Life history and group composition of melon-headed whales based on mass strandings in Japan

Abstract We examined melon-headed whales that mass-stranded live in two events inJapan: (1) 171 animals at Tanegashima Island in 2001 and (2) 85 animals at Hasakiin 2002. We report here the results of life history traits and group composition ofthese strandings, and compare them to another mass stranding with 135 individualsat Aoshima in 1982. In the Hasaki event, most stranded animals, including thosereleased were sexed and measured. The proportion of live males released was muchhigher than that of females, and larger animals, especially females, were more likelyto have died. Females were estimated to attain sexual maturity at around 7 yr andgive birth every 3–4 yr. The sex ratio was significantly different between the Hasakiand Aoshima events. Among dead specimens, females of various age classes wereincluded in all strandings, while age distribution of males varied considerablyamong strandings. This suggests females show group fidelity while males movebetween groups. Asymptotic body length of females from Hasaki was significantlysmaller than that from Tanegashima, suggesting that more than one population ofmelon-headedwhales existoff Japan.Key words: melon-headed whale, Peponocephala electra , life history, social structure,massstranding,Japan.