DNA fingerprinting evidence of nonfilial nursing in grey seals

Maternal responses to pup calls in a high-cost lactation species

Bonding between mothers and their young is fundamental to mammalian reproductive behaviour and individual fitness. In social systems where the risk of confusing filial and non-filial offspring is high, mothers should demonstrate early, strong and consistent responses to their kin throughout the period of offspring dependence, irrespective of maternal traits. We tested this hypothesis through playback experiments in the northern elephant seal Mirounga angustirostris, a phocid species that breeds in high-density colonies. We found that mothers recognized their offspring throughout lactation and as early as 1-2 days after parturition. Measures of experience (age) and temperament (aggressivity) did not predict their response strength to filial playback treatments, nor did pup age or sex. Some mothers showed great consistency in behavioural responses throughout the lactation period, while others were less predictable. The strength of a female's response did not influence her pup's weaning weight; however, more consistent females weaned pups of higher mass. This is a rare demonstration of individual recognition among phocid mothers and their offspring, and suggests that consistency in maternal responsiveness may be an important social factor influencing the pup's growth and survival.

Allo-suckling occurrence and its effect on lactation and nursing duration in harbour seals (Phoca vitulina) in Orkney, Scotland

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 statement

Allo-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.

Maternal Oxytocin Is Linked to Close Mother-Infant Proximity in Grey Seals (Halichoerus grypus)

Maternal behaviour is a crucial component of reproduction in all mammals; however the quality of care that mothers give to infants can vary greatly. It is vital to document variation in maternal behaviour caused by the physiological processes controlling its expression. This underlying physiology should be conserved throughout reproductive events and should be replicated across all individuals of a species; therefore, any correlates to maternal care quality may be present across many individuals or contexts. Oxytocin modulates the initiation and expression of maternal behaviour in mammals; therefore we tested whether maternal plasma oxytocin concentrations correlated to key maternal behaviours in wild grey seals (Halichoerus grypus). Plasma oxytocin concentrations in non-breeding individuals (4.3 ±0.5 pg/ml) were significantly lower than those in mothers with dependent pups in both early (8.2 ±0.8 pg/ml) and late (6.9 ±0.7 pg/ml) lactation. Maternal plasma oxytocin concentrations were not correlated to the amount of nursing prior to sampling, or a mother’s nursing intensity throughout the dependant period. Mothers with high plasma oxytocin concentrations stayed closer to their pups, reducing the likelihood of mother-pup separation during lactation which is credited with causing starvation, the largest cause of pup mortality in grey seals. This is the first study to link endogenous oxytocin concentrations in wild mammalian mothers with any type of maternal behaviour. Oxytocin’s structure and function is widely conserved across mammalian mothers, including humans. Defining the impact the oxytocin system has on maternal behaviour highlights relationships that may occur across many individuals or species, and such behaviours heavily influence infant development and an individual’s lifetime reproductive success.

Allosuckling in reindeer (Rangifer tarandus): milk-theft, mismothering or kin selection?

Allosuckling, the suckling of offspring from females other than their own mother, has been reported in a number of mammalian species, including reindeer. The causes and function of this behaviour are still being investigated. We monitored 25 doe-calf pairs of semi-domestic reindeer Rangifer tarandus over 10 weeks to test three allosuckling/allonursing hypotheses: (1) milk theft, calves opportunistically allosuckle; (2) mismothering, misdirected maternal care; and (3) kin-selected allonursing. A calf soliciting an allosuckling bout was categorized as non-filial (NF), and a calf soliciting a suckling bout from its mother was categorized as filial (F). We recorded 9757 solicitations, of which 5176 were successful F bouts and 1389 were successful NF bouts. The rejection rates were greater for NF than F calves. The proportions of antiparallel positions adopted were greater for F than NF calves. The odds of an allobout were lower for calves arriving 1st, 3rd, 4th, 5th or 6th than for those arriving 2nd, but the odds did not vary with position adopted and relatedness. Our results provided support to the milk-theft hypothesis, whereas limited support for the mismothering hypothesis was found. Our results did not support the hypothesized kin selection function of allosuckling in reindeer. This article is part of a Special Issue entitled: insert SI title.

Kin in space: social viscosity in a spatially and genetically substructured network

Population substructuring is a fundamental aspect of animal societies. A growing number of theoretical studies recognize that who-meets-whom is not random, but rather determined by spatial relationships or illustrated by social networks. Structural properties of large highly dynamic social systems are notoriously difficult to unravel. Network approaches provide powerful ways to analyse the intricate relationships between social behaviour, dispersal strategies and genetic structure. Applying network analytical tools to a colony of the highly gregarious Galápagos sea lion (Zalophus wollebaeki), we find several genetic clusters that correspond to spatially determined 'network communities'. Overall relatedness was low, and genetic structure in the network can be interpreted as an emergent property of philopatry and seems not to be primarily driven by targeted interactions among highly related individuals in family groups. Nevertheless, social relationships between directly adjacent individuals in the network were stronger among genetically more similar individuals. Taken together, these results suggest that even small differences in the degree of relatedness can influence behavioural decisions. This raises the fascinating prospect that kin selection may also apply to low levels of relatedness within densely packed animal groups where less obvious co-operative interactions such as increased tolerance and stress reduction are important.

Beyond habitat requirements: individual fine-scale site fidelity in a colony of the Galapagos sea lion (Zalophus wollebaeki) creates conditions for social structuring

Site fidelity has been widely discussed, but rarely been related explicitly to a species' social context. This is surprising, as fine-scale site fidelity constitutes an important structural component in animal societies by setting limits to an individual's social interaction space. The study of fine-scale site fidelity is complicated by the fact that it is inextricably linked to patterns of habitat use. We here document fine-scale site fidelity in the Galapagos sea lion (Zalophus wollebaeki) striving to disentangle these two aspects of spatial behaviour. Regardless of sex and age, all individuals used small, cohesive home ranges, which were stable in size across the reproductive and non-reproductive season. Home ranges showed a large individual component and did not primarily reflect age- or sex-specific habitat requirements. Site specificity could be illustrated up to a resolution of several metres. Long-term site fidelity was indicated by home range persistence over 3 years and the degree of site fidelity was unaffected by habitat, but showed seasonal differences: it was lower between reproductive and non-reproductive periods than between reproductive seasons. We further examined static and social interaction within mother-offspring pairs, which constitute a central social unit in most mammalian societies. Regardless of the occupied habitat type, adult females with offspring had smaller home range sizes than non-breeding females, demonstrating the importance of spatial predictability for mother-offspring pairs that recurrently have to reunite after females' foraging sojourns. While social interaction with the mother dropped to naught in both sexes after weaning, analysis of static interaction suggested female-biased home range inheritance. Dispersal decisions were apparently not based on habitat quality, but determined by the offspring's sex. We discuss the implication of observed fine-scale site fidelity patterns on habitat use, dispersal decisions and social structure in colonial breeding pinnipeds.

Reproductive performance in grey seals: age-related improvement and senescence in a capital breeder

1. Three hypotheses have been advanced to account for age-related improvement in performance: the selection hypothesis predicts improved due to the loss of lower quality phenotypes, the constraint hypothesis predicts individuals improve function, and the restraint hypothesis predicts younger individuals forego or reduce effort because of mortality risks. A decline in age-related performance (i.e. senescence) is predicted by mutation accumulation, antagonistic pleiotropy and disposable soma (wear and tear) hypotheses. 2. Using five measures of performance - birth rate, maternal and pup birth mass, pup weaning mass, weaning success and lactation length - we tested these hypotheses concerning age-related change in reproduction in 279 female grey seals (Halichoerus grypus), ages 4-42 years, over a 23-year period between 1983 and 2005 on Sable Island, Nova Scotia. These females produced 2071 pups. 3. Although body mass of primiparous females increased with age (4-7 years) birth mass of their pups did not, but pup weaning mass did. Second- and third-parity females of the same age as primiparous females gave birth to and weaned heavier pups. However, parity and age were dropped from models when maternal body mass was included. 4. The proportion of females giving birth varied significantly with maternal age, increasing in young females and then declining late in life. Weaning success rate also increased rapidly to about 8 years and subsequently declined in females > 32 years. 5. Generalized additive models indicated nonlinear changes in 3 day body mass (i.e. approximately birth mass) and weaning mass of pups as a function of maternal age, after accounting statistically for the effects of maternal body mass. Mixed-effects, repeated-measures models fitted to longitudinal data further supported the conclusion that pup birth mass and weaning mass vary nonlinearly with maternal age and indicated nonlinear changes in lactation duration. 6. We found some support for the constraint hypothesis, but our findings were not consistent with the selection hypothesis or the restraint hypothesis as the basis for improvement in reproductive performance. 7. Senescence was evident in multiple female and offspring traits, indicating the degeneration in function of several physiological systems as predicted by the disposable soma hypothesis.

Does kin selection influence fostering behaviour in Antarctic fur seals (Arctocephalus gazella)?

A recent observation that female Antarctic fur seals foster pups born to related females raises the fascinating possibility that kin selection may promote altruistic behaviour even on a crowded breeding beach, where individual interactions are frequent and complex. However, the use of genetic markers to identify small numbers of unusually highly related individuals is fraught with difficulty due to the likely presence of genotyping errors and related problems. Consequently, we examined an enlarged dataset where errors had been reduced to an absolute minimum by a combination of close scrutiny and repeat genotyping. We find no support for the idea that females preferentially suckle pups born to female relatives. Instead, the previously reported pattern can be explained by a combination of genotyping errors and de novo mutations. Our study emphasizes the need for caution when interpreting rare events that occur at a rate approaching that expected for normal genotyping errors.

The rate of fertilization in male mating tactics of the polygynous grey seal

Studies using molecular markers have shown that some grey seal males may be gaining success through exhibiting alternative mating tactics. We estimated the probability of fertilization success of grey seal males exhibiting the primary tactic of female defence and one alternative tactic of mating with departing females on Sable Island, Nova Scotia, Canada, during the breeding seasons of 1997-2002. Although the fertilization rate of the primary tactic (27-43%) was greater than that of the alternative tactic (10-12%), these low rates indicate the potential fitness value of alternative mating tactics in this size-dimorphic pinniped species.

Male reproductive strategy and the importance of maternal status in the Antarctic fur seal Arctocephalus gazella

Although mammalian mating systems are classically characterized in terms of male competition and polygyny, it is becoming increasingly apparent that alternative male strategies and female choice may play important roles. For example, females who mate with males from a dominant dynasty risk producing inbred offspring. Many pinnipeds are highly polygynous, but in some species alternative male strategies such as aquatic mating appear to be important, even when behavioral observations suggest strong polygyny. Here, we analyze male reproductive success in the Antarctic fur seal Arctocephalus gazella, an otariid described behaviorally as being highly polygynous, by combining a microsatellite paternity analysis spanning seven consecutive breeding seasons with detailed behavioral data on both sexes. Territorial males fathered 59% of 660 pups analyzed from our study colony. Male reproductive skew was considerable, with a quarter of all paternities assigned to just 12 top individuals on a beach where mean annual pup production was 635. Most males were successful for only a single season, but those able to return over successive years enjoyed rapidly increasing success with each additional season of tenure. We found no evidence of alternative male reproductive tactics such as aquatic or sneaky terrestrial mating. However, paternity was strongly influenced by maternal status. Females observed on the beach without a pup were significantly less likely to conceive to a sampled territorial male than equivalent females that did pup. In addition, their pups carried combinations of paternal alleles that were less likely to be found on the study beach and exhibited lower levels of shared paternity. Thus, from a territorial male's perspective, not all females offer equal opportunities for fertilization.

Kin selection may influence fostering behaviour in Antarctic fur seals (Arctocephalus gazella)

Fostering confers obvious advantages to the offspring but is seemingly costly to the caregiver. Such behaviour is particularly paradoxical in seals where the energetic investment in milk is very high and has led to the suggestion that this behaviour may have evolved through either kin selection or reciprocity. We used a combination of genetic and behavioural data to investigate whether kin selection plays a role in the fostering behaviour observed in a well-studied population of Antarctic fur seals (Arctocephalus gazella) from Bird Island, South Georgia. Genotypic data from eight highly polymorphic microsatellite markers were used to estimate relatedness among mother-pup pairs, foster mother-pup pairs and the total population. Mean relatedness was found to be significantly higher for foster mother-pup pairs than that observed for the total population, suggesting that kin selection could have a role in the maintenance of fostering behaviour in this species.

Mother-Offspring vocal recognition in northern fur seals is mutual but asymmetrical

During the 4-month period of offspring dependence, northern fur seal, Callorhinus ursinus, mothers and pups use a well-developed multimodal recognition ability to routinely find one another within large and dense breeding aggregations. I studied the vocal/auditory aspect of this ability to look at operational differences between the two members of a recognition dyad. If parent-offspring conflict theory is applied to animal communication behaviour, we should expect unequal selective forces acting on parents and offspring. In northern fur seal maternal recognition dyads, I expected pups to expend more energy in the reunion process because they carry the greater burden of a failed reunion. Furthermore, in terms of signal detection theory, pups should have a lower rejection threshold (lower bias) than mothers. To address these questions, I conducted vocal playback experiments and behavioural observations on a natural population of northern fur seals in the Pribilof Islands, Alaska, U.S.A. Although playback results support mutual vocal recognition, pups were both more vocally responsive and made more recognition errors (i.e. false alarms). Behavioural observations, including search time, distance travelled, signalling behaviour and contact with nonoffspring show that pups expend more effort in the reunion process. These findings are consistent with expectations and begin to quantify how selection pressure on recognition behaviour can vary at different stages of development. Copyright 2001 The Association for the Study of Animal Behaviour.

Individually distinctive pup vocalizations fail to prevent allo-suckling in grey seals

In crowded aggregations that occur in breeding colonies, female pinnipeds commonly become separated from their pups and may use spatial, olfactory, or auditory cues to locate them. A system of mutual recognition based on vocalizations is known for otariids. Female phocids are known to use location and olfaction to help identify pups, but evidence for vocal recognition is weak. During the 1997 breeding season on the Isle of May, Scotland, vocalizations were recorded from grey seal, Halichoerus grypus, pups; playback experiments were carried out; and nursing of nonfilial pups was observed. Pup vocalizations were found to be both stereotyped and individually distinctive, features normally associated with a system of individual recognition. However, playback experiments revealed that mothers did not respond more to vocalizations of their own pups than to those of nonfilial pups. Furthermore, seventeen cases of allo-suckling were observed during 68 h of observation on the colony. High densities of animals and frequent separations present challenges to identification of pups by their mothers.

Proximate factors associated with high levels of extraconsort fertilization in polygynous grey seals

Behavioural estimates of male mating success in polygynous grey seals, Halichoerus grypus, may be misleading as females are known to be promiscuous. At Sable Island, Nova Scotia, we collected behavioural observations and skin samples for paternity analysis from 56 females and their attending males. Twenty-four of these females were found in the following year and their offspring were sampled. Using seven hypervariable microsatellite loci, we excluded the consort male as the father in 43% of the cases. The probability of exclusion of these seven loci was 98.2%. Contrary to expectations, inland females had higher rates of extraconsort fertilizations (ECFs) (70%) than beach females (23%). Younger females (<9 years) had slightly more ECFs than older females, but this was not significant. The duration of male consortship did not differ between females with ECFs and females fertilized by their consort male. Two explanations may account for the inland females having more ECFs: a higher ratio of females to tenured males inland may provide a greater opportunity for nonconsort males to obtain copulations; and inland females travel greater distances to depart for the ocean and may attract more males. These results are more consistent with the hypothesis that ECFs are a by-product for females of male strategies to maximize reproductive success than with hypotheses concerned with either material or genetic benefits gained by females. Copyright 1999 The Association for the Study of Animal Behaviour.