Seasonal variation in forage availability and quality is understood to affect the annual timing of parturition in large herbivores. The forage quantity and quality requirements of these large herbivores are determined by their body mass. In India—where seasonal variation in forage availability is defined by monsoonal rains—chital Axis axis exhibit stronger seasonality in parturition than the larger gaur Bos gaurus. We hypothesized that this difference is a function of 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.

Results/Conclusions

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. Since chital births were spread over three to four months, this implies low synchrony of their births and suggests that predation was a less important factor 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 of sufficient 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 additional support to our hypothesis that body mass offers a plausible explanation for why breeding cycles of different large herbivores species differ.