Seasonal shifts in sociosexual behaviour and reproductive phenology in giraffe

Abstract

Reproductive phenology (timing) is a heritable trait that confers a range of fitness or survival advantages. Giraffe (Giraffa spp.) breed year-round; however, some studies have suggested adaptive birth pulses, where demanding stages of reproduction coincide with seasonal increases in resource availability (phenological match). Here we use 3.5 years of demographic data to investigate the sociosexual behaviour and reproductive phenology of Angolan giraffe (G. g. angolensis) in the hyper arid northern Namib Desert, Namibia. We show that, in a highly seasonal desert ecosystem, giraffe gave birth to significantly more calves during the wet season. These calves were more likely to survive their first year of life, suggesting that season of birth may convey a fitness advantage. Furthermore, we show a decrease in sexual segregation between dominant (dark) adult males and adult females during the hot-dry season, suggesting a possible hot-dry season conception pulse. Finally, we demonstrate that the strongest correlation between the temporal pattern of births (wet-season pulse) and that of decreased sexual segregation (hot-dry season pulse) was time lagged by 15 months. This time lag corresponds to the period of gestation in giraffe, suggesting that a seasonal reduction in sexual segregation in this population may explain a seasonal birth pulse. These findings add to a sparse literature on the breeding phenology of giraffe, of asynchronously breeding megaherbivores, and of species with a gestation period of greater than 1 year. Results are discussed in terms of the possible environmental drivers of both season of conception and season of birth in this population. Furthermore, we highlight how predicted increases in seasonal instability due to climate change could reduce any putative fitness advantage associated with earlier birth dates. To maximize survival, many species align giving birth with periods where food resources are increased. We studied this phenomenon in giraffe, a large mammal that breeds year-round at all latitudes but, in seasonal environments, possibly aligns the most energetically demanding stages of reproduction with resource-rich periods. We demonstrate that in a highly seasonal desert ecosystem, giraffe gave birth to significantly more calves during the wet season. These calves were more likely to survive their first year of life, suggesting that the wet season birth pulse is adaptive. Furthermore, we show that dominant male giraffe are more likely to seek females in the hot-dry season, suggesting a conception pulse that helps to explain the wet season birth pulse. These results advance our understanding of giraffe social and mating behaviours and contribute to a better understanding of breeding phenology in megaherbivores.