Evol Ecol Res 13: 759-764 (2011)     Full PDF if your library subscribes.

Slightly male-biased sex ratios for the avoidance of extinction

Hiromu Ito1, Takashi Uehara1, Satoru Morita1, Kei-ichi Tainaka1 and Jin Yoshimura1,2,3

1Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan,  2Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, New York, USA and  3Marine Biosystems Research Center, Chiba University, Kamogawa, Chiba, Japan

Correspondence: J. Yoshimura, Department of Systems Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan.
e-mail: jin@sys.eng.shizuoka.ac.jp


Background: Slightly male-biased sex ratios occur commonly in animals with a higher male juvenile mortality, including humans. R.A. Fisher’s principle of equal sex ratios does not explain these slightly biased sex ratios. Tainaka et al. (2006) suggested that male juvenile mortality can cause a slight male bias.

Aim: To develop and analyse a model of sex ratio and extinction probabilities assuming sex-biased mortality.

Hypothesis: Slightly male-biased sex ratios reduce the probability of extinction of a population, leading to a shortage of one sex at maturity.

Mathematical method: Analyse the probability of extinction of a population exhibiting a shortage of one sex at maturity.

Key assumptions: Mothers give birth to a total of n children with sex ratio r at every generation on a small island. Males incur higher risks of mortality at the juvenile stage. The number of males and females at birth follows a binomial distribution.

Conclusions: A slightly male-biased ratio at birth is most persistent when higher rates of male-specific mortality apply. Male biases become larger with population size. Our results suggest that group selection helps to explain why slightly male-biased sex ratios are common in humans and other animal species.

Keywords: extinction probability, Fisher’s principle, male mortality, optimal sex ratio.

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