Adaptive Hypotheses of Birth Synchrony in Ruminants: An Interspecific Test

Abstract

The ecological causes of birth synchrony in ruminants are examined. Measures of seasonality, including latitude and seasonal distribution of evapotranspiration, correlate well with birth-season length, explaining about half the variance of a 27-species sample. With one exception, the African buffalo, species with precocious "following" young display short birth seasons in all climates. It is inferred that predation on newborns plays an important role in imposing tight birth synchrony on following species, especially those occupying tropical and subtropical climates. A crude classification of species by diet (browsers vs. grazers) did not significantly improve predictions of birth-season length among sample species. I argue that, as a consequence of the diverse and complex mechanisms used to control the onset of breeding, birth-season lengths are extremely variable both between and within species. The onset of reproduction in species whose birth-season lengths are adapted to seasonal fluctuations in food supply and weather should be governed by reliable predictors of these conditions, such as photoperiod and nutritional status. However, species specially adapted to synchronize births in order to reduce predation on newborns may also respond to lunar cycles, social signals, and other cues that do not predict environmental conditions.