Today I had the pleasure of attending an excellent talk by University of Florida undergraduate Griffin McNamara. I was really impressed with the work he presented, especially for an undergraduate.
McNamara was investigating the ratio in digit length between the 2nd and 4th digits. This is interesting because in mammals, especially humans, this ratio is sex specific, with men typically having longer ring fingers than pointer fingers. A lot of research has looked into the developmental reasons for this, with a likely relationship to hormone exposure of the developing fetus. Applying these ideas to anoles makes sense because the toe of anoles are somewhat unique in lizards, as we all know here. McNamara is looking into if sexual dimorphism in 2nd and 4th digit length was also present for anoles.
McNamara wasn’t the first person to measure this in anoles, but he was the first to use cleared and stained specimens, which likely greatly improved his ability to accurately measure digit length. Interestingly, he found that the pattern was reversed, with males having longer 2nd digits, not longer 4th digits as in mammals. In addition, this pattern didn’t show up until a lizard’s teenage years, with juvenile anoles not showing a difference between the sexes. Using cell staining to visualize dividing cells, he was able to narrow down the digit discrepancy to growth in the 2nd phalanx during sexual maturity.
Suspecting that this late onset dimorphism might still be related to hormone exposure, McNamara got his fingers on some testosterone-treated female anoles from collaborators and found that they had “masculinized” digit ratios, although not as much as true males.
I thought this was a great study, combining old school cleared-and-stained approaches with cell biology and experimental endocrinology. It also opens up lots of interesting questions. Is there an adaptive reason that mammals and lizards have sexually dimorphic digit lengths? Is it just a quirk of development? Does this digit length reversal have anything to do with the fact that the shape of anoles’ rear feet is already kind of mirrored as compared to our hands?
- Evolution 2019: How Many Species of Anolis distichus? - June 23, 2019
- SICB 2019: Anole Setal Morphological Diversity - January 7, 2019
- SICB 2019: Sexual Differences in Relative Lengths of Toes - January 5, 2019
Skip Lazell
Very interesting! I never noticed…. Thanks!