Evolutionary ecology of endocrine-mediated life-history variation in the garter snake Thamnophis elegans

The endocrine system plays an integral role in the regulation of key life-history traits. Insulin-like growth factor-1 (IGF-1) is a hormone that promotes growth and reproduction, and it has been implicated in the reduction of lifespan. IGF-1 is also capable of responding plastically to environmental stimuli such as resource availability and temperature. Thus pleiotropic control of life-history traits by IGF-1 could provide a mechanism for the evolution of correlated life-history traits in a new or changing environment. An ideal system in which to investigate the role of IGF-1 in life-history evolution exists in two ecotypes of the garter snake Thamnophis elegans, which derive from a single recent ancestral source but have evolved genetically divergent life-history characteristics. Snakes from meadow populations near Eagle Lake, California ( USA) exhibit slower growth rates, lower annual reproductive output, and longer median adult lifespans relative to populations along the lakeshore. We hypothesized that the IGF-1 system has differentiated between these ecotypes and can account for increased growth and reproduction and reduced survival in lakeshore vs. meadow snakes. We tested for a difference in plasma IGF-1 levels in free-ranging snakes from replicate populations of each ecotype over three years. IGF-1 levels were significantly associated with adult body size, reproductive output, and season in a manner that reflects established differences in prey ecology and age/size-specific reproduction between the ecotypes. These. findings are discussed in the context of theoretical expectations for a trade-off between reproduction and lifespan that is mediated by pleiotropic endocrine mechanisms.