Although anoles are one of the top model systems in evolutionary biology today, it took decades of dedicated and inquisitive research to lay the groundwork. The foundation of understanding that we draw upon today to set up hypotheses, build experiments, and infer the process of evolution was slowly built by numerous researchers, including Ernest Williams, Rodolfo Ruibal, Stan Rand, and Ray Huey, to name only a few. Tom Jenssen, Professor Emeritus at Virginia Tech, stands among these giants – his work on the ethoecology of anoles laid the foundation for how we understand anole behavior, particularly display behaviors, and set up the experimental framework for how we conduct behavioral studies in anoles even today.
If you’re familiar with Tom’s research, then you’ll know he’s worked on Anolis carolinensis for more than two decades and, before that, he studied several species of Caribbean anoles. But back when he was a graduate student, Tom’s main focus was on a little-known anole from Mexico, Anolis nebulosus. During this time, he tracked a single population of A. nebulosus for over three years, and examined the behavior of hundreds of lizards. In 1970 he published some of the results from this long-term study in the Journal of Herpetology.
I first read this paper when I was starting out in graduate school and have returned to it many times since. One reason is that the study is an impressive piece of natural history work, with plenty of attention to detail and interesting observations. Tom included very detailed information on habitat selection, thermal biology, and home range, among other characteristics. He even describes the various stages of skin shedding in this species.
But the most exciting part of Tom’s study, in my opinion, is the section on social behavior. Until Tom Jenssen started working on anoles, our understanding of their communication through head and dewlap movements was very scarce and some of the older accounts sounded almost fanciful. In a subsequent study, Tom describes these old studies in detail in his Introduction. The 1960s, however, saw the advent of quantitative descriptions of lizard social behavior through the time-motion analysis and display-action-pattern (DAP) graphs. DAP graphs basically summarize how the amplitude of head and dewlap motion change with time (see Figure 10 from Tom’s paper, left). Tom was among the first biologists to create DAPs for anoles and his study was the most thorough to date, as he created DAPs for hundreds of lizards of both genders and various age classes, spanning various years. Based on many observations in the field and in the laboratory, Tom was able to describe four different displays commonly observed in male A. nebulosus. By simultaneously considering how head motion and dewlap motion change during displays (Fig. 10), Tom was able to simplify complex patterns into fairly stereotyped display ‘types.’ This enabled Tom to decipher how anoles were communicating with each other and, more importantly, what they were ‘saying’ in a sophisticated way. For example, he found that, whether in the wild or in the lab, dominant males always performed assertion displays when they saw a male challenger (see Fig. 10C). The assertion display was never underused in competitive encounters – Tom found that the display was used up to 23 times in a single hour observation period. They sometimes performed assertion displays to females, as well. Interestingly, Tom found that females used the assertion display, as well! Even though their dewlaps are not as large as in the males, females exhibited aggressive displays that followed the same display pattern as in males. Male or female, the message was the same – this is my territory! – and they used the same combination of head bobs and dewlap extensions to communicate this message.
Further, the urge to assert is clearly strong in A. nebulosus – hatchlings only a day old could do the ‘assertion’ display. In one case, he even observed two lizards hatch and, within only a few minutes, execute the ‘assertion’ display at each other! This result is particularly intriguing for those interested in the development of behavior. In many vertebrates such as birds and mammals, vocal communication usually requires a ‘babbling’ phase, when juveniles practice the ‘language’ of their species or their father. My colleagues and I recently found that even so-called ‘simpler’ vertebrates, such as the African clawed frog, Xenopus laevis, also go through a babbling phase in which male froglets explore and practice trills of different frequencies, going from female-like trills to, eventually, fully masculinized adult trills. Although these lizards don’t vocalize, they clearly have the tools upon hatching to communicate with each other in the manner of adults.
Another display type that Tom observed was the ‘challenge’ display. Challenges are more intense displays usually performed by males in close proximity to each other. This display is part of a more complicated dance involving different postures and motions. This ritualized dance led to direct combat when neither of the challengers would back down. The lizards would spar with open mouths and eventually lock jaws. Once locked together, each lizard would try to violently toss its competitor. In the field, Tom found that the victor would chase the loser off with a few exaggerated ‘assertion’ displays and, in the laboratory, the loser would never challenge the victor again. Tom observed this in female A. nebulosus only twice, and so he included his field notes from one of those encounters in the study. He notes that, as with the ‘assertion’ display, females exhibited ‘challenge’ displays that were nearly identical to those observed in males.
Tom’s work on Anolis nebulosus laid the groundwork for how we should think about social communication in anoles. Since then, research on social communication in anoles has flourished, aided greatly by Tom Jenssen himself, who has been studying Anolis behavior for nigh on five decades. Finally, for your reading enjoyment, I’ve included a snippet of Tom’s field notes that were published in his study, in which he describes in rich detail a courtship encounter in Anolis nebulosus.