Sexual Size Dimorphism and Body Condition in the Australasian Gannet

Lauren P Angel, Melanie R Wells, Marlenne A Rodríguez-Malagón, Emma Tew, John R Speakman, John P Y Arnould

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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.

Original languageEnglish
Article numbere0142653
Number of pages16
JournalPloS ONE
Volume10
Issue number12
Early online date4 Dec 2015
DOIs
Publication statusPublished - 4 Dec 2015

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Body Size
dimorphism
Sex Characteristics
body condition
Adipose Tissue
Ulna
Fats
Nutrition
body fat
ulna
Breeding
Diet
Morus
Bass
South Australia
foraging
Body Composition
Ankle
tarsus (bone)
Isotopes

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Angel, L. P., Wells, M. R., Rodríguez-Malagón, M. A., Tew, E., Speakman, J. R., & Arnould, J. P. Y. (2015). Sexual Size Dimorphism and Body Condition in the Australasian Gannet. PloS ONE, 10(12), [e0142653]. https://doi.org/10.1371/journal.pone.0142653

Sexual Size Dimorphism and Body Condition in the Australasian Gannet. / Angel, Lauren P; Wells, Melanie R; Rodríguez-Malagón, Marlenne A; Tew, Emma; Speakman, John R; Arnould, John P Y.

In: PloS ONE, Vol. 10, No. 12, e0142653, 04.12.2015.

Research output: Contribution to journalArticle

Angel, LP, Wells, MR, Rodríguez-Malagón, MA, Tew, E, Speakman, JR & Arnould, JPY 2015, 'Sexual Size Dimorphism and Body Condition in the Australasian Gannet', PloS ONE, vol. 10, no. 12, e0142653. https://doi.org/10.1371/journal.pone.0142653
Angel LP, Wells MR, Rodríguez-Malagón MA, Tew E, Speakman JR, Arnould JPY. Sexual Size Dimorphism and Body Condition in the Australasian Gannet. PloS ONE. 2015 Dec 4;10(12). e0142653. https://doi.org/10.1371/journal.pone.0142653
Angel, Lauren P ; Wells, Melanie R ; Rodríguez-Malagón, Marlenne A ; Tew, Emma ; Speakman, John R ; Arnould, John P Y. / Sexual Size Dimorphism and Body Condition in the Australasian Gannet. In: PloS ONE. 2015 ; Vol. 10, No. 12.
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