Abstract
In reptiles, reproductive maturity is often determined by size rather than age. Consequently, growth early in life may influence population dynamics through effects on generation time and survival to reproduction. Because reproductive phenology and pre- and post-natal growth are temperature dependent, environmental conditions may induce multi-species cohort effects on body size in sympatric reptiles. I present evidence of this using 10 years of neonatal size data for three sympatric viviparous snakes, Dekay's Brown snakes (Storeria dekayi), Red-bellied Snakes (S. occipitomaculata), and Common Garter snakes (Thamnophis sirtalis). End-of-season neonatal size varied in parallel across species such that snout-vent length was 36%-61% greater and mass was 65%-223% greater in years when gestating females could achieve higher April-May (vs. June-July or August-September) operative temperatures. Thus, temperature had a larger impact during follicular enlargement and ovulation than during gestation or post-natal growth. Multi-species cohort effects like these may affect population dynamics and the magnitude of these effects may increase with climate change.