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Human digit ratios depend on birth order and sex of older siblings and predict maternal fecundity

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Abstract

In men, the length ratio of the second to fourth finger (2D:4D) is smaller, while the length of the fourth finger relative to body height (4D:H) is larger than in women. Inter- and intrasexual variations in 2D:4D and 4D:H may depend on variation in fetal androgen and oestrogen environment. As maternal physiology varies with parity and is differentially affected by gestation of either sex, offspring 2D:4D and 4D:H may change according to sex and number of older siblings and may predict subsequent maternal performance. We analysed 2D:4D and 4D:H in Caucasian university students. 2D:4D was smaller and 4D:H was larger in males than in females, but no sexual dimorphism existed in 2D:H. In males, length ratios did not vary with birth order. 2D:4D became more masculine with increasing proportion of males among older siblings, and 2D:4D and 4D:H became more feminine as the number of older sisters increased. In females, length ratios did not vary with the number of older sisters or brothers. 2D:4D was also not related to birth order, but 4D:H became more masculine with birth order. In females, residual maternal fecundity (number of maternal offspring after the participant) decreased as 4D:H became more masculine. These findings are partly consistent with those from previous studies and suggest that maternal fecundity co-varies with length ratios and thus possibly fetal hormone environment of older offspring. 2D:4D and 4D:H may therefore represent powerful tools to investigate the relationships between fetal environment, offspring phenotype and maternal life history at mechanistic and evolutionary levels.

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Acknowledgement

The present study comply with the Italian current laws. We thank John T Manning for valuable comments on an earlier version of the manuscript.

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  1. Dipartimento di Biologia, Università degli Studi di Milano, via Celoria 26, 20133, Milan, Italy

    Nicola Saino, Barbara Leoni & Maria Romano

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  1. Nicola Saino
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  2. Barbara Leoni
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  3. Maria Romano
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Correspondence to Nicola Saino.

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Communicated by T. Czeschlik

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Saino, N., Leoni, B. & Romano, M. Human digit ratios depend on birth order and sex of older siblings and predict maternal fecundity. Behav Ecol Sociobiol 60, 34–45 (2006). https://doi.org/10.1007/s00265-005-0137-1

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  • DOI: https://doi.org/10.1007/s00265-005-0137-1

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