Anolis maynardi. Photo by J. Losos
AA commentator and Jamaican student Kuti Ra remarks (links to previous posts added by me:
“I notice a lot of fuss on Anole Annals about the skull morphology of carolinensis clade anoles, so I thought I’d weigh in with a theory of my own. All that you are about to read comes from pure indirect observation and speculation, so please keep that in mind.
First of all, I believe that the skull morphology of these anoles is directly related to their arboreal lifestyle and, more specifically, to the diet that such a lifestyle would facilitate. There have been several posts [1,2] about nectivory in anoles, but all these instances seem to involve carolinensis clade anoles (A. maynardi and A. carolinensis) and possessing a long tapered skull would undoubtedly make such feeding behaviour easier; this could come very useful as a trunk crown anole would encounter several blossoms and such high up in the trees in addition to various small fruits. In this respect, the forceps-like jaws could function something akin to a fruit-eating birds’ bill; considering all this, it would seem that the jaw morphology of carolinensis clade anoles is simply a useful adaptation for life in the canopy. This conclusion seems even more likely when you consider that sexual selection very likely wouldn’t play a role here as observations of Anolis maynardi suggests that longer-snouted males aren’t any more successful at securing mates; and why would they be, having such a long jaw narrow jaw would translate to a lower bite force and thus a less likely chance of emerging as the victor in a territorial battle?
No long schnoz here. Anolis grahami. Photo by J. Losos
Of course, this theory begs the question ‘‘Why didn’t the trunk-crown anoles on other Greater Antillean islands evolve to look like their carolinensis clade counterparts?” In my opinion, the answer is that they didn’t need to. You see, on Jamaica the trunk crown anole, Anolis grahami, has a skull that is overall very similar to the sympatric A. lineatopus, and though they are traditionally classified as belonging to different ecomorphs, I can testify from personal observation that they often share the same micro-habitat and thus compete for the same resources; however since there are only a few ecomorphs present on Jamaica and wherever these two species occur they are of the only common species in that area, the resources available in a particular micro-habitat are often sufficient to support full populations of the two species; thus neither species has reached the point where it needs to adapt to consume different things than the other. Moving on to the neighbouring islands of Hispaniola and Puerto Rico, one can recognize three things: first off, there are a greater number of trunk-ground anoles than on Jamaica, and more trunk-ground anoles than trunk-crown; second, all or almost all of the other ecomorphs are present and third the jaws of the trunk-crown anoles are noticeably longer than that of sympatric trunk ground species (more so on Hispaniola than on Puerto Rico). On these islands, where there are often more species of anole competing for the resources of the trunk-ground niche, as well as additional ecomorphs to occupy other microhabitats, trunk crown anoles are becoming more and more specialized to take advantage of the resources available to them in the trees.
On Cuba, the largest island, there are often a great number of sympatric species of all ecomorphs and thus the habitat use of the anoles there is the most deeply segregated. I have read that in the Cuban rainforest, Anolis porcatus is almost never found out of the canopy; this seems to also be true of A.allisoni. This greater restriction to a particular microhabitat and need to capitalize on the resources found there is what I believe caused the Cuban trunk-crown to evolve such drastic adaptations to a trunk crown lifestyle. Well that’s my theory in a nutshell. I don’t know if all this has been said before but I haven’t been seen it mentioned. Like I said, most of it is based on indirect observation (looking at pictures on Google images) and speculation so I f you see any problems or holes in the theory, or if you’d like me to elaborate on or better explain a particular point, please let me know.
I think it is also important to note that the heads of trunk-crown anoles start out short on Jamaica and steadily get longer until you get to Cuba.
All the scenarios above would apply only to anoles in their natural habitats and not to populations inhabiting urban areas where more resources are available.
On a side note, have you ever noticed that trunk crown anoles have relatively tiny dewlaps? Perhaps one should look into how the ecomorph dewlaps are affected by those of neighboring ecomorphs. For example no matter what island you go to, the dewlap of any trunk-ground anole will always be comparatively larger than the dewlap of any trunk-crown anole on that island. I haven’t seen all species for Cuba, but I’m still pretty confident about it. I think these two ecomorphs in particular may have some very big effects on each other that no one has noticed yet.
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K.ra
Thanks to Jonathan Losos for putting this up,cant wait to see replies!I am by no means an authority on anoles, or much else for that matter, and I don’t think I will become one but I do find them interesting so I cant wait to see what the AA community makes of this.
K.ra
Thomas Sanger
As the instigator for many posts on anole heads, I greatly appreciate your insightful comments. At this time we have largely focused on recovering the pattern of cranial evolution. Testing ecological hypotheses regarding the driving forces of skull shape evolution is in dire need of further study.
As a follow-up to your comments I want to pose a few more questions to you. Your ideas largely focus on comparisons between the ecomorphs in the Greater Antilles, yet the species with the longest faces are found on islands in the Cayman Islands and the Bahamas. Competition between species can’t easily explain this more nuanced pattern. Do you think that your nectar hypothesis is sufficient to explain this?
Also, what about sexual dimorphism? We recently published a paper on sexual differences in the anole head which showed that the carolinensis anole crania are highly dimorphic (the AA post is titled, Evo-Devo Meets Sexual Dimorphism). Again, the most dimorphic species are those on the smaller islands. Do you think that your hypothesis is consistent with this observation?
If you would like copies of any of our recently published papers please let me know. It would be my pleasure to send them to you.
K.ra
I had actually read the post before I came up with the idea but A.maynardi and A.longiceps are a mystery to me too, in fact it seems that these species should have evolved decreased snout length like A.carolinensis upon becoming solitary anoles.I tried to find more info about the vegetation on Navassa and Little cayman and if it was in any different from that found on Cuba ,this however to me seems unlikely.Taking this into consideration the best explanation I can come up with as it relates to the theory is that these two species live on islands with few or no other species and thus sexual dimorphism has evolved as a means to decrease competition between the sexes,but then again while the sexes are dimorphic it appears to me that the skull of female A.maynardi is roughly equivalent in length to that of a male porcatus or allisoni from cuba and thus both would still classify as trunk crown anoles.Perhaps the snout allows the males to eat something the females cant.
Of course it is also possible that something other than environment played a role in driving the elongation of these species snouts,I doubt sexual selection is the answer though.Do you have a different idea about these two species?
K.ra
P.S. to clarify I doubt that sexual selection would be entirely responsible as if environment did not play a role then why didn’t the anoles on Cuba evolve the same level of dimorphism (the mechanism by which the elongation was achieved in A.maynardi has to be at homologous to that in the closely related Cuban species,and the Cuban species evolved first), why did females of only two species choose to select for this?
Finally it is important to note that female choice is not very influential in anoles as females don’t have a chance to mate with many adult males outside of the one who controls their territory and this would be decided by territorial battles which a long snouted male is more likely to loose if anything.
Levi Gray
I’m not sure about long-snouted males more likely to lose territorial battles. In anole species that have been looked at (lineatopus, Herrel et al. 2006; carolinensis, Herrel et al. 2007), larger males tend to have longer (and wider) heads, and see an increase in bite force. The males with longer heads would seem to have an advantage in territorial battles, though sorting out exactly what is responsible (maybe head width or musculature is more important?) is not the easiest thing to do. Some of the data does suggest that head length can’t tell the whole story, because larger lineatopus had disproportionally powerful bites despite an overall negative allometry.
In any case, I’m a little curious about the allometries in lineatopus. Overall there was a negative (sublinear) slope, but looking at the graph suggests the males have a much steeper slope than males and females combined. Anyone involved in that study care to comment? Do males have isometric or superlinear scaling?
K.ra
I would imagine that if you wanted to outfit a lizard for a lizard for winning a fight then in addition to increasing skull length you would also widen the end of the snout as much as possible so as to have a greater surface area over which to apply the power being generated by the jaw muscles; after all it doesn’t make much sense to have huge jaw and skull muscles if you can barely grip on to your enemy in order to bite his face off.This appears to be the case in carolinensis clade anoles,particularly A.longiceps-the skull of this species is very narrow so any extra force generated by jaw muscles should be rendered relatively ineffective,not to mention that the extra skull length means that the bite force has to be applied over a longer distance.
Thomas Sanger
Sorry for the delay. Its tough to keep up with these conversations while I am traveling.
Unfortunately, I have few answers for the ecologically-based questions. But, the hypotheses are readily testable for someone dedicated to pulling some long hours in the field. For example, do species with elongate faces eat distinct prey from species with shorter faces? Likewise, do males and females of these species feed on distinct prey or live in different parts of the environment? Someone could even address whether males with longer faces hold larger territories and, in turn, mate more often. To do it thoroughly is a lot of work, but it is all within reach.
As your alternative ideas also suggest, the questions regarding allometry are also in need to further investigation. For starters, we have no idea whether the posterior skull and jaw muscles scale uniquely in the carolinensis anoles compared to other species. We chose not to focus on this part of the skull in our dimorphism paper and instead focused on facial length because of its convergence with the hendersoni clade. It was clear, however, that the primary axes of skull shape dimorphism vary among species.
The allometric slopes were not published as part of our 2013 paper. The pairwise comparisons of slope between species showed us that nearly all non-carolinensis males have the same scaling (including lineatopus), all of which exhibit negative allometry. The carolinensis males deviate from this pattern, however, significantly increasing the slope of their facial allometries. I am not sure what Levi specifically mean about the slope of “males and females combined.” Males and females are discrete biological entities, and it seems unwise to average them together in my opinion.
So no, facial length “doesn’t tell us the whole story.” It is only part of a big pattern. We are starting to understand the developmental bases of facial length dimorphism, but have a long way to go before we understand the selective forces that reshaped head evolution of the carolinensis anoles.
As a parting image, lets try to image what a lizard would look like whose face elongates at a rate faster than body length (with positive allometry). If my imagination is working correctly the lizard would eventually get top heavy and fall over on to its chin. This is one time where the animal is probably thankful to maintain its negative allometry.
Levi Gray
Sorry, I probably should have been more clear. The slope from the Herrel et al. 2006 paper is what I was referring to. It appears the slope in figure 1 incorporated males and females, and has a slope of 0.89. Looking at just the males, it looks like the slope would be much steeper than the overall allometry, which is what was presented in the paper.
Since anole heads can be considered weapons (which commonly exhibit positive allometry), it’s not out of the realm of possibility to see a slope slightly larger than 1. At least, in theory. I’m guessing you’ve never encountered that within males of any anole species?
K.ra
Thanks to everyone who has commented so far!
Sorry for the late reply but I wanted to do a little more research on A.longiceps before commenting again;To address questions posed about whether or not A.longiceps and maynardi may achieve a greater bight strength by increasing jaw width : Judging by the profiles of these species on Caribherp it is clear that the skull of A.maynardi is not actually any wider than that of any other Carolinensis group anole, and that the skull of A.longiceps is actually quite narrow and does not at all appear well suited for battle so for this species at least I think it is safe to say that competition with other males was not a deciding factor in the elongation process .
I also notice that the skulls of carolinensis clade anoles not only get longer on smaller islands they also become shallower (this even appears to be somewhat true of the A.allisoni on Roatan compared to those on Cuba) and vice versa with [relatively] robust skulled species like A.allisoni and A.carolinensis on larger landmasses.
Finally, I discovered that Navassa is populated by only four native tree species,all mid sized trees with small ovoid fruit and small flowers,as well as one other basically extinct palm species that bore numerous white flowers along a long inflorescence.
I hope that one day a more thorough ecological study on A.longiceps and A.maynardi will be carried out, and on a unrelated note I hope to find a picture of A.fairchildi other than that of the preserved specimen. Anyone ever been to Cay Sal?