Evol Ecol Res 10: 775-785 (2008)     Full PDF if your library subscribes.

Do humans adjust offspring sex according to the local operational sex ratio?

Samuli Helle,1 Pekka Käär,2 Samuli Helama3 and Jukka Jokela4,5

1Section of Ecology, Department of Biology, University of Turku, Turku, Finland, 2Turku Biological Museum, Turku, Finland,  3Department of Geology, University of Helsinki, Hensinki, Finland,  4EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland and  5Institute of Integrative Biology (IBZ), Swiss Federal Institute of Technology (ETH-Zürich), Zürich, Switzerland

Correspondence: S. Helle, Section of Ecology, Department of Biology, University of Turku, FI-20014 Turku, Finland.
e-mail: samuli.helle@utu.fi


Question: Do humans overproduce the rarer sex in response to skews in the local operational sex ratio (OSR)? And how should one define the OSR in humans?

Hypothesis: We predicted that instead of the current adult sex ratio used in previous studies as a cue for the local OSR, parents should use the offspring sex ratio of the immediate preceding cohorts (estimated by an average spousal age difference at marriage) that comprise the most potential mates for their offspring.

Organism: Two historical (1718–1890) Sami populations from Northern Finland.

Analytical methods: Multivariate linear dynamic regression models to examine the delayed ratio-dependence of annual offspring sex ratio at birth, while controlling for the potential effects of annual maternal and paternal age at reproduction and reconstructed mean ambient temperature.

Results: One of the two Sami populations studied showed evidence for the compensatory production of the rarer sex in relation to the skews in predicted local OSR.

Keywords: Fisherian sex allocation, maternal and paternal age, temperature, time-series analysis.

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