Biological and Environmental Influences on Parturition Date and Birth Mass of a Seasonal Breeder

Abstract

Natal features (e.g. Julian birth date and birth mass) often have fitness consequences and can be influenced by endogenous responses by the mother to seasonal fluctuations in nutritional quality and photoperiodic cues. We sought to further understand the biological and environmental factors that influence the natal features of a polytocous species in an environment with constant nutritional resources and limited seasonal variation. During a 36-year study we assessed the influence of biological factors (maternal age and litter type [i.e., litter size and sexual composition]) and environmental factors (total precipitation and mean maximum temperature during months encompassing conception, the last trimester of gestation, and the entire length of gestation) on Julian birth date and birth mass using linear-mixed effects models. Linear and quadratic functions of maternal age influenced both natal features with earliest Julian birth dates and heaviest birth masses occurring at prime-age and older individuals, which ranged from 5–9 years of age. Litter type influenced Julian birth date and birth mass. Interestingly, environmental factors affected Julian birth date and birth mass even though mothers were continuously allowed access to a high-quality diet. Random effects revealed considerable variation among mothers and years. This study demonstrates that, in long-lived polytocous species, environmental factors may have a greater influence on natal features than previously supposed and the influence from biological factors is also complex. The documented responses to environmental influences provide unique insights into how mammalian seasonal reproductive dynamics may respond to current changes in climate.

Birth dates vary with fixed and dynamic maternal features, offspring sex, and extreme climatic events in a high‐latitude marine mammal

“…, ; Wolcott et al. ) and lend support to recent studies that reported that fixed maternal traits account for more of the variation in birth dates than do dynamic attributes (Plard et al. ; Wolcott et al.…”

“…; Wolcott et al. ), we found evidence of a parabolic relationship where birth dates were latest for younger and older mothers and earliest for prime‐age mothers. Sydeman et al.…”

“…; Wolcott et al. ) greatly outnumbers those that have not found a relationship (Guinness et al. 1978a; Ellis et al.…”

“…; Wolcott et al. ). Thus, birth timing for a given population depends on the individuals in the population and the environmental conditions those individuals face (Sydeman et al.…”

“…) and in captivity (Temte ; Wolcott et al. ). Such repeatability could be tied to individual variation in acquisition of nutritional resources.…”