Many migratory species have within population variation in migration distance or timing (differential migration), while others vary in the proportion of the population that migrates (partial migration). Species with migrant and resident individuals provide a natural system for testing hypotheses concerning the evolution of migration. At Mountain Lake Biological Station, in southwestern Virginia, the red-spotted newt (Notophthalmus viridescens) is partially migratory. A portion of the population migrates to the terrestrial habitat after the breeding season, while the remainder stay as residents in the breeding pond overwinter. We examined the effects of habitat quality and gender on migration decision by manipulating population density and sex ratio in a factorial field experiment using large aquatic enclosures. In a separate experiment we investigated the reproductive costs of migrating for females by repeatedly sampling eggs from enclosed resident and migrant females.

Results/Conclusions

We found that migration can be a plastic response influenced by both local density and gender. In high density enclosures 63% of newts migrated, compared to 39% in low density enclosures. Females migrated earlier than males and 64% more females migrated, suggesting important sex-based trade-offs of migration. Our first experiment demonstrated experimentally that migration can be an adaptation for avoiding unfavorable environments. In the second experiment we found that resident females did not lay more eggs or begin laying eggs earlier than migrant females, despite having an early start to the breeding season and a higher mass than migrants. The prolonged breeding season of N. viridescens may have buffered the late arrival time of migrant females compared to species with short mating and egg laying periods or species that form breeding territories, where arrival time is more important to female fecundity. We expect that migration has a higher reproductive cost for males, where arrival time can determine competitive ability, the number of available mates, and sperm precedence. By directly measuring life history parameters of migrants and residents, our research provides evidence to explain the persistence of partial migration within a population.