Little evidence of inbreeding depression for birth mass, survival and growth in Antarctic fur seal pups

Abstract

Inbreeding depression, the loss of offspring fitness due to consanguineous mating, is generally detrimental for individual performance and population viability. We investigated inbreeding effects in a declining population of Antarctic fur seals (Arctocephalus gazella) at Bird Island, South Georgia. Here, localised warming has reduced the availability of the seal's staple diet, Antarctic krill, leading to a temporal increase in the strength of selection against inbred offspring, which are increasingly failing to recruit into the adult breeding population. However, it remains unclear whether selection operates before or after nutritional independence at weaning. We therefore used microsatellite data from 885 pups and their mothers, and SNP array data from 98 mother-offspring pairs, to quantify the effects of individual and maternal inbreeding on three important neonatal fitness traits: birth mass, survival and growth. We did not find any clear or consistent effects of offspring or maternal inbreeding on any of these traits. This suggests that selection filters inbred individuals out of the population as juveniles during the time window between weaning and recruitment. Our study brings into focus a poorly understood life-history stage and emphasises the importance of understanding the ecology and threats facing juvenile pinnipeds.

Figure 1

Interannual variation in three measures of season quality. (a) Map of Bird Island, South Georgia, showing the location of two adjacent breeding colonies, the Special Study Beach (SSB) and Freshwater Beach (FWB); (b) Annual numbers of breeding females on SSB; (c) The birth mass of female pups born on SSB; (d) The amount of time spent foraging at sea by mothers (data are from FWB). The squares show the means and the whiskers show the 95% confidence intervals. Data from 2019 and 2020 are already published by Nagel et al..

Figure 2

Model estimates and associated 95% confidence intervals for the fixed effects of (a) pup birth mass; (b) pup survival; and (c) pup growth in models including maternal effects; and (d) pup birth mass; (e) pup survival; and (f) pup growth in models excluding maternal effects. Statistically significant relationships are highlighted in salmon pink. Some of the significant relationships have small effect sizes and thus appear to overlap zero in the figure, but their 95% CIs do not overlap zero (see Supplementary Tables 1–3 for the exact values of the parameter estimates).

Figure 3

Results of genomic inbreeding analyses, including model estimates and associated 95% confidence intervals (CIs) for the fixed effects of (a) pup birth mass; (b) pup survival; and (c) pup growth. Statistically significant relationships are highlighted in salmon pink. Some of the significant relationships have small effect sizes and thus appear to overlap zero in the figure, but their 95% CIs do not overlap zero (see Supplementary Tables 4–6 for the exact values of the parameter estimates). Panels (d) and (e) depict approximately linear increases in pup mass over time for a representative subset of five pups and for all 98 pups respectively. The dark line in panel (e) indicates the global average rate of change over time (y = 0.081x + 5.617).