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

ABSTRACT

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.

DOWNLOAD A FREE, FULL PDF COPY
IF you are connected using the IP of a subscribing institution (library, laboratory, etc.)
or through its VPN.

 

        © 2011 Jin Yoshimura. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

       Subscribing institutions/libraries may grant individuals the privilege of making a single copy of an EER article for non-commercial educational or non-commercial research purposes. Subscribing institutions/libraries may also use articles for non-commercial educational purposes by making any number of copies for course packs or course reserve collections. Subscribing institutions/libraries may also loan single copies of articles to non-commercial libraries for educational purposes.

       All copies of abstracts and articles must preserve their copyright notice without modification.