Whiptails fueled by grasshoppers
The western whiptail is an active hunter that stalks and pursues prey, including grasshoppers. To see a larger image, click here. Photo by Jeff Mitton
By Jeff Mitton
Quite often, while walking in open space around Boulder, my slow advance triggers the explosions of grasshoppers frantic to escape, and it makes me wonder about what danger they are attempting to avoid.
Predators, no doubt, but which ones? At first I presumed that deer mice and voles keep the grasshoppers on the move, but now I have a different hypothesis.
I had stopped at a rest stop west of Grand Junction on a jaunt to the Colorado Plateau. I always stop there, for although it looks desolate from Interstate 70, the landscape usually has something worth seeing: rock squirrels, sego lilies, several species of lizards.
A western whiptail, Aspidoscelis tigris (formerly Cnemidophorus tigris), was at work beneath a Utah juniper, digging through a pile of accumulated debris in search of insects. After watching whiptails for several minutes, I concluded that they are chronically frantic, digging furiously, dashing about, flicking their forked tongues to taste fragrances. A study of their behavior reported that they flick their tongues an average of 456 times per hour.
I followed a whiptail that was stalking a grasshopper. Within a few minutes, the whiptail had captured it, and I grabbed a photo before the grasshopper disappeared forever. So now I think it is hunting lizards, more than mice and voles, that have selected for wary grasshoppers that flee at the slightest provocation.
Western whiptails are medium-sized, slightly over 4 inches from snout to the base of the tail, but their most distinctive feature is the long, slender tail, which is considerably longer than the body. As the lizard cruises about, looking for insects or acting aggressively with other whiptails, the tail is used for balance, especially when running upright on the back legs.
But the long, whipping tail also diverts the strikes of predators. Coyotes, foxes and a variety of hawks and falcons will take whiptails if they can. But quite often, these predators strike at the whipping tail, rather than the head or body. The lizard can jettison its tail, which continues its thrashing movement so that the predator usually presses the attack on the tail while the lizard escapes.
One of the most fascinating aspects of the whiptail lizards is that they have multiple modes of reproduction. The western whiptail, from what I have found in the literature, always reproduces sexually. But many whiptail species are all female and they produce eggs asexually, or more specifically, via parthenogenesis. Their specific mode of parthenogenesis produces eggs that are genetically identical to the mother and to each other, so a population of whiptail lizards is a clone.
The late University of Colorado professor Paul Maslin conducted some of the early work on parthenogenic whiptails, and he was the first to provide definitive evidence of asexual reproduction in a paper published in 1971. Two decades earlier, he had demonstrated that the lizards in an all-female population were genetically identical by successfully moving skin grafts among individuals. Later, Orlando Cuellar and Kevin Fitzgerald, who received their Ph.D. degrees at CU, conducted more work on parthenogens.
On this day in the field, I saw only the western whiptail, but it is frequently found living with other whiptail species. In New Mexico, it is found in the same localities with the little striped whiptail and the New Mexico whiptail (Aspidoscelis inornata and A. neomexicana, respectively). The New Mexico whiptail, the state reptile of the Land of Enchantment, is one of the parthenogenic species, and thorough genetic studies have clearly demonstrated that it is a hybrid produced by a cross between western and little striped whiptails. This cross did not occur just once, for 15 different clones of the New Mexico whiptail have been identified.
About 80 parthenogenic species have been reported, including other whiptails, geckos, rock lizards, Komodo dragons and snakes.
Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
November 2013