Reproductive roles as likely drivers of sexual dimorphism in New Zealand’s endangered mountain parrot, the Kea

Abstract

Abstract New Zealand’s endangered mountain parrot, the Kea (Nestor notabilis), exhibits moderate male-biased sexual size dimorphism in linear body measurements (~5%) and a pronounced dimorphism in bill size (12–14%). Using stable isotope analyses of carbon and nitrogen in Kea feathers and blood sampled from a significant portion (~10%) of the extant population, we determined that Kea bill dimorphism may be an ecologically selected trait that enhances male Kea’s ability to forage at a higher trophic level in order to provision females and offspring during nesting. Sexual dimorphism can arise through sexual selection, ecological drivers, or a combination of both. Ecological selection is associated with foraging niche divergence between the sexes to reduce inter-sexual competition or due to differing dietary needs associated with reproductive role. Despite the widespread occurrence of sexual dimorphism throughout the animal kingdom, empirical evidence for ecological causation is rare. We conducted the first molecular confirmation of sexual size dimorphism in Kea. We then employed Bayesian mixing models to explore potential correlations between diet and bill size to determine whether the dimorphism is linked to diet partitioning throughout all age classes (fledgling, juvenile, subadult, and adult). Female Kea foraged at a consistent, relatively low, trophic level throughout their lifetime, whereas male trophic level increased with age to a maximum at subadult stage, prior to breeding for the first time—a time in which males may have been actively learning extractive foraging techniques associated with a high protein diet. Adult males foraged at a high trophic level relative to all groups except subadult males. As males provision females on the nest, which in turn provision young, these results highlight that the evolution of morphology and reproductive output may be linked in circuitous ways.