Decreasing body size is associated with reduced calving probability in critically endangered North Atlantic right whales

Abstract

Body size is key to many life-history processes, including reproduction. Across species, climate change and other stressors have caused reductions in the body size to which animals can grow, called asymptotic size, with consequences for demography. A reduction in mean asymptotic length was documented for critically endangered North Atlantic right whales, in parallel with declines in health and vital rates resulting from human activities and environmental changes. Here, we tested whether smaller body size was associated with lower reproductive output, using a state-space model for individual health, survival and reproduction that quantifies the mechanistic links between these processes. Body size (as represented by the cube of length) was strongly associated with a female's calving probability at each reproductive opportunity. This relationship explained 62% of the variation in calving among reproductive females, along with their decreasing health (20%). The effects of decreasing mean body size on reproductive performance are another concerning indication of the worsening prospects for this species and many others affected by environmental change, requiring a focus of conservation and management interventions on improving conditions that affect reproduction as well as reducing mortality.

Keywords: Bayesian state-space model; Eubalaena glacialis; body size; capital breeding; health; length.

Figure 1.

Individual calving probability in available years over the study period (a), length of reproductive females at reproductive opportunities (b) and estimated cubic effect on the asymptote of calving probability (c). In (a,b), each thin grey line corresponds to the time series for one female, while the thick coloured line and ribbon indicate the posterior mean and 95% credible interval across females. In (c), the green line and ribbon represent the posterior mean and 95% credible interval.

Figure 2.

Estimated components of calving probability in available years over the study period. Top row: (a) mean contribution of health to calving probability, given mean length (13.3 m) and random effect equal to the mean (−0.86); (b) mean asymptote of calving probability across females. Bottom row: (c) mean contribution of length to the asymptote of calving probability, calculated by setting the intercept to the mean (−0.86); (d) mean individual random effect on the asymptote of calving probability. The lines and ribbons indicate the posterior means and 95% credible intervals in each year.