When I first encountered Anolis baleatus, this Hispaniolan crown-giant was mostly an inconvenience. At the time I was gathering data for my doctoral thesis by cycling preserved anoles through a µCT-scanner. Most of the adult specimens of A. baleatus were just too large to easily fit into the scan chamber, so it took a lot of patience and creativity to acquire any decent images of the appendicular girdles, which are the body parts I was interested in.
During that process I also acquired radiographic images of the head skeleton, and found unusual patterns of crenulation in this species. The cranium of Anolis baleatus displays a great degree of seemingly asymmetrical (or at least somewhat irregular) ornamentation across its dorsal surface. This is especially pronounced on the prefrontal and frontal bones, and completely obscures all superficial distinction between them in adult lizards. In adults, cranial ornamentation is also borne by the paired nasals, maxillae, and postorbitals, and the parietal (see figure).
Both Steven Poe (1998) and Susan Evans (2008) mentioned this ossified garnish, but a thorough account of their variation among anoles remains absent from the primary literature. Richard Etheridge and Kevin de Queiroz (1988) were probably the first to report on skull ornaments in anoles (as part of a discussion of several iguanian lizards with similar cranial adornments), and remarked that the distribution patterns of dermal rugae may reflect those of the topographically associated epidermal scales.
Overall, this ornamentation appears to be relatively uncommon among anoles, especially to the degree expressed in Anolis baleatus (and several other crown-giant ecomorph anoles). Considering the osteologically robust appearance of crown-giants, even at early stages of ontogenetic development, this gives rise to questions regarding the development of these ornamental patterns. Thanks to the collection efforts of Luke Mahler (University of Toronto), and a postdoctoral position in his lab, I was able to acquire CT-image data representing an ontogenetic series of this species, ranging from very young juveniles to skeletally mature adults.
While parts of the paired frontals of juveniles are covered in modest eminences, prominent cranial ornamentation is absent from small specimens (see figure). Likely, growth of these ornaments begins very late during ontogenetic development. Ornaments on the prefrontals and parietal are only evident in specimens that, to the best of our judgement, are approaching sexual maturity. We looked at fifteen specimens per sex, representing a range of juvenile and subadult sizes, and this general pattern is consistent throughout the image data. Schwartz (1974) inferred that anoles in the ricordii group reach sexual maturity between 100 and 110 mm snout-vent length (SVL), and we observed the first prominent ornaments at sizes between 90 and 95 mm SVL. Assuming that differences in size directly represent ontogenetic growth, these findings imply that Anolis baleatus starts to grow elaborate ornamentation as it approaches sexual maturity, and that expansion and growth of these ornaments then continues into skeletal maturity. Interestingly, both males and females appear to develop them at roughly the same body size.
The function and evolutionary cause of these structures remain unknown, and these are questions we are currently investigating. Body size is an important correlate for the occurrence of cranial ornaments, but these structures may also conceivably play roles in defense, feeding, or intraspecific agonistic interactions. Stay tuned!
Videos
References
Etheridge, R. & de Queiroz, K. (1988): A phylogeny of Iguanidae.─ [In:] Estes, R.D. & Pregill, G.K. (eds.): Phylogenetic Relationships of the Lizard Families: Essays Commemorating Charles L Camp, 283-367; Stanford: Stanford University Press.
Evans, S. (2008): The skull of lizards and tuatara.─ [In:] Gans, C., Gaunt, A.S. & Adler, K. (eds.), Biology of the Reptilia, vol. 20:1-347; Society for the Study of Amphibians and Reptiles, Ithaca, New York.
Poe, S. (1998): Skull characters and the cladistic relationships of the Hispaniolan dwarf twig Anolis.─ Herpetological Monographs, 12:192-236; The Herpetologists’ League.
Schwartz, A. (1974): An analysis of variation in the Hispaniolan giant anole, Anolis ricordi Dumeril and Bibron.─ Bull. Mus. Comp. Zool., 146:89-146.
- Shape Variation of the Pectoral Girdle of Anolis Ecomorphs - March 16, 2022
- Morphology of the Scapulocoracoid of Anolis Ecomorphs - September 3, 2020
- Cranial Ornamentation in Anolis baleatus - August 28, 2019
James T. Stroud
Very cool! Excellent post, thank you for sharing. It always occurred to me that this robust ossification of the skull is to facilitate male-male aggression, explaining why it’s especially pronounced in the males, rather than representing an ornament (although for the same reasons it might well also do that..). During agonistic interactions, males really chomp down hard on each others heads. My own experience with equestris in FL would definitely support this. Some ossification could be reactive to such interactions; local bone growth resulting from injuries, which may also explain why they aren’t as pronounced in juveniles which have not yet had such an interaction history. However I agree that there is much more likely a strong proactive genetic signature of ontogenetic growth. Similar ossifications/striations can be seen some trunk-crown anoles (e.g. porcatus/carolinensis), but not as dramatic as the crown-giants. Presumably they serve similar functions.
Anyway, fascinating post – thanks!
Kevin de Queiroz
For the record, skull ornamentation in anoles was discussed earlier by Etheridge (1959: see esp. pp. 74-78). This kind of ornamentation is present not only in crown giants but also in other giant anoles, notably the twig giants in the Chamaeleolis clade. However, it also occurs in some smaller species, such as some of the species in the Phenacosaurus clade and Anolis/Chamaelinorops/barbouri.
Mark
Hi Alex, nice post. This is something that we have discussed briefly on SquaMates podcast episode 8 http://squamatespod.com/archives/328, as it was brought up in the paper on Chamaeleolis by Alfonso et al. in the Anolis Newsletter VII. I would tentatively say that these structures are rather accessories than ornaments, per se. As you point out, their position is strongly related to the scales that lay atop them, and their expression is strongly related to the size of the skull and the maturity of the animals. They are a manifestation of the co-ossification of the skin to the skull, which proceeds as the lizards mature. I suspect that this is related to the aggression behaviour of these lizards (especially male-male competition); fights involving a lot of biting with a lot of force leads to cranial reinforcement, and one part of that reinforcement is the development of this kind of structure. Iguana iguana also develop similar superficial skull accessories, as do several frogs with reinforced skulls, such as Triprion, Hemiphractus (https://www.morphosource.org/Detail/ProjectDetail/Show/project_id/240), and especially the two venomous frogs Aparasphenodon brunoi and Corythomantis greeningi (https://www.cell.com/current-biology/pdfExtended/S0960-9822(15)00788-5), which, however, seem to have coopted these structures for more malicious purposes.
These are all very different from the ornamental ridges that we see in some animals, like chameleons, which are more than just co-ossification, but are ‘true’ ornaments (whatever that means). One way to distinguish the two is the consistency of fine details of pattern among individuals. I’d be willing to bet that this is much greater in chameleons than these large anoles.
Cheers,
Mark