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).
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)
4. C. dixoni (2N) - (see note)
5. C. grahamii* (2N) - (see note)
6. C. neomexicanus (2N) - New Mexican whiptail.
|* indicates Colorado native species.|
Note: Species #5, C. grahamii, and probably #4, C. dixoni, are included within C. tesselatus by Hammerson, 1986).
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.
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.