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

Male size and alternative mating tactics in white-tailed deer and mule deer

Within populations, individual males adopt different courtship tactics due to differences in their competitive ability, which may vary depending on the animal’s age and size. To test the hypothesis that mule deer (Odocoileus hemionus) and white-tailed deer (O. virginianus) males vary their courtship behavior based on their size, we conducted focal observations of 144 mule deer and 85 white-tailed males that varied in size, at a large grassland site in southern Alberta. The smallest mule deer males devoted more time to feeding, were less likely to engage in late-stage courtship than larger males and were less likely to move among female groups. Other males, including small white-tailed males, appeared to use a roving strategy to search for estrous females in different groups, which is consistent with recent research on male movements. Both medium and large males increased the time they spent in one-male groups, and specifically isolated pairs, as courtship advanced, presumably to reduce competition with other males. However, this trend was most pronounced for medium mule deer males, and for all size classes of white-tailed deer. In contrast, large mule deer males spent a similar proportion of time tending females in all group types. Our results identified potential size-dependent tactics for mule deer males. In contrast, white-tailed males of all sizes appeared to rely on a tactic of finding and tending estrous females in isolation from other males.

Multiple mating is linked to social setting and benefits the males in a communally rearing mammal

Individuals in social species may mate with multiple opposite-sex individuals, including members of the same or different social groups. This variation may be linked to genetic benefits, where multiple mating decreases risk of inbreeding. Multiple mating may also be constrained by the sociospatial setting through its effect on availability of mates. Because multiple mating with individuals from same or different groups may determine sex-specific fitness effects, we also examined how multiple mating modulates social benefits of females and males. We used 7 years of data on demography, social organization, and genetics of a natural population of the group-living and colonial rodent, Octodon degus, to determine how kin and sex composition within social groups, and spatial relations between these groups (i.e., colonial habits) influence multiple mating and its fitness consequences. Males (81.3%) and females (64.9%) produced offspring with multiple opposite-sex individuals within groups and with individuals of neighboring groups. Thus, polygynandry was the dominant mating system in the degu population examined. Multiple mating in degus was high when compared with estimates reported in other social mammals. Variation in female and male multiple mating was better explained by social setting through its effect on availability of potential mates rather than by benefits derived from decreasing risk of inbreeding. Finally, our study revealed how multiple mating enhances male, but not female reproductive success.

Factors influencing and consequences of breeding dispersal and habitat choice in female grey seals (Halichoerus grypus) on Sable Island, Nova Scotia

Selection of breeding location can influence reproductive success and fitness. Breeding dispersal links habitat use and reproduction. This study investigated factors affecting breeding dispersal and its reproductive consequences in grey seals (Halichoerus grypus) on Sable Island, Nova Scotia. Breeding dispersal distance was determined in 692 individually marked, known-age female grey seals observed from 2004 to 2014. We used generalized linear mixed-effects models to test hypotheses concerning environmental and demographic factors influencing breeding dispersal distance and the consequences of dispersal distance on offspring weaning mass. Grey seal females rarely exhibited fidelity to previous breeding sites. Median dispersal distance between years was 5.1 km. Only 2.9% of females returned to a previous breeding site. Breeding dispersal distance was affected by parity and density, but effects were small and are presumably of no biological significance. Variation in dispersal distance among adult females was large. Dispersal distance had no significant influence on offspring weaning mass; however, as previously found, pup sex and maternal age did. Although breeding location was not important, heavier pups were born in habitats with no tidal or storm-surge influence indicating that breeding habitat type did influence offspring size at weaning. The lack of site fidelity in grey seals on Sable Island is associated with an unpredictable and changing landscape (sand dunes) that could make it difficult for females to locate previous breeding locations. Although breeding location within habitat type had small consequences on offspring weaning mass, we detected no evidence that breeding site selection within the habitat had consequences to females.

Longitudinal changes and consistency in male physical and behavioural traits have implications for mating success in the grey seal (Halichoerus grypus)

We examined age-related changes and consistency in physical and behavioural traits of 20 male grey seals ( Halichoerus grypus (Fabricius, 1791)) and implications for a proxy of mating success (number of oestrous females attended) over four successive breeding seasons on Sable Island, Canada. Across the study, young males (10–15 years) gained body mass, while old males (23–31 years) lost body mass. Body length was an important determinant of tenure (time spent at a site among females) and males of all ages exhibited a high level of consistency in duration of tenure (r = 0.40–0.50). In young males, our proxy of success showed a strong relationship with arrival body mass and also exhibited a high level of consistency (r = 0.50). None of the physical traits measured explained variation in success by exhibiting mating tactics that did not involve tenure, which is likely due to the opportunistic nature of those tactics. Whereas young male grey seals exhibited age-dependent improvements in success owing to changes in their physical state, later in life physical traits were less influential and suggest that nonphysical traits may compensate for a deteriorating physical state and its impact on male success.

The genetic mating system, male reproductive success and lack of selection on male traits in the greater bilby

The greater bilby (Macrotis lagotis) is the sole remaining species of desert bandicoot on the Australian mainland. The mating system of this species remains poorly understood, due to the bilby’s cryptic nature. We investigated the genetic mating system of the greater bilby in a five-year study of a semi-free-ranging captive population that simulated their wild environment. Morphological traits were examined to determine whether these influenced patterns of male reproductive success and whether selection was acting on them. In any given year more than half the males (59.2 ± 9.3%) failed to sire any offspring. Approximately 70% of sires fathered one offspring, and 30% two or three offspring. Since paternity was not dominated by few males, and given the species’ solitary nature, lack of territoriality and large home ranges, it is likely that males adopt a roving strategy to find receptive females. These results are consistent with an overlap promiscuous mating system. Sires and non-sires could not be distinguished by their morphological traits, and there was no evidence for strong linear or non-linear selection on male traits. These data increase our understanding of bandicoot life-history traits and will assist conservation and management efforts.

Alternative mating tactics in the New Zealand fur seal (Arctocephalus forsteri): when non-territorial males are successful too

In polygynous mammals, the status of many males does not allow them to have a high social rank and theory predicts selection for alternative mating tactics. Alternative tactics were suggested to explain discrepancies between mating and paternity successes in several pinniped species. However, information on alternative tactics in fur seals is limited. Here, we focus on the polygynous New Zealand fur seal, Arctocephalus forsteri, predicting that competition for females is likely to cause a diversification of male mating tactics and that non-territorial tactics can yield reproductive success. We describe the behaviour of 38 males in a medium to large colony. Paternity success was assessed using CERVUS and PASOS, from a pool of 82 pups sampled at the study site and at neighbouring breeding areas. To see whether size is correlated with mating tactic, the length of 17 males was estimated using photogrammetry. Cluster analysis identified three male behavioural profiles: one corresponding to large territorial males and two illustrating alternative tactics employed by smaller non-territorial males. Of the 13 pups born at the study site that were assigned a father, eight were sired by three territorial males and five were sired by non-territorial males. Our study highlights that holding a territory is not a necessary condition for reproductive success in all otariids.

The implications of stress on male mating behavior and success in a sexually dimorphic polygynous mammal, the grey seal

Studies on primates and other taxa have shown that the physiological response of an individual to stress reflects their social status. We combined behavioral observations with measures of stress to test the hypothesis that stress is an important physiological determinant of mating behavior and success in the male grey seal. Known-age males (N=19) were studied during the breeding seasons of 2004 and 2005 at Sable Island, Canada. The stressor was a capture and restraint period of 35 min and serial samples of cortisol and testosterone were taken as measures of stress. The mean baseline concentrations of cortisol and testosterone were 9.7+/-0.5 ug/dl and 6.2+/-0.6 ng/mL, respectively. The baseline cortisol concentration was negatively correlated with the duration of time a male spent at a site (r=-0.507, P=0.027), which was a strong correlate of mating success (r=0.659, P=0.002). All males experienced an increase in the concentration of cortisol during the restraint period (79.1+/-8.4%; CV=46.1%). The percentage rise in cortisol during restraint was correlated with the mean duration of time spent at a site (r=0.544, P=0.016) and thus success. The concentration of testosterone also increased during the restraint period (32.8+/-9.7%). This might be an adaptive response to maintaining the ability to reproduce while under stress. Our study indicates that stress is an important determinant of success in male grey seals. More successful males might exhibit an adaptive response to stress by maintaining low concentrations of cortisol during breeding.

Mix and match - hybridization reveals hidden complexity in seal breeding behaviour

Not so long ago, mammalian breeding systems were seen as dominated by males fighting each other for the right to mate with passive females. Genetic parentage analysis has been instrumental in changing this view and exposing the key role of female choice. Some of the most interesting discoveries have emerged from work on seals, where extreme polygyny is common but females often seem to have a bigger say than was previously thought. A remarkable case in question involves Macquarie Island, where three species of fur seal recently formed a mixed breeding colony (Goldsworthy et al. 1999). Here, the true colours of both sexes lie unusually exposed, because classical models predict that males of the biggest species will dominate the beach and force females of smaller species to conceive mainly hybrid pups. In a fascinating paper in this issue of Molecular Ecology, Lancaster and colleagues (Lancaster et al. 2007) show that females are not this naïve. Although happy to gain protection for most of the season by sitting in the territory of one of the largest males, regardless of whether he is the same species, females almost always conceive to one of their own kind. The females do this, not because any hybrid male offspring they conceive will be sickly and fail to hold good territories, but because females who pup in their hybrid son's territories will be disproportionately likely to mate elsewhere. Hybrid males seem physically fit but sexually unattractive!

Finding fathers: spatio-temporal analysis of paternity assignment in grey seals (Halichoerus grypus)

Molecular studies of pinniped breeding systems exhibit a broad range of agreement and disagreement with observational indices of male breeding success. Grey seal studies have reported considerable discrepancies between genetic and behavioural paternity measures that have been interpreted as evidence of previously unidentified male strategies and/or tactics. Therefore, these studies have the power to fundamentally alter our perceptions of mating systems. However, other pinniped studies exhibit no such disagreements, and one possible explanation for disparities may be sampling biases in space and time. Therefore, it is essential that potential sampling biases are examined to evaluate the likelihood of previously unidentified male strategies. We examined paternities assigned at the North Rona grey seal colony between 1999 and 2002 in relation to concurrent detailed behavioural and locational data for males and females. We found that (i) for females observed in sexual interaction(s) during their oestrus period, it was highly probable that one of the interacting males fathered their next pup; (ii) over 80% of assigned paternities agreed with observations of the in-colony behaviour and spatio-temporal proximity of the males and females involved; and (iii) a minority of females exhibit mate choice and seek sires outside their local male's home range, although evidence suggests that these females mate on the colony rather than at sea. In conclusion, nearly all paternities assigned agreed with expectation based upon detailed knowledge of the spatio-temporal patterns of individuals during the breeding season. We found little evidence of unidentified male strategies at North Rona, Scotland, whereas further examination of mechanisms of female choice may be productive.

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.

Diving behaviour during the breeding season in the terrestrially breeding male grey seal: implications for alternative mating tactics

We examined the diving behaviour of breeding male grey seals (Halichoerus grypus) at Sable Island, Nova Scotia, from 1997 to 2001. The proportion of time spent at sea varied between 0 and 78% (N = 30). Males engaged in deep (43.4 ± 3.3 m (mean ± SE), N = 27) diving, and these dives were clustered into bouts, which mostly occurred during long trips (62.2 ± 14.7 h). We suggest that males spent time foraging during deep dives. Shallow diving (5.9 ± 0.1 m, N = 27) accounted for 40.8% of dives, which were also clustered into bouts that mostly occurred during short trips (2.1 ± 0.37 h). We suggest that shallow diving comprised a suite of behaviours, but included little foraging behaviour. Phenotypic traits had little influence on diving behaviour. Further work is required to understand the extent to which foraging behaviour enhances reproductive success, and whether shallow diving is a component of the mating tactics of male grey seals at Sable Island.

Genetic mating system and the significance of harem associations in the bat Saccopteryx bilineata

We analysed the polygynous mating system of the bat Saccopteryx bilineata using behaviour observations and genetic data on 11 microsatellite DNA loci. Basic social units in S. bilineata are harem groups that consist of single males and up to eight females. Colonies comprise several harem groups, and the composition of colonies and harems is often stable over several reproductive seasons. The combination of parentage exclusion and likelihood-based parentage assignment in this study produced detailed parentage information for a large colony of S. bilineata. Reproduction occurred mostly within the colony (17% extra-colony paternity), but social associations in harems within the colony did not represent reproductive units (70% extra-harem paternity). The latter finding was consistent over three reproductive seasons. Spatial association of the roosting sites of males and females could not explain parentage patterns in the colony. Even though intra-harem paternity was less frequent than expected, it contributed significantly to reproduction of harem males. On average, the number of offspring sired by a male with females in his harem territory increased significantly with harem size, which corresponds to the higher energetic investment that is related to the maintenance of large harems. However, extra-harem paternity was not correlated with a male's harem size or intra-harem reproductive success. This suggests that individual preferences of females rather than male traits associated with the ability to defend large harems are most likely to cause the detected differences between social association and genetic mating system.