Maternal resource allocation adjusts to timing of parturition in an asynchronous breeder

Abstract

Environmental variation affects foraging decisions and resources available for allocation among competing life-history traits. In seasonal environments, variation in breeding phenology leads to differences in relative timing of resource intake and expenditure, which can lead to variation in maternal allocation tactics. Monitoring maternal allocation to fetal growth in wild mammals is challenging, however, and few studies have linked seasonal effects of forage and maternal condition to early offspring development. Asynchronous parturition and short gestation make kangaroos ideal for studying phenological effects on very early growth, since pouch young born in different seasons can be measured during stages equivalent to in utero development for eutherian mammals. Over 4 years, we recaptured 68 eastern grey kangaroo mother-young pairs with parturition dates spanning 5 months to evaluate how birthdate affects maternal allocation to offspring growth before pouch exit. Structural equation modeling revealed that mothers that gave birth in autumn gained mass during lactation, and their young grew faster than young born in early summer. When later lactation coincided with poor winter forage and cold temperatures, mothers prioritized maintenance of their own mass over offspring growth. Differences in maternal mass change and allocation to early and late-born young suggest that seasonal resource availability influenced tactics of resource storage and expenditure. Our results provide a mechanistic link between reproductive phenology, seasonal forage, and allocation trade-offs in wild mammals, and demonstrate a clear effect of maternal mass change on growth of young during a phase that occurs in utero for eutherian mammals.

Significance statement

Capital and income breeding are often presented as opposing tactics of resource provisioning. Many species, however, use a combination of stored and concurrent resources to reproduce. In seasonal environments, reproductive phenology should affect the relative timing of resource acquisition and expenditure, which could affect maternal allocation to offspring. We used repeated captures of mother-young kangaroo pairs and path analysis to explain how maternal allocation tactics adjust to season of parturition. Mothers that timed later lactation with cold weather and low winter forage relied more heavily on stored resources for reproduction and allocated less to offspring growth. Flexibility in foraging tactics may explain the variability in kangaroo parturition date by allowing mothers to use stored energy to sustain reproduction during periods of scarce forage.