Digit length may reveal unusual breeding behaviour in a seabird

Abstract

The hormonal environment experienced during prenatal development may affect adult phenotype and behaviour. Digit lengths may provide an estimate of steroid levels encountered during embryonic development in humans and other vertebrates. Finger patterns in humans have been shown to reveal sexual orientation or cooperative behaviour. We explored individual breeding behaviour in a monogamous seabird, the Balearic shearwater Puffinus mauretanicus and unexpectedly detected some cooperative breeders. Furthermore, we show evidence of correlation between digit lengths and cooperative breeding in this species. Additionally, we suggest that the first digit could be a possible indicator of prenatal steroid levels. These results are the starting point for further tests of the hypothesis that first digit length is an indicator of prenatal hormone levels in other vertebrate species. Moreover, these results may offer practical use in wild populations to study the implications of the changes in prenatal environment for adult social behaviour.

Figure 1

Morphometric comparisons of (a) bill depth, (b) head plus bill depth, (c) tarsus length and (d) wing, between males (solid circles) and females (open circles) and between monogamous birds and cooperative breeders. Bars represent confidence intervals (95%) for each measure (in mm) and sample sizes are shown above bars.

Figure 2

Confidence intervals (95%) for (a) first digit length, (b) third digit length and (c) 1 : 3 digit ratios, in monogamous and cooperative breeders. Sample sizes are shown above bars.