Decreasing Sociality With Body Size in All‐Male Groups of Sperm Whales ( Physeter macrocephalus )

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 occurred 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.