Report of Two Rare Cases of Allonursing and Fostering in the Cape Fur Seal (Arctocephalus pusillus pusillus) in Namibia

Abstract

Allomaternal care includes a nonfilial conspecific individual provisioning care to a dependent young, for example, feeding them, protecting them, or training them (Wilson 1975). This care can be given during a short time window (hours to days), a longer but still temporary period, or on a long-term basis until weaning. Although different terms are used to describe allomaternal care and there appears to be no consensus on what behaviors it includes, we distinguish here between the term “allonursing” referring to the provision of care over a short period (a few hours or days) and the term “fostering” which refers to the provision of care over a longer period, including until weaning (Maniscalco et al. 2007). Nonfilial care has been reported in many mammal and bird species as they can, in some cases, represent an advantage by improving the fitness of the mother and/or the chances of survival of the young (Riedman 1982). In pinnipeds, allonursing and fostering behaviors are known to be far more common in phocids than in otariids (Boness et al. 1998; Riedman 1990; Sepúlveda and Harcourt 2021). Except in the New Zealand sea lion ( Phocarctos hookeri ) and the Antarctic fur seal ( Arctocephalus gazella ) for which fostering can reach 6% and 7%–11% of nursing females, respectively (Childerhouse and Gales 2001; Gemmell 2003; Lunn 1992), studies have only reported fostering or allonursing as isolated cases. The behavior has been described as “abnormal” in the South American fur seal ( Arctocephalus australis ) (Franco-Trecu et al. 2010), the New Zealand fur seal ( Arctocephalus forsteri ) (Dowell et al. 2008), the Steller sea lion ( Eumetopias jubatus ) (Maniscalco et al. 2007), the Subantarctic fur seal ( Arctocephalus tropicalis ) (de Bruyn et al. 2010), and the Australian sea lion ( Neophoca cinerea ) (Pitcher et al. 2011) (although it should be noted that an allonursing rate of 13% was observed during an unusual season at Seal Bay in Australia; McIntosh and Pitcher 2021).

The Cape fur seal ( Arctocephalus pusillus pusillus ) is an otariid species distributed along the southwest and south coasts of Southern Africa, from Baia dos Tigres in southern Angola to Algoa Bay in South Africa (Huisamen et al. 2011). Similar to other fur seals, maternal care in the Cape fur seal is provided on land, in the breeding colonies where females exclusively nurse their own pup for a long duration, that is, 10–12 months (Rand 1955; Riedman 1990). Mothers and pups are frequently separated throughout the lactation period as pup attendance times on shore are interspersed with maternal foraging trips at sea lasting an average of 2.3 days (David and Rand 1986; Rand 1955). As in other otariid species, females are generally very aggressive towards nonfilial pups and often do not accept their presence nearby. Females chase nonfilial pups away by displaying behaviors ranging from growling to violent biting. To our knowledge, allomaternal behaviors have never been described in the Cape fur seal. Here, we report a case of fostering and a case of allonursing seen at Pelican Point breeding colony (central part of the Namibian Coast; 25° 52.2′ S, 14° 26.6′ E), from November 2021 to January 2022 and in November 2022, respectively. Although we provided observational data only and that key contextual factors may be missing, based on our observations, we tried to identify what could have led to the emergence of these allomaternal behaviors.

Case 1: Fostering: During the 2021–2022 breeding season, a female and her male pup were marked at Pelican Point as part of a study involving the deployment of a multi-sensor tag on the female. On November 12, the female was equipped with the tag (glued to the fur on the back, between the shoulders) and marked with two large rectangles of blond hair dye on either side of the tag. The same day, a long strip of dye was made along the pup's back (the pup was a male) (Figure 1A) and he was also equipped with a small GPS tag (CatLog Gen 2 GPS/GNSS) of size 2.5 × 6 × 1 cm attached on his back. Based on the absence of an umbilical cord and body size, the pup was estimated to be at least 1-week-old at that time (Martin et al. 2022). The pup and its mother's presence and absence at the colony were monitored for 53 days, from the marking and tagging day to January 3 (Figure 1). Mother and pup remained together at the colony for the first three days during which the pup was seen suckling several times. The female left during the night of November 14 or early hours of the 15th for a foraging trip, as the pup was seen alone on the morning of November 15. From then until November 21, the pup was seen alone every day. He wandered around the colony, sometimes calling or resting amongst a group of pups left alone while their mothers were at sea. The pup approached adult females appearing to look for suckling opportunities. Most of these females were aggressive and would chase him away. On November 22, the pup was seen suckling a female who was not his mother; she had no dye marks on her back and no sign of the deployed tag. In addition to nursing the pup, the female was highly protective of it, that is, she displayed aggression toward approaching seals. It is likely that this female had given birth to a stillborn pup within the past few hours since we found a dead newborn pup (still wet from the amniotic fluid and its umbilical cord was long and very fresh) less than 5 m away and that the female's genitals were wet and covered in sand with light bloodstains. Both the pup and the foster female remained together in the colony for at least 3 days and the pup was seen suckling the foster female (Figure 1B). From November 26 to January 3 when dedicated observations ended, the pup was observed alternately alone or with the foster female (Figure 1). Opportunistic monitoring by local collaborators after this date revealed no additional resights of the pup or his foster mother, suggesting that they might have moved to another part of the colony. No sightings of the pup's biological mother (marked on the back) were made between her initial departure on November 14–15 and February 21, 2022, when she was resighted back at the colony and seen alone.

Case 2: Allonursing: On November 14, 2022, an adult female was seen with two pups that were previously marked as part of another study: “pup 38” and “pup 39.” Both pups had a very fresh umbilical cord stump and were thus estimated to be less than 24 h old. Despite their similar age, pup 38 was larger than pup 39. The female was not aggressive with either of the two and permitted both pups to suckle (Figure 2). She regularly sniffed the pups and responded vocally to their calls. During the 14th, the female was marked with a hair-dye rectangle on her chest to allow us to recognize her (no other female in the colony was marked at this time). We were also intrigued by the behavior of a young-looking adult female standing alone a few meters from the triad: she was constantly staring at the two pups and responded vocally to their calls. On the second and third days, pups 38 and 39 were still with the marked female. On the fourth day, the marked female was only observed with pup 38. Pup 39 was about 30 m away with a small/young-looking adult female. We saw no further interactions between the marked female and pup 39. The marked female showed typical maternal behaviors with pup 38, as did the young-looking female with pup 39 (including milk provision), which suggested that each of the pups was back in a pair with their mother.

As mentioned in the introduction, observations of fostering and allomaternal care in otariids are sparse. A case of shared nursing, that is, two females nursing the same pup, as we report on here, has also been reported in the Antarctic fur seal (Acevedo et al. 2016). Although the few published reports are likely an indication of the rarity of allomaternal care events, their frequency may be underestimated because they are rare events. Indeed, due to the density of their breeding colonies and the fact that births peak synchronously over a short period of time, and marked animals are not available for observation, allomaternal behaviors can be difficult to assess. A medium- to long-term marking of a large number of individuals (females and pups) is critical to determine the prevalence of such behavior.

The continuous monitoring of marked pups over several breeding seasons at Pelican Point (central Namibia) allowed us to at least note that allonursing and fostering do occur in the Cape fur seal. However, we cannot provide insights into its prevalence. Various constraints (such as the absence of markings on females, the impossibility of observing the animals day and night, the difficulty of sighting them every day, and the lack of physiological data) limit our ability to fully understand the context in which these behaviors occur.

Whilst the observation of these two cases of allomaternal care in Cape fur seals was opportunistic and information was lacking to explain the details of these events, we have added another otariid species to the list of ones in which fostering and allomaternal care behavior do occur. There is still much to be learned about these behaviors in this species. Future studies on individual recognition or genetic studies to determine filial relationships could provide a deeper understanding of the proximate and ultimate causes of fostering and allomaternal care in this otariid species as well as others.

Mathilde Martin: conceptualization, data curation, formal analysis, investigation, writing – original draft. Tess Gridley: project administration, supervision, writing – review and editing. Simon Elwen: project administration, writing – review and editing. Isabelle Charrier: conceptualization, data curation, methodology, project administration, resources, supervision, writing – review and editing.

The authors declare no conflicts of interest.