Thursday, December 14, 2017


Image result for rock paper scissors

Biological Evolution
Environmental & Science Education
Edward Hessler

You probably have played the game "rock-paper-scissors." The small (~2.5" or ~6 cm) Side-blotched lizards (Uta stansburiana)  have "played" this game over a long period of time.

These lizards are what evolutionary biologists refer to polymorphic, i.e., they occur in different forms or morphs.  The three color morphs of male side-blotched lizards differ in throat-and-side color: yellow, orange and blue.

In the rock-paper-scissors game, rock beats scissors, paper beats rock, and scissors beats rock.  In the side-blotched lizard game the orange-throated males beat blue-throated males; the orange-throated males are beaten by yellow-throated males; and the yellow-throated males are beaten by the blue-throated males. 

These involve strategies that keep the cycle going.  This short film shows each of them and how they work. The orange-throated males are very-dominant, the yellow-throated males use a "sneaker" strategy, and the blue-throated males a mate-guarding strategy. Blue-throated males cooperate with each other in defense of their territories and closely-guard the females.

As the interactions occur over time the morph frequencies vary.  First there is a dominance hierarchy. Orange, blue and yellow in that order (a testosterone driven community of lizards). Orange-throated males have the largest territories and harems. Orange-throated males have large territories and female harems. Blue-throated males have smaller territories and harems but cooperate with eachother in defense of their respective territories and closely-guard their harems. Yellow-throated males do not have territories or harems.

When orange-throated males are common their breeding success begins to drop (there are very few harems of blue-throated females with which to mate). The yellow-throated males take advantage by sneaking into the orange-throated territories and their numbers increase. However, they are not successful in mating with females in blue-throated territories and the blue-throated population grows.

This mating pattern and population highs and lows are shown in this short--3 minutes--film.

Keep in mind that the rock-paper-scissors game is an analogy and like many analogies breaks down or requires a deeper explanation when it is studied in the natural world or in different areas over the range of the lizards. There are some populations where only one morph survives. The evolutionary biology of side-blotched lizards has captured the research interest of Dr. Barry Sinervo, a professor at the University of Santa Cruz, his graduate students and post-doctoral students for some 25 years.  There you can learn more about the research and how it is performed.

These variations and details about the evolution of these morphs are described in a University of Santa Cruz news release which describes the work of one of Sinero's post-doctoral students. There is also more information on the mating strategies of these lizards from a course blog, Cornell University.

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