The large, colourful dewlap is an obvious defining characteristic of the anole. Understandably, then, there has been a lot of investigation (and speculation) on what the dewlap is used for. Without doubt it’s for social communication, but to communicate what. Historically, the dewlap was thought to be used for species recognition, which remains a reasonable explanation today. But a typical assumption made by many anole researchers and evolutionary ecologists alike is the dewlap, and specifically its size, is effectively an ornament used to attract mates or advertise potential fighting ability among territorial rivals. In other words, the evolution of the dewlap is the product of sexual selection.
If that’s the case, then dewlap size should be linked to some aspect of an individual’s ‘quality’ or physical condition, especially in males who seem to be the ones courting females (not vice versa) or defending territories. This is because a male’s quality or condition can be hard to assess by general appearance alone, unless there is a key feature that provides an honest indicator of that quality. In anoles, this is assumed to be a large dewlap that’s physiologically costly to produce.
One easy way that has been proposed to test for sexual selection in the origin of a morphological structure like the dewlap, is to look how it scales with body size. Structures that are honest indicators of condition will be costly to develop and maintain. Large males are often in better condition than small males because of the underlying factors that result in bigger bodies (e.g., a history of successful foraging, superior growth rate, having ‘good’ genes). This means larger males can invest more in exaggerating the size of the dewlap than smaller males. There would be a clear evolutionary incentive to do so as well, because having a larger dewlap would attract more mates and appear more threatening to male rivals. The outcome of this should be disproportionately larger dewlaps in larger males. This is called positive allometry or hyper-allometry. If dewlap size has a hyper-allometric scaling relationship with body size, then it probably resulted from sexual selection. Or at least that’s the idea. And you can find this out by just measuring a bunch a males.
The dewlap of anoles featured heavily in the original formulation of this idea, with the conclusion being that dewlap size was hyper-allometric and assumed to be the product of sexual selection. Anoles have therefore become a classic example of how sexual selection drives hyper-allometric scaling in ornament size.
Tom Summers
Tom Summers was a graduate student who thought about hyper-allometric scaling a lot. He looked at the scaling relationship of ornaments that he had confirmed experimentally to be the target of sexual selection in fish, and found they were hyper-allometric…sometimes. Tom found natural selection on ornament size can often work in the opposite direction to sexual selection. This is because large ornaments can interfere with locomotion and often be conspicuous targets for predators. When these pressures are high, species tend not to show hyper-allometry in ornaments. Those ornaments were still the product of sexual selection, but their allometric scaling was dampened by opposing natural selection.
Tom turned this attention to the anoles, and found overwhelmingly that dewlap size was not hyper-allometric but hypo-allometric. That is, larger males have disproportionately smaller dewlaps than smaller males. He even looked at another group of lizards that have independently evolved a dewlap, the southeast Asian Draco, and found the same pattern. His results have just been published in the Journal of Evolutionary Biology.
The scaling relationship of the dewlap in both groups varied from one species to another, but never was it hyper-allometric. In the case of the anole dewlap, this variation in dewlap size was predicted by factors important in signal detection (receiver distance and habitat light). This was consistent with the general hypo-allometry of the dewlap as well.
The effectiveness of a visual flag (like the dewlap) in attracting the attention of a receiver (another lizard) is dependent on the gross size of that flag, not how big it is relative to the signaller’s body (i.e., allometric scaling is irrelevant). Beyond a particular threshold size, which is dependent on the visual acuity of the animal in question, there are diminishing returns for detection with increasing size. Even a large increase in dewlap size beyond a certain point wouldn’t really improve signal detection, a phenomenon known as ‘Weber’s Law’. The resulting pattern when comparing dewlap size among males is hypo-allometric scaling. Larger males have generally reached the size threshold for reliable detection, so there’s little point in further elaboration.
It also fits with the extensive amount of work showing that the dewlap is likely to be most important in signal detection, rather than a cue of quality.
So why such a dramatically different finding to earlier investigations of the anole dewlap? All studies prior to Tom’s measured dewlap size by catching the lizard and manually pulling out the dewlap using forceps. Simon Lailvaux has discovered that the skin of the dewlap varies in its elasticity. Larger dewlaps are going to be more stretchy than smaller dewlaps. This means you can probably pull the dewlap out to a larger size in larger males. This would subsequently generate the artifact of hyper-allometric scaling when comparing dewlap size across males of different size.
Tom had measured dewlap size from high-definition videos of free-ranging males fully extending their dewlaps during display. There are various analyses in his paper that confirm this approach provides an accurate measure of dewlap size. His logic at the time was this view of the dewlap would be how lizards actually see and evaluate the size of the dewlap relative to body size. It also meant animals didn’t have to be caught, so the approach was less intrusive for the animal (always a plus). It just happened he avoided the potential problem of over stretching the dewlap if he had caught the animals and manually extended the dewlap by hand.
What does this mean for all that data that has been based on researchers pulling out the dewlap using forceps to measure its size? Honestly, I don’t know. Maybe nothing depending on what the data are being used for. Maybe everything if the data are being used in allometry studies.