Abstract
Adult male sperm whales (Physeter macrocephalus) are long distance runners of the marine realm, feeding in high latitudes and mating in tropical and subtropical waters where stable social groups of females and immatures live. Several areas of uncertainty still limit our understanding of their social and breeding behavior, in particular concerning the potential existence of geographical and/or social fidelities. In this study, using underwater observation and sloughed-skin sampling, we looked for male social fidelity to a specific matrilineal sperm whale group near Mauritius. In addition, we captured a wider picture of kin relationships and genetic diversity of male sperm whales in the Indian Ocean thanks to biopsies of eight individuals taken in a feeding ground near the Kerguelen and Crozet Archipelagos (Southern Indian Ocean). Twenty-six adult male sperm whales were identified when socializing with adult females and immatures off Mauritius. Sloughed-skin samples were taken from thirteen of them for genetic analysis. Long-term underwater observation recorded several noteworthy social interactions between adult males and adult females and/or immatures. We identified seven possible male recaptures over different years (three by direct observation, and four at the gametic level), which supports a certain level of male social fidelity. Two probable first- and thirty second-degree kin relationships were highlighted between members of the social unit and adult males, confirming that some of the adult males observed in Mauritian waters are reproductive. Male social philopatry to their natal group can be excluded, as none of the males sampled shared the haplotype characteristic of the matrilineal social group. Mitochondrial DNA control region haplotype and nucleotide diversities calculated over the 21 total male sperm whales sampled were similar to values found by others in the Indian Ocean. Our study strongly supports the existence of some levels of male sperm whale social fidelity, not directed to their social group of birth, in the Indian Ocean. Males sampled in breeding and feeding grounds are linked by kin relationships. Our results support a model of male mediated gene flow occurring at the level of the whole Indian Ocean, likely interconnected with large-scale geographical fidelity to ocean basin, and a small-scale social fidelity to matrilineal social groups.
Highlights
Sexual dimorphism, defined as differences in external appearance or other characteristics between the two sexes of a species (Mesnick and Ralls, 2018), is widespread among animals, and especially in vertebrates (Shine, 1989)
Using 9 years of monitoring based on underwater observation of sperm whale social groups off Mauritius paired with genetic information collected on individuals from both this area and the Crozet/Kerguelen region, our aims were to: (i) Assess the association patterns and genetic relatedness of adult males with the members of a resident social group with which they associate; (ii) determine the extent of genetic relatedness across adult males, and, (iii) analyze possible social and geographical fidelity of adult male sperm whales, including whether they show fidelity to their natal social group
Since 2015, fieldwork has been conducted on a regular basis between February and May, and some sporadic observations made during the rest of the year, except in January (Supplementary Table 1)
Summary
Sexual dimorphism, defined as differences in external appearance or other characteristics between the two sexes of a species (Mesnick and Ralls, 2018), is widespread among animals, and especially in vertebrates (Shine, 1989). Sperm whales certainly display some of the most striking sexual dimorphism among cetaceans, both in terms of body size with adult males growing up to 18 m long and a weight of 45 t, while females usually remain around 11 m long for 13 t (Best, 1979; Cantor et al, 2019); and in terms of feeding ecology, geographical distribution and social organization (Rice, 1989; Whitehead and Kahn, 1992; Teloni et al, 2008; Kobayashi et al, 2020). Males disperse from their natal group after 6–8 years, before their sexual maturity, and move poleward to areas abundant in food (Rice, 1989) After their twenties, they make periodic forays to warmer waters for mating, with no known clear frequency, seasonal agendas nor migration routes (Best, 1979). We know that adult male sperm whales can travel thousands of kilometers across ocean basins (Lyrholm et al, 1999; Engelhaupt et al, 2009; Mizroch and Rice, 2012; Steiner et al, 2012), no recurrent migration routes between feeding and breeding areas have so far been identified (Cantor et al, 2019)
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