Sexual and geographic dimorphism in northern rockhopper penguins breeding in the South Atlantic Ocean

Abstract

The Endangered northern rockhopper penguin Eudyptes moseleyi, like all penguins, is monomorphic, making sex determination of individuals in the field challenging. We examined the degree of sexual size dimorphism of adult birds across the species' breeding range in the Atlantic Ocean and developed discriminant functions (DF) to predict individuals' sex using morphometric measurements. We found significant site-specific differences in both bill length and bill depth, with males being the larger sex on each island. Across all islands, bill length contributed 78% to dissimilarity between sexes. Penguins on Gough Island had significantly longer bills, whilst those from Tristan da Cunha had the deepest. Island-specific DFs correctly classified 82−94% of individuals, and all functions performed significantly better than chance. The model for Nightingale Island correctly classified the greatest proportion of individuals (94−95%), while that for Tristan da Cunha performed the poorest (80−82%). A discriminant function derived from all sites accurately sexed 86−88% of northern rockhopper penguins achieving similar accuracy to island-specific functions. While molecular techniques conclusively determine an individual's sex, morphometric measurements can provide a reliable estimate with close to 90% accuracy using a method that is less invasive and requires little technical expertise. Sexing is an important tool for meaningful interpretation of ecological data. Consideration of sex-specific differences in future studies will aid investigation of a potential sex-dependent vulnerability in this Endangered species.

Pitfalls and advances in morphometric sexing: insights from the Adélie penguin Pygoscelis adeliae

“…Nevertheless, discriminatory power may depend on geographical, population-specific differences (e.g. Valenzuela-Guerra et al 2013;Steinfurth et al 2019, for penguins). Indeed, applying to our study population those morphometric functions previously implemented for the Adélie penguin in other colonies would not provide a sufficient classification power to sex penguins at Edmonson Point.…”

Factors affecting adult body condition in the endangered northern rockhopper penguin

“…Individuals were sexed according to their arrival date at the colony after winter migration (males 20 July-3 August; females 10-26 August). The frequency distribution of the culmen length of sexed birds was then used to determine a culmen length threshold used to sex individuals captured during the moulting period when both sexes arrive together (Warham 1970;Steinfurth et al 2019). Since there is a slight overlap between male and female culmen length, we removed individuals belonging to the 90th percentile of the culmen length frequency distribution for females and the 15th percentile for males to remove potential sexing errors.…”

“…Wikelski and Wrege (2000)). As northern rockhopper penguins are sexually size dimorphic, with females being generally smaller than males (Warham 1970;Steinfurth et al 2019;Cuthbert 2013), low resource availability would disproportionately affect males, which is suggested by the higher sensitivity to variability in SSTa (Figs. 4,5).…”