Everything You Wanted to Know About
Whiptail Lizards (Genus Cnemidophorus)

and Quite a Lot that You Didn't

by Donald L. Blanchard

A slightly different version of this paper was published in The Cold Blooded News,
the newsletter of the Colorado Herpetological Society, Vol 23, #10 October, 1996.

Contents:
Introduction
Taxonomy
Behavior
Current Problems
References


Introduction:

The book "Biology of Whiptail Lizards (Genus Cnemidophorus)," available in the CHS library, is a collection of scientific papers presented at a symposium by the American Society of Ichthyologists and Herpetologists at the University of Oklahoma in August, 1984. If you have never read scientific papers, you probably don't want to. They tend to be very dry and highly technical, full of obscure words specific to their particular discipline. What is worse, scientists seem to delight in using words not in your home dictionary wherever possible, where two or three small words you wouldn't need a dictionary for would say the same thing better. And if that isn't bad enough, whenever a number is included in the data, it must be subjected to a whole battery of statistical analyses to prove that the number has some significance. If you aren't a statistician, this might as well be written in Greek. (If you read Greek, it might as well be written in Swahili!)

I don't claim to be expert in reading technical papers (I read neither Greek nor Swahili), but I have had some practice in ignoring words I can't find in my dictionary and still deducing something of what (I think) is being said. The following article is a summary of things I found interesting about the "Biology of Whiptail Lizards" (with a few of my own observations thrown in).


Taxonomy:

Lizards of the American genus Cnemidophorus, known commonly as whiptails and/or racerunners, have posed problems for scientists for nearly as long as they have been known. Edward Drinker Cope, known more for his work as a paleontologist and discoverer of dinosaurs, but also regarded as our first cnemidophorologist, wrote of them in 1900 that "The discrimination of the North American species of this genus is the most difficult problem in our herpetology." In 1906, Gadow wrote in the Proceedings of the Royal Society of London (75:227-375) that "Most of the 'species' are so plastic, so variable, that they may well drive the systematist to despair. Not two authorities will, nor can, possibly agree upon the number of admissible species." (Quoted in Wright, 1993)

The taxonomy of the genus did not start to become clear until the 1950's and early '60's. It was reported in 1958 that no male lizards had been discovered in a collection of specimens of C. tesselatus. In the same year, parthenogenesis was reported in the genus Lacerta of the exclusively Old World family Lacertidae (which corresponds closely with the Cnemidophorus of the New World family Teiidae). Quickly thereafter, it was discovered that there were also no males in C. exsanguis, C. neomexicanus, or C. velox (Lowe, 1993).

In 1931, 14 species of Cnemidophorus were recognized; now at least 45 have been identified, with other populations known but still awaiting formal recognition. Over 30% of the known species are parthenogenic (Wright, 1993). It is now known that parthenogenic species originate in whiptails when a female of one bisexual species mates with a male from another. Instead of producing sterile offspring (the normal result of interspecific matings), the young are all female, contain double the normal number of chromosomes (2N), and are 'born' fertile (i.e. capable of producing fertile eggs without benefit of male fertilization). In many parthenogenic species, a subsequent mating with a male from another species has led to a parthenogenic population having triple the normal chromosome count (3N).

All of the presently identified whiptail species are classified into six "species groups", two of which contain only bisexual species, two of which contain only parthenogenic species, and two which contain both bisexual and parthenogenic species. These six species groups are as follows (only U.S. species are listed): (from Wright, 1993)

C. lemniscatus Species Group:
Bisexual and parthenogenic species. Argentina north to Honduras. 11 named species.
C. cozumela Species Group:
Parthenogenic only. Southern Mexico. 2 named species.
C. deppii Species Group:
Bisexual only. Honduras to Southern California. 5 species.
1. C. hyperythrus - Orange-throated whiptail.
C. tigris Species Group:
Bisexual only. Northern Mexico and western U.S.A. 1 species.
2. C. tigris* - Western whiptail.
C. tesselatus Species Group:
Parthenogenic only. Northern Mexico, Arizona, New Mexico, West Texas, Western Oklahoma, and Colorado. 4 species.
3. C. tesselatus* (3N) - Colorado checkered whiptail.
4. C. dixoni (2N) - (see note)
5. C. grahamii* (2N) - (see note)
6. C. neomexicanus (2N) - New Mexican whiptail.
C. sexlineatus Species Group:
Bisexual and parthenogenic. Mexico and U.S.A. 22 named species.
Bisexuals:Parthenogens:
7. C. sexlineatus* - Six-lined racerunner. 11. C. exsanguis (3N)
8. C. burti 12. C. flagellicaudus (3N)
9. C. gularis 13. C. innotatus (2N)
10. C. septemvittatus 14. C. laredoensis (2N)
15. C. sonorae (3N)
16. C. uniparens (3N)
17. C. velox* (3N) - Plateau striped whiptail.
* indicates Colorado native species.

Note: Species #5, C. grahamii, and probably #4, C. dixoni, are included within C. tesselatus by Hammerson, 1986).


Behavior:

All whiptail species employ essentially the same feeding strategy. Most dry-climate lizards spend the majority of their active time stationary, waiting for prey to come within range. Prey is usually detected visually. This is referred to as sit-and-wait, or ambush predation. Whiptails, on the other hand, forage actively, rarely sitting in one place long enough for a collector to slip a noose over their heads (personal observation). Prey may be detected visually or by smell. This strategy is known as widely ranging predation. (Gila monsters and monitors are other widely ranging predators.)

While many sit-and-wait lizards are strongly territorial, fighting to defend a prime basking rock or a favored tree branch, whiptails appear to be totally non-territorial; foraging ranges conflict (personal observation). This is good, because whiptails generally have vastly larger home ranges than other lizards in the same locales, and so much area would be very costly to defend (Etheridge & Wit, 1993).

Whiptails divide their foraging time between open areas in full sun and the shade beneath bushes according to their temperature requirements. Whiptails (C. exsanguis and C. velox) in one study were able to maintain a body temperature of 101.5 degrees F [38.6 C] within 1 or 2 degrees. (Bowker, 1993) Whiptails seem to travel faster when in the open, relying primarily on vision to detect prey. When under bushes, they typically move slower, scratching and digging with their forefeet into the leaf litter, apparently detecting prey as much by smell as by sight. The principal dietary item of dry climate whiptails is termites, although beetles and beetle larvae, caterpillars, grasshoppers, ant lions, leafhoppers, and arachnids are also taken. (Anderson, 1993)

A seeming corollary of wide foraging appears to be reduced activity time; wide foragers are typically active for only 1/2 to 1/3 as many hours per day as sit-and-wait predators living in the same areas. They also are typically active for fewer days per year. (Etheridge & Wit, 1993) On a typical summer day in Colorado, six-lined racerunners are only active for about three hours in the late morning and early afternoon (personal observation). Activity is reported to occur for only about 4 months per year.

In spite of reduced activity time, wide foragers expend considerably more metabolic energy per day than sit-and-wait predators; the deficit is apparently made up by higher foraging efficiency. (Etheridge & Wit, 1993) In other words, they find more to eat by actively hunting for it, yet take far less time to do it. Thus, they are out, and therefore vulnerable to predation themselves, for considerably less time. Whiptails also have the habit of closing off the opening to their burrows when they retire (personal observation), which consumes added energy but may also discourage potential predators.


Current Problems:

Now that the classification of whiptail lizards has been established to an acceptable degree (taxonomy is always subject to change at a moment's notice), new problems in cnemidophorology have been discovered. One of them is the definition of a parthenogenic "species." The usual definition of a biological species is an interbreeding population of organisms sharing a common ancestor. Parthenogenic whiptails don't interbreed. Furthermore, while some parthenogens (e.g. C. exsanguis) are sufficiently alike genetically to be clones of a single hybrid ancestor, others (e.g. C. tesselatus) differ enough to appear to have descended from multiple separate hybrid matings between the same two parent (bisexual) species. Current practice, although still hotly debated, is to retain the traditional binomial nomenclature (i.e. genus and species, but no subspecies) for the former, but to refer to the latter as a binomial "species complex." This implies a degree of uncertainty about the origin of the group; did the population actually have multiple origins, or did mutations occur during parthenogenic reproduction?

The apparent resolution of the taxonomy problem has revealed yet another problem, this time in ecology. It is a fundamental tenet of evolution that two or more species cannot simultaneously occupy the same ecological niche in the same locality. Stated differently, when two species share the same area and have identical ecological requirements, one species is supposed to out-compete and drive the other species from the territory. In bisexual whiptail species, this appears to hold true; where the geographic ranges of two bisexual species overlap, they seem to prefer differing microhabitats -- one preferring grassy areas and the other more open terrains, as an example. The same seems not to hold true among parthenogenic species.

All species of whiptails in the western United States are sufficiently similar in food preferences, foraging habits, and periods of activity as to provide no discernable difference in the ecological niches occupied. Adult size variation in western species is less than two to one, and most prey ingested by the largest species is small enough for even the smallest adults to eat. Yet frequently, where one bisexual species is found, one or even two parthenogenic species will be found intermingled in the very same fields. Where two bisexual species occupy distinctive microhabitats, the parthogens are likely to occupy both indiscriminately. Several papers (Schall, 1993; Cuellar, 1993; Price, et al, 1993) present analyses and discussions of this problem, but the data is ambiguous and the conclusion remains unclear.

One suggestion is that the parthenogenic species are newcomers on the scene, having existed for only hundreds of years, rather than the hundreds of thousands or millions of years of most reptile species (Wright, 1993). It is noted that the geographic ranges of parthenogenic whiptails is significantly less than that of bisexual species (Schall, 1993). Perhaps the parthenogens haven't been around long enough to displace their bisexual competitors.

Another suggestion is that the parthenogenic species are opportunistic 'weeds,' adaptable enough to quickly exploit new or disturbed ecosystems. In support of this hypothesis is the fact that the reproductive capacity per generation for an all female population is (nominally) double that of a population comprising half males. The studies reported in the present work were not of long enough duration to convincingly confirm or refute this notion. The issue remains unresolved.