Latitudinal and seasonal variation in reproductive effort of the eastern fence lizard (Sceloporus undulatus)

Geographic variation in life-history traits among populations of wide-ranging species is influenced by both spa­tial and temporal aspects of the environment. Rarely, however, are the effects of both aspects examined concur­rently. We collected gravid female lizards (Sceloporus undulatus) from northern (Indiana), central (Mississippi) and southern (Florida) populations, spanning nearly the full latitudinal range of the species, to examine among-population differences in strategies of reproductive energy allocation. Adult females from the southern popula­tion were smaller, and produced fewer and smaller eggs in their first clutches than did females from the more northern populations. Southern females were more likely to produce a second clutch, and second clutches were smaller than first clutches for females from the 2 northern populations. Together these trends eliminated popu­lation differences in overall reproductive output after accounting for body size. The trend for greater reproduc­tive energy to be allocated to first clutches at higher latitudes, and to later clutches at lower latitudes is corrobo­rated by published data from field studies on multiple populations. Distributing reproductive effort by producing more clutches of smaller eggs may be an adaptive response to the long season available for egg incubation and lizard activity in sub-tropical southern environments. In contrast, allocating greater resources to early reproduc­tion may enhance maternal fitness in the relatively short activity seasons that characterize more northern sites.