Sex Ratio in an Island Population

A long-term study of an island population of the bank vole showed sex ratio among trappable voles close to 1, though at low population numbers numcr.eal prevalence of females over males was fvtquent. Among mature individuals the females were usually more numerous than the males. Mortality rate of mature males was higher than that of mature females. A lower maturation rate was found for males than for females. Relative numerical stability with different levels for ma-ture males and females was evidenced. Hence, sex ratio in mature voles showed tendency toward prevalence of females during population increase. Spacing behaviour (a possible mechanism for regulation of natality) could be responsible for the changes in sex ratio among ma-ture individuals. [Institute of Ecology, PAS, Dziekanów Leśny, 05-092 Łomianki, Po-land] 1