A comparison of five methods for assignment of sex in the takahe (Aves: Porphyrio mantelli)

Abstract

The takahe (Porphyrio mantelli) is an endemic New Zealand avian species which was once thought to be extinct. Its rediscovery in 1948 heralded an intense species recovery programme, which is ongoing. Early in this programme, difficulties in the assignment of sex became apparent. Subsequently several approaches have been used in order to develop accurate methods to identify males and females in this species. We discuss the value of these approaches and report the results of our studies into DNA-based, as well as morphometric and hormonal methods. We demonstrate that sex-linked DNA sequence variation within both the chromo-helicase-DNA binding (CHD) and ATP synthase α-subunit, (ATP5A1) genes allows the correct assignment of known-sex adults. We also report the development of a discriminant function using body weight, bill depth, shield width, length from nares to bill-tip, tarsus length, mid-toe length and culmen length. This method had a maximum accuracy of 89% on a sample of 37 known-sex takahe, using culmen and tarsus lengths. In relation to hormonal methods of sexing takahe, plasma testosterone and oestradiol concentrations in adult and juvenile male and female takahe were measured in spring. The oestradiol/testosterone ratio discriminated between all 15 adults sampled within 38 min of capture. However, males experiencing increased capture stress had reduced testosterone levels and hence an uninformative oestradiol/testosterone ratio. Testosterone and oestradiol levels were generally undetectable in juvenile takahe less than 10 months old. Consequently, plasma sex steroid analyses in the breeding season are useful for the determination of sex in recently captured adults only.