Survival improvements in zoo-housed giraffes lead to surplus males: A long-term analysis of longevity and mortality in Japanese zoos

Abstract Surplus animals exceeding housing capacity pose a major challenge for zoo population management. In polygynous species such as giraffes (Giraffa camelopardalis), where fewer males than females can be accommodated per facility, male surpluses are particularly problematic. Although the proportion of males among zoo-housed giraffes has increased globally in recent decades, the demographic drivers remain unclear. Here, we analysed Japanese zoo studbook data (1980–2023) to examine population trends, birth sex ratios, and sex-specific survival to elucidate factors influencing the population sex balance. Our analysis showed that the number of males rose sharply after 2012 and surpassed females for the first time in 2021, indicating the emergence of a pronounced male surplus. Survival analyses showed that both sexes experienced higher survival following husbandry reforms around 2010, but the improvement was disproportionately greater in males. Although females still exhibited higher survivorship, the survival gap between sexes narrowed and disappeared in the most recent cohorts, shifting the adult sex ratio toward parity and accelerating the male surplus. This study demonstrates that male surplus can emerge as an unintended consequence of successful husbandry improvements rather than biased birth ratios or reduced culling. These findings provide key implications for proactive sex ratio management in zoos.