Sexual Size Dimorphism and Body Condition in the Australasian Gannet

Abstract

Sexual size dimorphism is widespread throughout seabird taxa and several drivers leading to its evolution have been hypothesised. While the Australasian Gannet (Morus serrator) has previously been considered nominally monomorphic, recent studies have documented sexual segregation in diet and foraging areas, traits often associated with size dimorphism. The present study investigated the sex differences in body mass and structural size of this species at two colonies (Pope's Eye, PE; Point Danger, PD) in northern Bass Strait, south-eastern Australia. Females were found to be 3.1% and 7.3% heavier (2.74 ± 0.03, n = 92; 2.67 ± 0.03 kg, n = 43) than males (2.66 ± 0.03, n = 92; 2.48 ± 0.03 kg, n = 43) at PE and PD, respectively. Females were also larger in wing ulna length (0.8% both colonies) but smaller in bill depth (PE: 2.2%; PD: 1.7%) than males. Despite this dimorphism, a discriminant function provided only mild accuracy in determining sex. A similar degree of dimorphism was also found within breeding pairs, however assortative mating was not apparent at either colony (R2 < 0.04). Using hydrogen isotope dilution, a body condition index was developed from morphometrics to estimate total body fat (TBF) stores, where TBF(%) = 24.43+1.94*(body mass/wing ulna length) - 0.58*tarsus length (r2 = 0.84, n = 15). This index was used to estimate body composition in all sampled individuals. There was no significant difference in TBF(%) between the sexes for any stage of breeding or in any year of the study at either colony suggesting that, despite a greater body mass, females were not in a better condition than males. While the driving mechanism for sexual dimorphism in this species is currently unknown, studies of other Sulids indicate segregation in foraging behaviour, habitat and diet may be a contributing factor.

Fig 1. Location of Australasian gannet colonies in the present study, Point Danger (indicated by black square) and Pope’s Eye (indicated by black circle).

Fig 2. Distribution of discriminant scores for male and female Australasian gannets at two breeding colonies.

Scores given for (a) Pope’s Eye and (b) Point Danger colonies, where males are in black, and females are in white. Scores are based on the discriminant functions provided in text.

Fig 3. Estimated total body fat(%) reliably predicts actual total body fat(%) in Australasian gannets based on a body condition index.

Measurements of body mass (kg), wing ulna and tarsus length (mm) were selected. Plot shows the predicted model (solid line) and the 95% confidence interval (dashed line).

Fig 4. Mean body condition (%) ± SE for males and females from Pope’s Eye and Point Danger gannet colonies.

Where males are in black and females are in white.