Snake Mating Systems, Behavior, and Evolution: The Revisionary Implications of

Recent FindingsRivas, Jesus A., Burghardt, Gordon M. (2005). Snake Mating Systems, Behavior and Evolution: The Revisionary Implications of Recent Findings. Journal of Comparative

Psychology. 119(4), 447-454.

Laura Cain, Taylor Casillas, Sarah Fukui and Nick O’BrienPsychology of Sex Differences

October 20, 2010

Difference between Polyandry and Polygyny?

• Polyandry: A mating pattern in which a female mates with more than one male in a single breeding season.

• Polygyny: few males monopolize access to many females– Benefit = mate with more females, spread genes

• Ex: male elephant seals (dominant one gets to mate)

• Previous to this study, it was widely believed that the primary form of reproduction in snakes is polygyny

• Males have upper hand/advantage (sexual selection) – In mating (male-male competition)– Attractiveness to females (body size, morphological

behavior, physiological attributes)

Framework for understanding diversity of mating systems

• Relies on…– Degree males can monopolize– Distribution of resources– Availability of mates

• Thus… sexual selection > for the sex that benefits more from increased number of matings (more offspring)– Polygyny = for males

• Four types of polygyny that were believed to accompany all snakes– Female defense (mate guarding)– Hot-spot – Prolonged male searching– Explosive mating assemblage

Initial research on snake mating systems

• Disproportionate Research– done on one species – in a specific climate and location- inaccurate

findings– Diverse data for comparative findings unavailable– Poor database/invalid research due to:

• Secretive nature of snakes & other difficulties

– Contrary to prejudiced info available, polyandry has been found to be the primary, if not universal form of mating system in snakes, not polygyny

• Snakes are the first species in which polyandry best describes their mating system

Findings on snake mating systems

– Contrary to prejudiced info available, Polyandry has been found to be the primary, if not universal form of mating system in snakes, not polygyny

• Snakes are the first animal in which polyandry documented

– Evidence: males spend more time courting and mating• Choosier (larger more fertile female)• Less reproductive investment• Female multiple mating: found in all species (anaconda not

the rare exception anymore)• Males are smaller than females (would be opposite if

polygyny)

– Males rarely if ever feed during reproductive period• Suffer higher mortality rate

QuickTime™ and a decompressor

are needed to see this picture.

Evidence for polyandry in snakes

• Garter snake– DNA and molecular studies proved several sires in the litters of

garter snakes – Even with molecular methods to assess paternity( have found that

multiple paternity is the norm for snakes), the word polyandry was still avoided

• Although this species still recognized as polygynous

• Female Green Anaconda– One female lays in mud or water– 13 males come over

• Coil around her and attempt to mate

– This mating can last for up to a month• Males may stay with female until the end of her attractive period

http://www.youtube.com/watch?v=7TF7d4jvays

Female Snakes

• OSR (operational sex ratio) = <1:1 for most snake species– OSR is the ratio of available females to

available males

• Female snakes: large reproductive investment– Can’t always mate every year

• Leads to male-biased OSR– potential for females to mate numerous times

Conclusion

• Snakes are more likely to exhibit polygynandry (group mating) or polyandry than polygyny– Polgynandry is similar to promiscuity– When looking at their ancestors and sister taxa,

evidence for polyandry or serial monogamy– Explains paradoxical behavioral differences

• Ex: lack of territoriality or male-based sexual size dimorphism