Lucky To Be Alive, Luckier to Breed: Lifetime Reproduction in Weddell Seals.

We utilized a long-term study of Weddell seals to compute lifetime reproductive success (LRS) distributions using a theoretical approach and an empirical approach. These comparisons are often difficult to achieve among natural populations but are important for disentangling sources of variation in lifetime measures. We performed three independent analyses: the first compared LRS and age-at-death distributions among populations living under different environmental conditions, the second focused on populations differing in individual heterogeneity (i.e., high or low reproductive strategy), and the third compared a theoretical LRS distribution with an empirical LRS distribution. Iceberg conditions increased the probability of LRS = 0 while individual heterogeneity had little influence on LRS distributions. Age-at-death distributions were also highly skewed and only early life mortality was affected by environmental condition. Our theoretical LRS distribution was strikingly similar to the empirical distribution estimated from females experiencing natural intrinsic trait variability and extrinsic environmental variability. We also examine inequality measures which show that females must be 'lucky' to survive past maturity and of those who do, only 74% breed. These findings contribute to ongoing research that reveals how diversity in lifetime outcomes is largely governed by chance alone.