Center of Expertise in Marine Mammology: Scientific Research Report 2012-2014

Long-term Measures of Reproductive Performance Reveal Marked Variation in Fitness in Grey Seals

Bowen, W. D., den Heyer, C. E., McMillan, J. I., Lang., S., Lidgard, D.C. and Iverson, S. J.

Marine mammals are long-live vertebrates with delayed maturation and low reproductive rates. Therefore, long-term studies are needed to understand both the sources and extent of variation in reproductive success among individuals and the effect of this variation on population dynamics. We studied the number and weaning mass of offspring of 619 known-age, grey seal (Halichoerus grypus) females annually between 1978 and 2012 (Figure 7). Individual females varied significantly in reproductive quality. The percentage of females that failed to produce offspring increased with population density. Modal number of births by individual females was 20, but varied from 1 to 30 in 242 females aged 25 years or greater. Number of offspring produced was negatively correlated with age at primiparity, but positively correlated with age at last birth (Figure 8), underscoring the influence of longevity on fitness. Individual variation in maternal quality, as judged by birth rate, was not significantly correlated with maternal energy investment during lactation (i.e., mean weaning mass of offspring) for 212 females in which three or more of their pups had been weighed at weaning. Our long-term study reveals considerable heterogeneity in reproductive performance among females. Such heterogeneity has implications for understanding the response of populations to environmental forcing.

Figure 7. Adult female grey seal with her pup just prior to weaning at about 18 days after birth. (Photo: W.D. Bowen).

Figure 7. Adult female grey seal with her pup just prior to weaning at about 18 days after birth. (Photo: W.D. Bowen).

Figure 8. Relationship between longevity and the number of pups produced by grey seal females at Sable Island.

Figure 8. Relationship between longevity and the number of pups produced by grey seal females at Sable Island.

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