Group structure and mating strategies of Cuvier's (Ziphius cavirostris) and Blainville's (Mesoplodon densirostris) beaked whales off the island of Hawai`i

Citation

Mahaffy, Sabre D., R. W. Baird, D. J. McSweeney, D. L. Webster, G. Schorr. 2015. Group structure and mating strategies of Cuvier's (Ziphius cavirostris) and Blainville's (Mesoplodon densirostris) beaked whales off the island of Hawai`i. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.

Abstract

Small, long-term resident populations of Cuvier's and Blainville's beaked whales exist off Hawai`i Island; however, little is known about their mating systems. We examine group structure using 97 encounters with Blainville's beaked whales over 29 years (mean group size=3.2, range=1-10) and 89 encounters with Cuvier's beaked whales over 25 years (mean group size=2.0, range=1-5). Adult sex ratios were not significantly different from 50:50 (Cuvier's - 29 adult females, 21 adult males; Blainville's - 38 adult females, 35 adult males). Adult females were encountered significantly more frequently than adult males in both species, and were re-sighted significantly more frequently in Blainville's. The difference in re-sightings between adult male and female Cuvier's was not significant but may be a result of small sample size. Re-sightings of adult females spanned 25 years for Blainville's and 20 years for Cuvier's, while adult male re-sightings spanned 15 years and 10 years, respectively. Adult female Blainville's were rarely seen alone (5.6% of encounters with females, n=72), whereas more than one third of encounters with adult male Blainville's (n=55) and adult male and female Cuvier's (n=36, n=66, respectively) were solitary. In groups of four or more, two adult males were often present in Cuvier's encounters (37.5%), but were rare in Blainville's encounters (6.5%). Males present in the same encounter typically avoided each other and one male generally had more prominent teeth and extensive scarring. Groups containing two adult females were not uncommon for either species; however groups with three or more females comprised 15.2% of Blainville's sightings with females but only 3% of Cuvier's sightings. Results suggest the formation of groups in both species may reflect different mating strategies: female defense polygyny has been posited for Blainville's elsewhere and is supported here. Mating strategies have not been previously described for Cuvier's, though there is some evidence indicating that sperm competition plays an important role. Our observations of multi-male groups when females are present support this; however, the presence of erupted teeth in males and extensive scarring from agonistic encounters is contrary to other species demonstrating sperm competition, suggesting Cuvier's may employ a uniquely modified system.

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