Growth rates in spotted hyenas living in a highly seasonal environment: Socio-ecological factors and life history consequences

Background/Question/Methods

Morphological growth rates are critical life history traits in their own right, reflecting multivariate life history patterns interspecifically, and both genetic and environmental variation intraspecifically. Growth rates also commonly have important consequences for the timing of life history events, and future fitness effects within populations.Many factors can affect nutrition and other determinants of growth rate during development, including food abundance in the environment, and for mammals especially, maternal quality and investment, as well as cohort-related variables. We present analyses using mechanistic growth models focusing on the role of maternal social rank, litter size, maternal parity, prey availability, and intra-litter rank on growth rate in spotted hyenas living in the Masai Mara game reserve in Kenya, an environment with highly seasonal prey availability. Prey availability in this system is so variable due to a massive ungulate migration that moves through the area for a few months each year. Second, we estimate size-at-age residuals from a basic mechanistic growth model for individuals under 24 months, and using these as predictors of the age at which different developmental milestones are reached, we investigate the effect of growth rate on age at weaning, age at dispersal, and age at first parturition.

Results/Conclusions

Our results suggest that subordinate cubs from twin litters grow more slowly than either singleton cubs or dominant cubs from twin litters, females with higher social rank and thus priority access to food at kills bear offspring that grow more rapidly, as do mothers that have reared more than one litter. Surprisingly, despite the incredible seasonal variation in prey availability in the Masai Mara/Serengeti ecosystem, we do not find an effect of prey availability on growth rate. We also find strong support for a role of higher growth rate in advancing weaning age, and some evidence that males exhibiting higher growth rates disperse earlier. We do not, however, find evidence for a role of early-life growth rate in influencing age at first parturition of females. Our results highlight the importance of access to food in this system, but leave open the question of why variation in maternal access to food due to social rank and variation in access to food among littermates overshadows seasonal variation in prey availability. Finally, our results suggest specific hypotheses concerning how socioecological and maternal factors early in life may influence life history milestones and fitness later in life in spotted hyenas.