News Release

Unequal sex ratios in Darwin's finches

Peer-Reviewed Publication

Proceedings of the National Academy of Sciences

A <em>Geospiza Fortis</em> in the Island of Pinta -- Galapagos in 2011

image: A ,Geospiza fortis in the Island of Pinta -- Galapagos in 2011 view more 

Credit: Image courtesy of Ruben Heleno (University of Coimbra, Coimbra, Portugal).

Environmental fluctuations can strongly bias the adult sex ratio of populations, altering mating patterns and reproductive success, a study finds. The adult sex ratio of populations is influenced by social and environmental forces. However, whether the adult sex ratio changes over time, and if so, how such changes affect mating patterns and fitness remain unclear. Peter Grant and B. Rosemary Grant analyzed 21 years of data on two species of Darwin's finches - the medium ground finch (Geospiza fortis) and the cactus finch (Geospiza scandens) - in the Galápagos archipelago. Environmental perturbations caused starvation, especially among females, resulting in strongly male-biased sex ratios. The 1983 El Niño event, which primarily affected G. scandens due to the destruction of cactus bushes, resulted in males of this species outnumbering females in 1987, but no bias in the sex ratio was observed for G. fortis during that year. Due to the increased opportunity for females to choose high-quality males, the frequency of mate changes in 1987 was three times higher in G. scandens than in G. fortis. Females that changed mates gained a clear fitness advantage in terms of numbers of offspring that survived to the following year. According to the authors, the findings demonstrate that environmental fluctuations can strongly bias the adult sex ratio of populations, altering mating patterns and reproductive success.

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Article #19-03838: "Adult sex ratio influences mate choice in Darwin's finches," by Peter R. Grant and B. Rosemary Grant.

MEDIA CONTACTS: Peter Grant, Princeton University, Princeton, NJ; tel: 609-258-5156, 609-924-5533; e-mail: prgrant@princeton.edu


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