Evaluating strategic offspring sex allocation in the Southern Rockhopper Penguins Eudyptes chrysocome chrysocome, a species with obligate brood reduction

Abstract

Southern Rockhopper Penguins Eudyptes chrysocome chrysocome exhibit marked intra-clutch egg size dimorphism andengage in obligate brood reduction of the first and smaller laid egg, with the rearing of the two nestlings occurring only onrare occasions. Studying the sex ratio of hatchlings within a brood in this species, under this breeding strategy context, isessential to understanding population dynamics. The objectives of this study were to analyze egg size dimorphism withinthe broods, determine the offspring sex ratio, and investigate if the hatchling sex ratio was determined by hatching orderand egg volume to discuss potential biases in the context of brood reduction. We molecularly assessed hatchlings’ sex ratioaccording to the hatching order at Isla Pingüino, Argentina (3 years, n = 68 broods). Sex ratio was not different from theexpected 50:50 proportion in all the studied years. In this sense, we found a balanced sex ratio in the first hatchlings, typicallythe survivors of the brood reduction process. Although not significant, we found a bias toward females in the secondhatchlings. Assuming less energy is required to raise a female compared to a male, because of its smaller size; this strategymight enhance the chances of raising two nestlings if conditions are optimal. Southern Rockhopper Penguins at Isla Pingüinoappear to exhibit a strategic sex allocation evident in the second but not in the first hatchlings, which might be consistentwith its out-of-the-norm exponential growth population.