Contrasting timing of parturition of chital Axis axis and gaur Bos gaurus in tropical South India - the role of body mass and seasonal forage quality

Seasonal variation in forage availability and quality is understood to affect the annual timing of parturition in large herbivores. In India–where seasonal monsoonal rains define variation in forage availability and quality–chital Axis axis exhibit stronger seasonality in parturition than the larger gaur Bos gaurus. We hypothesized that this difference can be explained by forage requirements determined by body mass. We developed a model to simulate changes in leaf biomass and nitrogen content based on plant available moisture and nutrients, and calibrated our model with field data. Our results show that the minimum forage nitrogen content required by lactating gaur was available throughout the year, but that required by lactating chital was available for less than 40% of the year, i.e. during the early wet season, which coincides with the annual peak period of chital births. The three to four month spread of chital births, which begins in the dry season, implies that the period of highest quality is also important for females to replenish maternal reserves for future reproduction and help maximize the growth rate of neonates. This spread also indicates low synchrony of chital births and suggests that predator swamping was less important in influencing their timing of parturition. As monsoonal rain exhibits annual temporal variation, we analyzed our model under different rainfall patterns while keeping the total annual rainfall constant. We found that the difference between the durations of how long forage quality is available to satisfy the minimum requirements of lactating gaur and lactating chital is similar for all simulated patterns. This insensitivity to variable rainfall patterns lends support to our hypothesis that forage requirements determined by body mass is one plausible explanation for the variation in parturition strategies among large herbivores species. Many animals live in environments where the availability and quality of food follow cyclical patterns of seasonal variation. In the temperate zone, food of adequate quantity is available only during the summer and not in winter. In tropical regions with seasonal rainfall, abundant food of high quality during the wet season is replaced by less abundant food of low quality during the dry season. This seasonality in food availability and quality is understood to influence the timing of parturition of mammalian herbivores (Rutberg 1987, Wittemyer et al. 2007). The ‘seasonality’ hypothesis proposes