Five decades on: Use of historical weaning size data reveals that a decrease in maternal foraging success underpins the long-term decline in population of southern elephant seals (Mirounga leonina)

Abstract

The population of Southern elephant seals (Mirounga leonina) at Macquarie Island has declined since the 1960s, and is thought to be due to changing oceanic conditions leading to reductions in the foraging success of Macquarie Island breeding females. To test this hypothesis, we used a 55-year-old data set on weaning size of southern elephant seals to quantify a decrease in weaning size from a period of population stability in 1950s to its present state of on-going decline. Being capital breeders, the size of elephant seal pups at weaning is a direct consequence of maternal foraging success in the preceding year. During the 1940-1950s, the mean of female pups at weaning was similar between the Heard and Macquarie Island populations, while the snout-tail-length length of male weaners from Heard Island were longer than their conspecifics at Macquarie Island. Additionally, the snout-tail-length of pups at weaning decreased by 3cm between the 1950s and 1990s in the Macquarie Island population, concurrent with the observed population decline. Given the importance of weaning size in determining first-year survival and recruitment rates, the decline in the size at weaning suggests that the decline in the Macquarie Island population has, to some extent, been driven by reduced maternal foraging success, consequent declines in the size of pups at weaning, leading to reduced first-year survival rates and recruitment of breeding females into the population 3 to 4 years later.

Lifetime reproductive success of northern elephant seals (Mirounga angustirostris)

“…The data suggest that foraging success affects fecundity, consistent breed-ing, and the size of pups weaned. Foraging success and weanling size have implications for the growth and dynamics of the population (Clausius et al 2017). It is not clear if mortality is affected directly by the inability of seals to gain sufficient nourishment.…”

Climate influences on female survival in a declining population of southern elephant seals (Mirounga leonina)

“…The SAM has often been hypothesized to affect foraging conditions for other aspects of this population, including the relative number of breeding females (van den Hoff et al, 2014), adult female foraging success (McMahon et al, 2017), and juvenile weaning mass (Clausius et al, 2017). In the Southern Ocean, SAM reflects different effects depending on latitude (Lovenduski & Gruber, 2005).…”

Modelling southern elephant seals Mirounga leonina using an individual-based model coupled with a dynamic energy budget

“…The pups weigh around 45 kg at birth, and 117 kg when they wean 23 days later ( e.g . [ 51 , 52 , 53 ]). This rapid weight gain is possible due to the extreme ‘fattiness’ of southern elephant seal milk (16.1±6.98% fat on day one, up to 39.5±15.2% fat at weaning [ 54 ]).…”

“…These modifications take into account the expected length and weight of pups at birth (110 cm and 45 kg) and weaning (125 cm and 117 kg; e.g . [ 51 , 52 , 53 ]), as well as pregnancy and weaning durations (217 and 23 days, respectively; [ 48 ]).…”