Comparative Life Histories of Holbrookia Maculata and Sceloporus Undulatus in Western Nebraska.

The life histories of the iguanid lizards Holbrookia maculata and Sceloporus undulatus were studied in western Nebraska during 1978-1984. The two species differ in their reproductive strategies and age-specific survival. The reproductive strategy of H. maculata is invariant from year to year. Yearlings consistently produced a single clutch and older females (≥2 yr) consistently produced two clutches. The reproductive strategy of S. undulatus is opportunistic. Clutch frequency of S. undulatus varied among years, ranging from 1-3 for yearlings and 2-3 for older females. This variation in clutch frequency was a function of the length of the breeding season. Significant differences in the age-specific survivals of H. maculata and S. undulatus were found for juveniles and yearling females. In both cases, survival of H. maculata was greater. Despite differences in their life histories, H. maculata and S. undulatus are exposed to similar selective pressures in western Nebraska. Food does not appear to be limiting and mortality due to predation is low. Mortality is density-independent and is highest during the overwintering period. Differences in the reproductive strategies of H. maculata and S. undalatus are most likely due to their phylogenetic histories rather than to adaptation to different environments. The different reproductive strategies of the two species may account for differences in their age-specific survival. The high fecundity of yearling S. undulatus may explain its lower survival relative to H. maculata. Evidence for a cost of reproduction is provided by a negative relationship between clutch frequency of yearling S. undulatus and postreproductive survival. Because the response of lizards to specific environments may be constrained by their phylogenetic histories, comparison of life history strategies among syntopic species may not provide adequate tests of life history models. We conclude that identification of the mechanisms underlying variation in life history traits will most likely come from intraspecific comparisons.