Reproduction involves costs and benefits to a female snake. Costs include increased predation risk as well as lower future survival and reproduction probability, whereas benefits are the current production of offspring. Reproduction also is highly energy demanding: at each reproductive event, a female must decide between the allocation of energy to female condition or to reproductive output. Moreover, energy allocation linked to reproduction may be influenced by environmental features, especially climate and food availability. During 2008 and 2009, we studied the female reproductive tactics of Thamnophis scaliger, a viviparous and earthworm-eating snake distributed at rather high altitudes in central México. Gravid females were collected in the field and maintained in captivity until giving birth. The low percentage of females that were gravid each year suggested they did not reproduce annually. This result probably was driven by the seasonal constrained availability and low-energy content of earthworms, which preclude females from acquiring sufficient energy to fuel more frequent reproduction. Females also produced rather large litter sizes, implying a significant investment in reproduction. Univariate and multivariate analyses (path analysis) found the expected relationships between maternal body size and litter size and mass, and there was no trade-off of energy allocation between postpartum females and their litters. We found, however, a noteworthy relationship between parturition date and postpartum female body condition. Females that reproduced later benefited most from foraging opportunities during the short rainy and high prey-availability period and remained in better body condition.

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