Dear colleagues, We are pleased to announce the publication of our new open-access article in Marine Mammal Science.
Kobayashi, H., Aoki, K., & Amano, M. (2026). Decreasing Sociality With Body Size in All-Male Groups of Sperm Whales (Physeter macrocephalus). Marine Mammal Science, e70297. The article is open access and can be found here: https://doi.org/10.1111/mms.70297 Abstract The costs and benefits of group living can influence animal group size and social relationships, and these balance points may shift as animals mature. Young male sperm whales (Physeter macrocephalus) appear to form stable all-male groups unrelated to direct defense of females or territories, but quantitative data on how sociality changes with growth are limited. We examined associations among individually identified males in the Goto Submarine Canyon and Nemuro Strait, Japan, and tested whether social relationships varied with body length. Smaller males showed higher association indices, and both association strength and maximum association index declined with increasing body length. The decline appeared to occur around 13–14 m, near the size at which males typically reach sexual maturity. Standardized lagged association rates indicated both temporary and longer-lasting associations among smaller males in the Goto Submarine Canyon. These results are consistent with a reduction in male sociality during growth, likely reflecting changing physiological state and shifting ecological costs and benefits of sociality. Increased energetic demands, improved individual foraging capacity, and reduced predation risk in larger males may all contribute to weaker associations with increasing size. Best regards, Hayao Kobayashi Faculty of Bioindustry Tokyo University of Agriculture Abashiri, Hokkaido, Japan
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