California, US: University of California Press.
Necas, P. (1999/2004). Chameleons: Nature's Hidden Jewels(2 ed.). Germany: Chimaira.
Necas, P. (2018a, July 27). Veield Chameleon Feeding. The Chameleon Breeder Podcast. (B. Strand, Interviewer)
Necas, P. (2018b, March 8). Veiled Chameleon Taxonomy and Ecology. The Chameleon Breeder Podcast. (B. Strand, Interviewer)
Necas, P. (2019, 09 24). Why it is NOT true that the veileds eat leaves neither for the purpose of additional hydration nor for nutrition?Retrieved 12 17, 2019, from chameleons.info: https:///www.chameleons.info
Parsons, T. S., & Cameron, J. E. (1977). Internal releif of the digestive tract. In C. Gans, & T. S. Parsons, Biology of the reptilia volume 6. morphology(Vol. 6, pp. 159-223). New York, New York, US: Academic Press.
Pleguezuelos, J. M., Poveda,, J. C., Monterrubio, R., & Ontiveros, D. (1999). Feeding habits of the common chameleon, Chamaeleo chamaeleon in the southeastern Iberian Peninsula. Israel journal of zoology, 45, 267-276.
Reilly, S. M., McBrayer, L. D., & Miles, D. B. (2007). Lizard Ecology: The Evolutionary Consequences of Foraging Mode.(S. M. Reilly, L. D. McBrayer, & D. B. Miles, (Eds.) Cambridge, UK: Cambridge University Press.
Takahashi, H. (2008). Fruit feeding behaviour of a chamelon Furcifer oustaleti: comparison with insect foraging tactics. Journal of Herpetology, 42, 760-763.
Tolley, K. A., & Herrel, A. (. (2014). The Biology of Chameleons.Berkeley/Los Angeles, California, US: University of California Press.
Related Questions:
- Are all/some chameleons omnivores?
- What exactly makes an animal an omnivore?
- Should I be feeding my chameleon vegetables?
- Do some chameleons need plant material in their diet?
- Are chameleons able to digest/extract nutrients from plant material; do they have the right kind of digestive system?
- Do some chameleons use plants a supplemental water source?
- Why do some chameleons—veileds, in particular—eat the plants in their cages? Do wild veileds do the same thing?
While even addressing—no less answering—all of the above questions is beyond the scope of this blog, I would like to discuss some, and I want it to be as clear as possible what I am saying, what I think the evidence suggests, and where I think the evidence is inconclusive.
------------------
[1]This is Necas’ point…more to follow.
------------------
Data point:
Some captive chameleons have been observed consuming the plants in their cages.
While captive Chamaeleo calyptratus (plural calyptratii?) provide the most famous example, this behaviour has also been noted in captive specimens of C. chamaeleon(Ibrahim, 2013), (Herrel, 2007). Moreover, this behaviour does not appear to be restricted to captivity. Wild examples of C. calyptratus calyptratus (Necas, 1999/2004), C. calyptratus calcarifer (Necas, 1999/2004), C. chamaeleon (Karen-Rotem, Bouskila, & Geffen, 2006) (Herrel, 2007) (Ibrahim, 2013), C. namaquensis (Burrage, 1973)and Furcifer oustaleti (Takahashi, 2008) have either been observed eating plant material, or else significant plant material was found in their faecal samples. Takahashi, Burrage and Ibrahim have noted not only vegetation, but fruit in the faeces of their respective species. Given this, it is reasonable to amend our original data point:
Some chameleons eat plant material, including fruits.
Several theories have been proffered to explain this (Measey, Raselimanana, & Herrel, 2014). Among these are:
A) accidental ingestion (Cooper & Vitt, 2002);
B) as a dietary tool to move soft prey (e.g. caterpillars) through the digestive tract (Necas, 2018a);
C) as a means of supplementing hydration in dry habitats (Burrage, 1973), and
D)to supplement some nutritional element missing from the diet.
Theory a) may explain some instances. Hornworms, for instance, are notorious for their grip. A delicate leaf may give way before the worm’s grip, in which case both worm and leaf fragment are ingested. In fact, I have observed this very situation, but the chameleon actually spit out the leaf. Likewise, I can imagine certain species of Brookesia or Rhampholeon ingesting the occasional fallen leaf while hunting tiny bugs on the forest floor; but such cases seem implausible as a comprehensive explanation here. For one thing, the species in question—namely C. calyptratus, calcalifer, chameleon, namaquensis and Furcifer oustaleti—are all large chameleons. And with the exception of namaquensis—whose hunting grounds are often not covered by leaf litter—all are arboreal feeders, so the Brookesia/Rhampholeon example does not apply. More importantly, chameleons are precision hunters—highly adapted to make accurate strikes on their prey. It strains credulity to think that they are accidentally ingesting enough plant material to account for the data. A fortiori, it has been observed that in at least some examples of herbivory, the chameleon uses a non-lingual feeding behaviour (Takahashi, 2008)(and pretty much everyone who has watched their chameleon eating a hibiscus flower), which seems inconsistent with accidental ingestion.
Theory B) has been suggested by Petr Necas (2018a),(2019)from his first hand experience with C. calyptratus both in Yemen, and in his decades of keeping, breeding, and researching this and many other species. The idea here is thus: Calyptratus has a relatively short intestinal tract (small intestine and colon) that lacks the curves and bends characteristic of other animals (Necas, 2018a). Consequently, loose or liquid gut contents are difficult to pass. During certain times of year, the most prolific prey items for C. calyptratus are soft-bodied caterpillars, and the nymphs of various orthopterans (Necas, 2019). Such prey items are low in chitin, and relatively soft bodied—the very kinds of prey items that one would think would create the kind of loose homogenous gut content in question. In the absence of spiny, firm body parts to help the intestines get a “grip” on these looser contents, calyptratus will ingests plant material to fill this role. Basically, plant matter replaces chitin as dietary fibre. I must admit that I have had a certain intuitive draw to this position. First, it posits a novel and creative explanation of the data. Second, it pulls together facts about chameleon biology, ecology and morphology in a cogent and intuitive way. Necas goes on to explain that the consumption of plant material as a means of hydration is inconsistent with the data: the majority of the herbivorous activity takes place during the wet season (Necas, 2019). This jives with our observations of captive calyptratus, which consume plants even when they are well hydrated.
Again, I have been sympathetic to this account, but there are some shortcomings here. First, the theory appears to be light on the peer-reviewed support, and my admittedly brief survey of the relevant literature suggests the same. Although nothing I have read directly addresses Necas’ hypothesis, I have also found no other researcher actively pursuing this line. That being said, Galileo didn’t enjoy a lot of public support in his time either. Second, the theory and its companion assertion (the part about hydration) fail to account for the other data about F. oustaleti, C. chamaeleon and C. namaquensis. It’s not entirely clear that any of these latter species experience a season wherein the insect prey consists mostly of soft-bodied insects thus necessitating the consumption of plants as fibre. Ibrahim (2013) noted equal amounts of plant matter in the faeces of C. chamaeleon throughout the year. Likewise, tying together data from (Ibrahim, 2013) (Pleguezuelos, Poveda,, Monterrubio, & Ontiveros, 1999)(Burrage, 1973)(Karen-Rotem, Bouskila, & Geffen, 2006), we see a rather interesting correlation between the aridity of the environment and the level of plant material in the faeces of C. chamaeleon. Plant matter was found in the faeces of specimens from the relatively arid climates of the Northern Sinai (Egypt), Israel and Lybia, but conspicuously absent from specimens taken from a significantly less arid area of Spain (Measey, Raselimanana, & Herrel, 2014, p. 108). For clarity’s sake: there seems to be a correlation between the ingestion of plant matter by C. chamaeleon and the aridity of its habitat. Intuitively, this seems to partially undermine Necas’ hypothesis, and his discounting of the hydration view (theory c), above). Finally, Necas’ account assumes not only a particular view of the digestive capacity of C. calyptratus—something that stands in contrast to the scant treatment digestion gets in a volume dedicated to chameleon biology (see (Tolley & Herrel, 2014)), but would also require that we generalize to all the abovementioned species. That being said, the evidence in not conclusive either way.
As I mentioned in the previous section, there is some empirical support for c)—the theory that the ingestion of plant matter by chameleons is an adaptation to supplement hydration. Chameleons of the same species present levels of plant material in their faeces that appear to vary with the aridity of their environments (see above). However, this data is equally challenged by the findings Takahashi (2008) and Necas (2019). We will discuss Takahashi’s data below, but he has observed (and re-created) instances of Furcifer oustaleti seemingly seeking out fruit regardless of hydration. And anecdotal though the following evidence is, we have all witnessed well-hydrated chameleons consuming plant matter. So, while it is not surprising that, e.g., C. namaquensis regularly consumes fleshy plant material (Burrage, 1973), dehydration seems an unlikely immediate cause of all instances of herbivory. Although, herbivory could turn out to be a well-entrenched vestigial genetic adaptation—still present, but no longer necessary…More on this later.
Theory d)—the view that chameleons are supplementing their nutrient intake via the ingestion of plants—is perhaps the most contentious and demanding of the four. To be clear, this account is not merely arguing that plant material is important for digestion, but that the body is actually able to extract certain nutrients from the plant material—nutrients that form an important or necessary part of the diet. It is both contentious and demanding for the following reasons. First, there is so much community pushback against it (and I sympathize with this intuition, but admit it is merely an inductive leap that admits of many inconsistent explanations). Second, the view either involves a number of contentious assumptions about chameleon digestion, or else demands a massive research project into the same. The problems are myriad. From what I can tell, given the cursory treatment of digestion in multiple sources e.g. (Anderson & Higham, 2014), we just don’t know enough about digestion in chameleons to know whether nutrient extraction from plant material is even possible. Indeed, the small intestine and colon are given a whopping two paragraphs in an entire volume about chameleon biology see (Anderson & Higham, 2014, p. 52)—the most cited source therein coming from over forty years ago (Parson and Cameron, 1977)! That alone testifies to the state of our knowledge. There are probably some compounds that any digestive system can sequester from plant matter (even celery adds something), but the question is whether anything important/necessary is plant-derived. To this, I am less than optimistic about something forthcoming in the near future. However, thoroughly unqualified though I undoubtedly am, I can (at the very least) bring conceptual analysis to bear here…
From other vertebrates, we can glean something about herbivory and omnivory. Obligate herbivores, as well as frequently herbivorous omnivores, tend to have longer, more complex digestive systems. These systems employ various means of breaking down plant matter. Having several stomachs, chewing cud, having a long small intestine and a colon capable of fermentation, excessive, sometimes day-long eating, and symbiotic relationships with certain microorganisms are all adaptations for breaking down plant material. More relevantly, herbivorous reptiles such as iguanas often display teeth capable of efficient chewing/shredding of leaves, a long intestinal tract and/or special intestinal valves that slow the movement of food through the gut, as well as various digestive aids including enzymes and micro organism symbiosis (Cooper & Vitt, 2002); (Baer, Oftedal, Rumpler, & Ullrey, 1997) (Iverson, 1988) (Mcbee & McBee, 1988). While these adaptations are present in herbivorous (and some omnivorous reptiles), reptiles that consume only small amounts of plant matter tend to lack these more complex adaptations, and consequently consume more of the easily digested plant parts such as fruit, nectar, flowers and pollen (Cooper & Vitt, 2002).
So, how does this information help us? Well, for starters, it is commonly held that chameleons have short digestive tracts (Necas, 2018a) (Necas, 2019) (Anderson & Higham, 2014). Photos available online at Chameleons! Online E-Zine—courtesy of Bill Strand and Tom Greek DVM—bear this out: The esophagus and stomach appear to be as long as the entire gastro-intestinal tract. Likewise, there does not appear to be any textual support for chameleons being capable of the kind of gastro-intestinal fermentation that occurs in herbivorous reptiles like iguanas, or even omnivores such as the bearded dragon[2]. Chameleon dentition and chewing also seem inconsistent with an ability to break down plant matter in a way conducive to digestion (Anderson & Higham, 2014)—something for which their herbivorous reptilian relatives are well adapted (Cooper and Vitt, 2002). That being said, the photo evidence from Strand and Greek comes from dissected Trioceros deremensis, and Calumma parsonii—neither of which species are commonly thought to have herbivorous tendencies. It might very well turn out that veiled chameleons are anatomically different—displaying specific adaptations for plant digestion. However, I cannot find any literature that supports this view (nor any that tells against it)[3]. For the most part, the literature appears to assume digestive anatomical homogeneity across Chamaeleonidae. However, one would expect that if something novel were discovered in the digestive tract of C. calyptratus it would have made the journals.
------------------------
[2] Is this why chameleon poop doesn't stink?
[3]I am indebted to Kinyonga for pointing this out. In personal correspondences, she has repeatedly asked me whether veiled chameleons, in particular, have short digestive tracts. Her point is particularly topical here, as it really is veileds that we want to know about. Unfortunately, I have no answer here. The literature appears to lump all chameleons into one digestive-tract phenotype.
------------------------
Tying together the previous two paragraphs, there seems to be some prima facie reasons for doubting theory d)—the view that chameleons are able to extract certain nutrients from plant material—nutrients that form an important or necessary part of their diet. Again, this is hardly conclusive. Indeed, it conflates a distinction cut by Cooper and Vitt (2002)above; namely, that different plant parts are more or less easily digested. Folivorous reptiles generally have special adaptations to process the relatively hard to digest foliage of plants (Cooper & Vitt, 2002). However, reptiles that are not so endowed tend to feed on the more easily digestible parts of plants such as fruit, nectar and pollen. So, even if theory d) seems dubious in light of the previous two paragraphs, that does not tell against the possibility that chameleons are able to extract nutrients from e.g. fruit.
In a highly relevant paper, Takahashi (2008) examines the fruit eating habits of Furcifer oustaleti. Having observed a wild specimen feed on the fruit of Grangeria porosa, Takahashi devised a study wherein 24 wild-caught F. oustaleti were each presented with a horsefly in one instance, and the fruit of Chassalia princei in another. Takahashi found that tongue projection was employed in the case of the horsefly, but jaw prehension was used for the fruit. According to Takahashi (2008) and Measey, Raselimanana and Herbel (2014, p. 108), this suggests that the subjects distinguished between the two food items and used prey-appropriate methods of feeding. More clearly, it looks like the chameleons were able to distinguish the fruit as such and yet proceeded to ingest it. Takahashi is not alone in his findings. Burrage (1973) also observed C. namaquensis employing different feeding techniques for fruit. Moreover, Takahashi’s observations and subsequent experiments were conducted over several years between November and April (the wet season)—the implication being that hydration was not a factor. This makes theory d) less dubious, at least for fruit. If chameleons are purposefully consuming fruit, the mechanisms of evolution would seem to support their doing so for a reason. If that reason isn’t hydration, then it could be nutrition.
Prima facie, the scant evidence seems to support the view that (some/most?) chameleons do not have the hardware to extract nutrients from the hard to digest foliage of plants[4]. On the other hand, certain chameleons appear to be able to distinguish the more easily digestible fruit as a different kind of food item—appropriate to elicit a different kind of feeding behaviour. And the equally scant evidence here might support the view that these chameleons are doing it for nutritional reasons.
--------------------
[4] Let me stress that this last point is tenuous at best. Indeed, all I did was piece together some superficial anatomical facts about herbivores, compare them to some equally flimsy data about chameleon anatomy in a cursory sort of way, and adduce a half-formed hypothesis that is supported by as much intuition as it is empirical evidence. But the research just isn’t there.
--------------------
While we obviously need to amend theory d) to reflect the fruit/foliage distinction, there is a more pressing concern: What often prompts us to ask why chameleons sometimes eat plants is the observation that our captive veiled chameleons are often observed devouring the plants in their cages. And the plants in their cages are rarely covered in fruit or flowers—the more easily digested parts. So, even if wild specimens of F. oustaleti sometimes purposefully consume fruit, this does not help us understand why captive veiled chameleons eat leaves.
Earlier, during the discussion of theory c), I said that I would come back to a point about evolution. The point was that herbivory in chameleons might be an evolutionary adaptation that imbued certain members of a species with a slight survival advantage—thereby allowing them to pass on whatever gene was responsible for the behaviour. Just to spit ball some ideas here: Perhaps sometime in their evolutionary history, chameleons from arid climates did consume leaves to supplement their hydration needs. Similarly, perhaps famine did prompt certain individuals to consume plant matter. And, inefficient though the digestive system of a chameleon is for extracting nutrients from plants, the small energetic gain might have tipped the balance in favour of survival for those intrepid few. As I said, even celery probably has some energetic benefit given a particular metabolic rate. Perhaps during certain times of year, the winds carry small amounts of calcium carbonate and deposit it on the foliage, making herbivory incidental to the supplemental source of calcium[5]. Alternatively, it could be all of the above, or some other evolutionary quirk. The point is that herbivory in the evolutionary history of some chameleons might well have imbued certain individuals with a slight survival advantage over their conspecifics, and this was enough to ensure the passing on of whatever gene/s were responsible for the behaviour.
---------------------
[5] I credit the genesis of this idea to Kinyonga, who has observed more prevalent herbivory in female veileds. Perhaps sometime in their evolutionary history, gravid females went on to produce larger and healthier clutches if they ate leaves dusted in calcium rich sediment..
---------------------
What exactly does this mean for us? First, if herbivory turns out to be some adaptation that helped chameleons survive during drought or famine or else was incidental to the increase uptake of calcium, then it does not appear to apply to captive specimens. In other words, it does not appear to be a requirement for captive individuals, as we don’t typically subject them to these extreme conditions. So, according to this line, the adapted behaviour of herbivory is a vestigial relic of times past. If this is right, then the answer it delivers to our over-arching question is thus: Chameleons eat plants because sometime in their evolutionary history such behaviour helped them to eek-out a living during tough times. Both the behaviour and the gene responsible for it have been encoded in the genetic make up of certain species; and forty years of captivity are not enough to undo tens of thousands of evolution. In short, the view is that chameleons do not require plants in their diet, but continue to consume plant material because of a no longer necessary genetic adaptation.
Is this a satisfying account of every instance of herbivory in chameleons? I certainly find it a plausible account for many instances. Indeed, it actually ties together several of the theories we considered: Chameleons have used herbivory to supplement their hydration requirements, to gain just enough energetic benefit to get through a famine, even to supplement their calcium requirements. Indeed, nothing about this last theory/hypothesis (guess?) discounts Necas’ dietary fibre view (see above). Necas has said that in the absence of something to help move soft items through the gut, veiled chameleons have been observed vomiting (Necas, 2018a). Herbivory, in this context, might have helped them retain the precious contents of their guts, rather than loosing those potential nutrients by vomiting. So I suppose we need to amend our list of theories to include this fifth, evolutionary account. Luckily the next letter in our theory list is e)…fortuitous.
To be clear, this is not a novel account. Necas(2018a), Tolley and Herrel (2014), Reilly et. al (2007)and a host of others have suggested this view. However, it is interesting how the evolutionary account is compatible with many of the theories proffered to explain the phenomenon of chameleon herbivory. Indeed, theory e) stands as a partial vindication for those who speculate that chameleons consume plants for hydration, for nutrition, for fibre, etc. Chameleons probably did, in fact, use plants for all these reasons. For those still convinced that we ought to be offering fruits and veggies to our modern-day captive pets, feel free to stop reading now: Some species have adapted to be able to consume small amounts of these items, and it probably doesn’t hurt. For everyone else, it might be worthwhile considering that although herbivory might be a genetically entrenched behaviour, the purpose it used to serve (at least according to this line of thought) is no longer applicable, since our captive charges don’t face the kind of survival challenges that—in combination with the mechanisms of natural selection—resulted in their ancestors’ passing on the genes responsible for herbivorous behaviour[6].
------------------------
[6] Please note that I am not conflating the onto/phylogenetic distinction. Ontogenetic behavioral oddities that imbue an individual with a survival advantage often end up becoming phylogenetic adaptations.
-----------------------
Of course, a lot of this germinal theory depends on some pretty long logical leaps, quite a bit of speculation, and no small amount of plain old guesswork. Perhaps the most glaring of these shortcomings is the following inference:
- Chameleons appear to have short digestive tracts.
- Herbivores and some heavily herbivorous omnivores don’t.
- It looks like chameleons don’t have the right kind of stuff to process hard to digest plant matter.
- Bumblebees have small wings compared to their body size.
- The vast majority of good flyers do not.
- It looks like bumblebees don’t have the right kind of stuff to fly.
What of our list of questions? Well, I was thinking about going through them one by one, and noting where a possible answer occurs in this longwinded ramble. But I just broke 4000 words, and it’s New Year’s. I’ll leave the minutia to anyone patient enough to read this...
Works Cited
Anderson, C. V., & Higham, T. E. (2014). Chameleon Anatomy. In K. Tolley, & A. Herrel, The biology of chameleons(pp. 7-55). Berkeley/Los Angeles, California, US: University of California Press.
Baer, D. J., Oftedal, O. T., Rumpler, W. V., & Ullrey, D. E. (1997). Dietary fibre influences nutrient utilization, growth and dry matter intake of green iguanas. Journal of nutrition, 127(8), 1501-1507.
Burrage, B. R. (1973). Comparative ecology of Chamaeleo pumilis pumilis and C. namaquensis. Annals of the South African Museum, 61, 1-158.
Cooper, W. E., & Vitt, L. J. (2002). Distribution, extent and evolution of plant consumption by lizards. Journal of Zoology, 257, 487-517.
Herrel, A. (2007). Herbivory and oraging mode in lizards. In S. M. Reilly, L. D. Mcbrayer, D. B. Miles, & (eds.), Lizard Ecology: The evolutionary consequences of the foraging mode(pp. 209-236). Cambridge, UK: Cambridge University Press.
Ibrahim, A. A. (2013). Some Aspects of Ecology of the Common Chameleom, Chamaeleo chamaeleon musae (Squamata: Chamaeleonidae) in Northern Sinai, Egypt. Russian Journal of Herpetology, 20(3), 203-212.
Iverson, J. (1988). Adaptations to herbivory in iguanine lizards. In G. M. Burghardt, A. S. Rand, & (eds), Iguanas of the world: their behavior, ecology and conservation.Saddle River, NJ, US: Noyes Publications.
Karen-Rotem, T., Bouskila, A., & Geffen, E. (2006). Ontogenetic habitat shift and risk of canabalism in the common chameleon. Behavioral Ecology and Sociobiology, 59, 723-731.
Mcbee, R. H., & McBee, V. H. (1988). the hindgut fermentation in the green iguana, iguana iguana. In G. M. Burghardt, & A. S. Rand, Iguanas of the world: their behavior, ecology and conservation.Saddle river, NJ, US.
Measey, G. J., Raselimanana, A., & Herrel, A. (2014). Ecology and life history of chameleons. In K. A. Tolley, & A. Herrel, The biology of chameleons(pp. 85-109). Berkeley/Los Angeles, California, US: University of California Press.
Necas, P. (1999/2004). Chameleons: Nature's Hidden Jewels(2 ed.). Germany: Chimaira.
Necas, P. (2018a, July 27). Veield Chameleon Feeding. The Chameleon Breeder Podcast. (B. Strand, Interviewer)
Necas, P. (2018b, March 8). Veiled Chameleon Taxonomy and Ecology. The Chameleon Breeder Podcast. (B. Strand, Interviewer)
Necas, P. (2019, 09 24). Why it is NOT true that the veileds eat leaves neither for the purpose of additional hydration nor for nutrition?Retrieved 12 17, 2019, from chameleons.info: https:///www.chameleons.info
Parsons, T. S., & Cameron, J. E. (1977). Internal releif of the digestive tract. In C. Gans, & T. S. Parsons, Biology of the reptilia volume 6. morphology(Vol. 6, pp. 159-223). New York, New York, US: Academic Press.
Pleguezuelos, J. M., Poveda,, J. C., Monterrubio, R., & Ontiveros, D. (1999). Feeding habits of the common chameleon, Chamaeleo chamaeleon in the southeastern Iberian Peninsula. Israel journal of zoology, 45, 267-276.
Reilly, S. M., McBrayer, L. D., & Miles, D. B. (2007). Lizard Ecology: The Evolutionary Consequences of Foraging Mode.(S. M. Reilly, L. D. McBrayer, & D. B. Miles, (Eds.) Cambridge, UK: Cambridge University Press.
Takahashi, H. (2008). Fruit feeding behaviour of a chamelon Furcifer oustaleti: comparison with insect foraging tactics. Journal of Herpetology, 42, 760-763.
Tolley, K. A., & Herrel, A. (. (2014). The Biology of Chameleons.Berkeley/Los Angeles, California, US: University of California Press.
