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'.

Keywords: cetacean; female; kinship; social evolution.

Figure 1.

Social and matrilineal kinship structure in cetaceans—a conceptual organization. Cetacean societies across the Mysticeti (baleen whales) and Odontoceti (toothed whales) span a continuum between low and high modularity, increasingly structured into sets of highly connected individuals with the tendency of maternally related individuals to interact among themselves (female kinship organization ranging from matrifocal to more strictly matrilineal). Networks depict empirical data from long-term studies (after [4]) on photo-identified individuals (red nodes = females, blue nodes = males, yellow nodes = calves) connected by association (link thickness is proportional to association index). (Online version in colour.)

Figure 2.

Life-history processes and the central role of females as mothers in cetacean societies. (a) Duration of key life-history processes of female toothed whales (Odontoceti) and baleen whales (Mysticeti). Split violin plots indicate the probability distributions of log-transformed length of gestation and lactation (mean), inter-birth interval (mode), maturity (mean age at first birth) and longevity (maximum lifespan) across 13 Mysticeti species (red) and 43 Odontoceti species (blue). Dashed lines within violins indicate mean values. (b) Speed of female life-history processes relative to body length between toothed and baleen whales. Age at sexual maturity correlates highly with all other life-history traits and thus is used here as a summary measure of the other life-history processes. Shaded circles represent species of Odontoceti (red) and Mysticeti (blue) and solid icons indicate the mean values for each taxonomic family for which data were available. Icon sizes are suggestive of the average body length. Data and estimation methods in [11]. (Online version in colour.)