Diel activity patterns in the breeding migrations of winter-breeding anurans

Abstract

Diel activity patterns in the breeding migrations of four winter-breeding anurans (Hyla crucifer, Pseudacris nigrita, Pseudacris ornata, and Rana utricularia) were examined at a breeding pond in South Carolina, U.S.A., using a terrestrial drift fence with pitfall traps. Traps were censused at 0600, 1200, 1800, and 2400 during six 24-h periods from 1982 through 1984. Census dates were selected during which the weather was expected to remain uniformly warm and rainy throughout all four time intervals. This allowed us to separate the effect of time of day on migratory activity from the effects of temperature and rainfall. There were significant differences in migratory activity among time intervals, but no significant differences among time intervals in either mean air temperature or rainfall. Males and females of all four species migrated to and from the pond primarily during the two night intervals. The absence of daylight appears to be a necessary trigger for migratory behavior in these anurans. Temperature and moisture conditions often limit their breeding migrations, and the failure to utilize suitable diurnal weather conditions represents an additional constraint. We suggest that the observed diel pattern of migratory activity may be an adaptation to avoid visual predators.