The size of offspring is an important indicator of maternal size and allocation, offspring fitness and ultimately population growth, with larger offspring typically having higher survival rates. Obviously, higher survival confers greater potential fitness on mothers that will persist into future generations.
Southern elephant seals are a particularly good model species for investigating differences in maternal expenditure in offspring because: (i) they are highly sexually dimorphic, in extreme cases breeding males weigh up to 10 times the mass of breeding females, with the biggest males siring the most offspring; (ii) they are one of the most polygynous of all mammals and consequently lifetime reproductive output varies not only between sexes but particularly among individual males; (iii) they are capital breeders, with breeding females relying solely on energy reserves accumulated prior to breeding during their brief 24-day nursing period so that expenditure is not entangled with foraging efficiency and (iv) they live in the biologically heterogeneous environment, the Southern Ocean, which has highly variable resources.
The study, led by Dr Clive McMahon of the Sydney Institute of Marine Sciences, the Institute of Marine and Antarctic Studies and the Marine Predator Research Group at Macquarie University, quantified maternal size and pup size at birth and weaning for 342 elephant seal mothers at Macquarie Island. The study was conducted over 11 years of contrasting sea-ice and Southern Annular Mode values, both indices of the resources available to mother seals. The study also used tracking data from 94 adult females tracked between 1994-2016 (some of which is IMOS seal tracking data) to describe the spatial context of mother’s core foraging areas.
Overall large females weaned male pups that weighed 17kg more than female pups. Maternal condition varied by as much as 59kg among years, and was positively related to Southern Annular Mode, and negatively to maximum sea-ice extent.
Importantly, the study demonstrated that when conditions are favourable that small and medium sized mothers weaned relatively larger male pups, with this effect less apparent for larger mothers. This is because there is a non-liner relationship between pup size at weaning and survival. In fact, pup survival declines above a threshold weaning mass, so there is potentially a cost if the largest females produced overly large pups.
In conclusion, the study suggests that elephant seal mothers appear to adjust their reproductive effort according to prevailing environmental conditions, which thereby enhances their lifetime reproductive success.
The full paper is available online through Wiley Online Library.
McMahon, C. R., Harcourt, R. G., Burton, H. R., Daniel, O. and Hindell, M. A. (2017), Seal mothers expend more on offspring under favourable conditions and less when resources are limited. J Anim Ecol, 86: 359–370. doi:10.1111/1365-2656.12611