The fitness correlates of scalation asymmetry in garter snakes Thamnophis sirtalis parietalis

1. In a field population in Manitoba, about 16% of Red-Sided Garter Snakes (Thamnophis sirtalis parietalis) have a different number of ribs on either side of the body, as indicated by anomalous ventral scales. A study on congenerics shows that this asymmetry reflects disruption of embryogenesis by environmental factors, including suboptimal maternal thermoregulation during pregnancy. 

2. Correlates of this scalation asymmetry in adult snakes can clarify the degree to which (and pathways by which) the thermal environment that an individual experiences during embryogenesis affects its later fitness. 

3. Data on >4000 free-ranging snakes reveal no significant association between asymmetry and fitness-related traits such as body size, body condition, locomotor speeds or female ability to resist harassment from males. 

4. However, adult male snakes with scale asymmetry dispersed from the den-based mating aggregations sooner than did symmetric males. In outdoor arenas, asymmetric males were less able to obtain matings than were symmetric males; and if asymmetric males did mate, they consistently used the hemipenis on the side of the body opposite to their extra rib. 

5. Our data suggest that careful thermoregulation by a female garter snake during pregnancy provides a fitness benefit to her sons after they mature, via sexual selection.