Thursday, January 7, 2016

Side-Blotched Lizards

"Remind yourself that you don't have to do what everyone else is doing" -Rajat Dogra

A Side-blotched Lizard in the Sonoran Desert

The Side-blotched Lizard, Uta stanbursiana, is the most widespread and common lizard in the North American Deserts. One can observe them throughout the year, even during the cool winter months. Though they can be quite colorful, otherwise nothing jumps out about this typical lizard. I recall one guide book referring to them as "the popcorn of the desert", because they provide snacks to so many desert predators; snakes, birds, coyotes, and other lizards.

With this abundance, comes an increased chance that lizards of this species will cross paths and interact. This common lizard, which a seasoned herpetologist might ignore due to its abundance is part of a complex social hierarchy which lends insight into important ideas in evolution and physiology.

Males of this species are polymorphic (multiple "types" that look and behave differently). There are large males which control and defend large territories, smaller males which defend smaller territories, and males which control and defend no territories and resemble female Side-blotched Lizards. Females

Selective forces should shape evolution of males towards big, territorial males, able to control and monopolize large territories and thus female lizards. So why do the other two morphs exist?

If very large males become to abundant, they struggle. They compete with each other and have trouble defending the territory and the females in their respective territories. The smaller males are more able to defend their small territories and mate with the females there.

If either of the two males that defend territories become abundant the males that resemble females have an advantage. They are able to "sneak" into other males territories and mate with females unnoticed as the larger males fight with each other.

But when these "sneakers" become most common the large males are able to identify them and chase them away easily.

In other words, the relative fitness of each type of male is related directly to its relative abundance within the population. This is called density dependent natural selection. If one type of male becomes too abundant it increases the fitness of the other two types, and this is how all three are able to persist in the population. This is the hipster form of natural selection, you're most cool (fit) when you're doing things that no one else is doing.

Another interesting part of this social hierarchy deals with the length of each lizards tail. Recall that an important defense mechanism for many lizards is that they are able to drop their tails when threatened by a predator and that they eventually regenerate them. The predator might be then left with a tail, but no lizard.

Losing a tail appears to have social costs in Side-blotched Lizards. When a dominant lizard loses its tail, subordinate lizards recognize this and behave more aggressively towards the formerly dominant lizard, perhaps recognizing that this lizard no longer has the social status it once did due to the unfavorable encounter. The length of the tail and whether or not it was regenerated appears to be especially important for status in female lizards.

References and further reading:

Sinervo, Barry, et al. "Testosterone, endurance, and Darwinian fitness: natural and sexual selection on the physiological bases of alternative male behaviors in side-blotched lizards." Hormones and Behavior 38.4 (2000): 222-233.

Ferguson, Gary W. "Mating behaviour of the side-blotched lizards of the genus Uta (Sauria: Iguandidae)." Animal Behaviour 18 (1970): 65-72.

Sinervo, Barry, and Curt M. Lively. "The rock-paper-scissors game and the evolution of alternative male strategies." Nature 380.6571 (1996): 240-243.

Fox, Stanley F., Nancy A. Heger, and Linda S. Delay. "Social cost of tail loss in Uta stansburiana: lizard tails as status-signalling badges." Animal Behaviour 39.3 (1990): 549-554.

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