Abstract
Fostering and allo-suckling are widespread among pinnipeds, and several hypotheses have been formulated to explain their occurrence. Here, we describe the occurrence of allo-suckling in harbour seals from photo-identification data of females and pups in Orkney (Scotland) during the pupping seasons between 2016 and 2019. We used a generalised linear model framework to investigate the effect of allo-suckling on the duration of lactation (females) and of nursing period (pups). A generalised additive model framework was used to explore how the probability of allo-suckling varied throughout the pupping season, and with changes in mother-pup separation time. Allo-suckling was observed in 31 females, at higher rates (18–37% of lactating females and 18–47% of the pups every year) than those observed in other phocid populations, with 13 females allo-suckling in multiple years. The duration of the pups’ nursing period was not affected by allo-suckling occurrence. However, females in mother-pup pairs where both mother and pup allo-suckled had longer lactation duration than when only the pup allo-suckled, or than in pairs where no allo-suckling was observed. The probability of allo-suckling increased during the pupping season and with increased mother-pup separation time. However, the proximate causes and the consequences on future reproductive output and pup survival remain unknown.Significance statementAllo-suckling, where females nurse others’ young, is widespread in pinnipeds, particularly among true seals. Given the high costs of lactation in pinnipeds, allo-suckling is a puzzling behaviour. Using photo-identification and field observations, we examined the occurrence of allo-suckling in harbour seals at a colony in Orkney, Scotland. We found that allo-suckling is common among seals at the study site, and at rates higher than reported elsewhere. Our results show that allo-suckling does not appear to affect the duration of the pups’ nursing period but does increase the lactation duration of females who suckle other pups and whose own pups also allo-suckle. This study highlights an area which requires further investigation as the energetic costs and benefits of allo-suckling remain poorly understood.
Highlights
Lactation incurs elevated energetic costs in mammals (Lee 1996)
Of the mother-pup pairs with accurate pupping dates (n=77), weaning date could be estimated in 66 cases (16 in 2016, 19 in 2017, 17 in 2018 and 14 in 2019) to allow calculation of lactation duration in females and of nursing period in pups, in the different allo-suckling occurrence groups (Table 2)
Allo-suckling and fostering behaviour are frequently observed in pinniped species, among phocids, the proximate causes and the subsequent consequences on reproductive success and survival are not fully understood
Summary
Lactation incurs elevated energetic costs in mammals (Lee 1996). It can impose physiological stress on females which may reduce their survival and future reproductive success (Clutton-Brock et al 1989; Desprez et al 2018). Females have to balance the energy expenditure of lactation on land with the energy intake of foraging at sea (Bowen et al 2009). This has led to the evolution of three lactation strategies. Female phocids (i.e. true seals) are classified as capital breeders, characterised by a high milk fat content (40–60%; Boness et al 1994) and a fasting strategy where females use large body energy stores (i.e. blubber) accumulated prior to parturition to sustain themselves and to suckle their pups during a comparatively short lactation period (4 to 60 days; Bowen et al 2009), finishing with the abrupt weaning of the pup (Bowen 1991)
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