Timing of Birth and Body Condition in Neonates of Two Gartersnake Species from Central México

The body condition at birth of two gartersnake species occurring in Me´ xico increased with later periods of the birth season, but SVL and mass did not vary in similar fashion. Various hypotheses have been proposed to explain variation in the traits associated with neonate body condition, including maternal effects such as maternal size, condition, and investment on developing follicles. The birth date could be considered a maternal effect correlated with changes in ecological factors that the gravid female experiences during the birthing season (Stewart 1989; Weatherhead et al. 1999). Much of this variation, however, remains unexplained.In temperate zones, offspring born earlier in the breeding season might have more time to acquire resources before hibernation and consequently have higher survival rates. In this study, we tested the hypothesis that neonates born earlier in the birth season are smaller (in body size, mass, or both combined as body condition) than neonates born later, but that they have more time to acquire resources before hibernation. We used 43 laboratory-born litters of Thamnophis melanogaster and 21 litters of T. eques obtained from females collected from syntopic field sites. For both species, neonate body condition at birth differed within the birth season, such that sizes were smaller in the early portion, and larger in the later portion, of the season. When compared, only the snout–vent length and mass of neonates, averaged for each litter, did not change throughout the birth season. The temporal variation in neonate body condition found in the two species could explain increased offspring survival when neonates face challenging environmental conditions. If these differences reflect variations in the environment throughout the birth season, then body condition at birth could be relevant for neonate survival.Universidad Autonoma del Estado de Mexico (2865/2010, 2663/2013