Causes and consequences of female centrality in cetacean societies

Abstract

Cetaceans are fully aquatic predatory mammals that have successfully colonized virtually all marine habitats. Their adaptation to these habitats, so radically different from those of their terrestrial ancestors, can give us comparative insights into the evolution of female roles and kinship in mammalian societies. We provide a review of the diversity of such roles across the Cetacea, which are unified by some key and apparently invariable life-history features. Mothers are uniparous, while paternal care is completely absent as far as we currently know. Maternal input is extensive, lasting months to many years. Hence, female reproductive rates are low, every cetacean calf is a significant investment, and offspring care is central to female fitness. Here strategies diverge, especially between toothed and baleen whales, in terms of mother–calf association and related social structures, which range from ephemeral grouping patterns to stable, multi-level, societies in which social groups are strongly organized around female kinship. Some species exhibit social and/or spatial philopatry in both sexes, a rare phenomenon in vertebrates. Communal care can be vital, especially among deep-diving species, and can be supported by female kinship. Female-based sociality, in its diverse forms, is therefore a prevailing feature of cetacean societies. Beyond the key role in offspring survival, it provides the substrate for significant vertical and horizontal cultural transmission, as well as the only definitive non-human examples of menopause. This article is part of the theme issue ‘The evolution of female-biased kinship in humans and other mammals’.

Kinship and reproductive condition correlate with affiliation patterns in female southern Australian bottlenose dolphins

“…This also suggests that some females could be dispersing from their natal areas into adjacent bays, which differs from the general pattern observed in inshore bottlenose dolphins, where females usually remain philopatric (i.e. 36,44,45,67 ). Under this scenario, kin clusters of preferred associates may relocate to different areas or embayment within Coffin Bay in response to social or ecological pressures, providing an explanation for the lack of correlation between home range overlap and genetic relatedness.…”

Prolonged maternal investment in northern bottlenose whales alters our understanding of beaked whale reproductive life history

“…Maternal investment in mammals varies based on an array of ecological and evolutionary factors resulting in a range of maternal strategies e.g. [1]. Nursing is critical to the survival and fitness of infant mammals; providing our earliest energetic and nutritional requirements, supporting maternal bonding, and initializing ongoing socialization [2][3][4].…”

“…Maternal investment in cetaceans (dolphins and whales) is known to be extensive and provides a key role in infant survival, however our appreciation of weaning strategies is challenged by the cryptic nature of nursing behaviour and their aquatic habitat [1]. What we do know can be generalized by sub-order; with a large degree of variability between species, odontocetes appear to prolong nursing and weaning over years (mean = 21 months), while mysticetes typically wean their young within the first year (mean = 11 months) [11] (S1 Table).…”

“…What we do know can be generalized by sub-order; with a large degree of variability between species, odontocetes appear to prolong nursing and weaning over years (mean = 21 months), while mysticetes typically wean their young within the first year (mean = 11 months) [11] (S1 Table). This difference in maternal investment has been linked to energetic resources available to income versus capital breeders [1,12].…”

“…Four different methods have been used to estimate nursing duration in cetaceans: stomach content analysis, cow-calf ratios, behavioural observations and stable isotope analyses, which may explain some of the discrepancies between estimates within and among species [1,11]. Across studies, behavioural observations typically reported the oldest average age at weaning (27 months), in contrast with stomach content analyses, which found average weaning age occurred much younger (16 months, S1 Table).…”