Digit Ratio in Birds

Abstract

The Homeobox (Hox) genes direct the development of tetrapod digits. The expression of Hox genes may be influenced by endogenous sex steroids during development. Manning (Digit ratio. New Brunswick, NJ: Rutgers University Press, 2002) predicted that the ratio between the lengths of digits 2 (2D) and 4 (4D) should be sexually dimorphic because prenatal exposure to estrogens and androgens positively influence the lengths of 2D and 4D, respectively. We measured digits and other morphological traits of birds from three orders (Passeriformes, house sparrow, Passer domesticus; tree swallow, Tachycineta bicolor; Pscittaciformes, budgerigar, Melopsittacus undulates; Galliformes, chicken, Gallus domesticus) to test this prediction. None were sexually dimorphic for 2D:4D and there were no associations between 2D:4D and other sexually dimorphic traits. When we pooled data from all four species after we averaged right and left side digits from each individual and z-transformed the resulting digit ratios, we found that males had significantly larger 2D:4D than did females. Tetrapods appear to be sexually dimorphic for 2D:4D with 2D:4D larger in males as in some birds and reptiles and 2D:4D smaller in males as in some mammals. The differences between the reptile and mammal lineages in the directionality of 2D:4D may be related to the differences between them in chromosomal sex determination. We suggest that (a) natural selection for a perching foot in the first birds may have overridden the effects of hormones on the development of digit ratio in this group of vertebrates and (b) caution be used in making inferences about prenatal exposure to hormones and digit ratio in birds.

Digit ratios in two lacertid lizards: sexual dimorphism and morphological and physiological correlates

“…Such a 'reversed' sexual dimorphism in digit ratio has been noted in another Podarcis species (P. muralis, Rubolini et al 2006). Moreover, in 16 of the 32 lizard species examined up to now, males have larger, rather than smaller, 2D:4D digit ratios (Rubolini et al 2006;Lombardo et al 2008;Direnzo and Stynoski 2012;Gomes and Kohlsdorf 2012). This, combined with evidence from birds (e.g.…”

“…We speculate that digit ratios will differ between males and females, but the sign of the difference is hard to predict. In about half of the (restricted) number of species of lizards examined in previous studies, males had larger 2D:4D ratios than females; in the second half, the reverse was the case (Lombardo et al 2008;Direnzo and Stynoski 2012;Gomes and Kohlsdorf 2012). If the difference in 'temperament' between the study species is mediated by testosterone, we expect the 'bolder' P. siculus to have a lower digit ratio than more 'shy' P. melisellensis.…”

“…Why tetrapod species vary so much in the size and direction of the dimorphism in 2D:4D ratio remains highly unclear. On a proximate level, species may vary in the effect that steroids have on the secretion of growth hormone, or in the way that such steroids are deposited in eggs that will give rise to male and female offspring (Rubolini et al 2006;Lombardo et al 2008). These ideas could be …”

“…Brown et al 2002;McFadden and Bracht 2002;Nelson and Shultz 2010; but see Roney et al 2004), the trait is less conserved in birds (e.g. Burley and Foster 2002;Navarro et al 2007;Lombardo et al 2008). Information available for other taxa is scant.…”

Relationships of Toe-Length Ratios to Finger-Length Ratios, Foot Preference, and Wearing of TOE Rings

“…This is particularly the case for 2D:4D research in avian species (Lombardo, Thorpe, Brown, & Sian, 2008). It is therefore of interest whether sex differences in human toe-length ratios, as suggested by the findings of McFadden and Shubel (2002), are replicable.…”