Some Cape fur seal (Arctocephalus pusillus pusillus) mothers may nurse their young for several years longer than previously thought, thereby potentially improving their offspring's chances of survival.
The observations of this unusual behaviour has now been published in the journal African Zoology in an article entitled “Prolonged nursing in Cape fur seals (arctocephalus pusillus pusillus) at Cape Cross colony, Namibia".
Researchers from the Namibian Dolphin Project, a research project run by Sea Search Africa, made the observation at the Cape Cross colony in Namibia. Cape Cross is one of the world's largest breeding colonies of Cape fur seals, hosting up to 210 000 animals. While adult males are only present during the breeding season, females stay at Cape Cross all year long with their offspring, hunting in the nearby waters.
Dr Anna Osiecka, lead author, says Cape fur seals normally wean their young within a year: “It appears that some may choose to keep the bond with their pups or even feed unrelated pups. As seal milk is very rich in fat and protein, this extra 'free' food can give their offspring an upper hand by allowing the young to grow larger and improve their chances of survival.
“For male pups, this can translate into better chances to defend a harem and father offspring when they grow up, as larger males tend to be more successful," she explains.
What we know about maternal care in Cape fur seals
Cape fur seals are the only seals which breed in southern Africa, with a range from southern Angola to the Port Elizabeth, in South Africa. “These animals were hunted to the brink of extinction in the last century, but with appropriate protection measures they have recovered and became numerous throughout their range", says Dr Simon Elwen, a marine mammal expert and director of Sea Search.
When they reach maturity, Cape fur seal females give birth to a single pup annually. Pups are weaned at 10-12 months and getting separated from their mother earlier often results in death. To sustain this long period of nursing, mothers spend about half of the time out at sea feeding, leaving their young alone onshore in big nursery groups.
According to Dr Tess Gridley, a postdoctoral fellow at Stellenbosch University's Department of Botany and Zoology, Cape fur seals are a fascinating species. She is leading the research project on their biology and behaviour in South Africa and Namibia.
Dr Osiecka says the observations prove that mother-and-pup relations are not as simple as previously thought. “Mothers can continue nursing their older offspring if the year's pup dies or is lost as a still birth. This is a great advantage to the older pups: they can grow larger faster, and this will ultimately increase their chances of survival and reproduction when they grow up."
This also implies that seals can recognise their family members over many years and maintain their bonds.
While physically costly, prolonged nursing may also benefit the mother. Removing excess milk helps to prevent mastitis, and in some cases it may be simply be more efficient to support older, healthy offspring, e.g. if the new pup is very sickly or lost.
“In some species nursing inhibits future pregnancies – we don't know yet if this is the case with Cape fur seals, but if so, prolonged nursing could also provide a year without a pregnancy, allowing the mother to recover her physical condition," says Osiecka.
Prolonged suckling has been observed in other fur seals, though it is often attributed to milk theft or mistaken identity with females nursing an unrelated pup.
“This is not the case in our observations. In all of the cases, the females were aware and allowing of the situation, and sniffed the sucklers. This is how these animals recognise each other, and it implies that the females know and accept the sucklers."
However, Dr Osiecka points out that there is still much to learn. “Our observations, and in fact all descriptions of unusual nursing in fur seals, are based on opportunistic sightings. We are still not sure how females decide on whether to extend nursing their young, or whether adoption takes place in this species. Longer, dedicated studies are needed to better understand the social dynamics of these animals."
On the photos above: A Cape fur seal mother nurses a year's pup (right) and a two-year-old juvenile simultaneously. The Cape Cross colony in Namibia hosts up to 210 000 seals. Photo: ©Anna N Osiecka/Sea Search
Full report: Osiecka, A. N., Fearey, J., Elwen, S., & Gridley, T. (2020). Prolonged nursing in Cape fur seals (Arctocephalus pusillus pusillus) at Cape Cross colony, Namibia. African Zoology, 1-7. https://doi.org/10.1080/15627020.2020.1768144
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