Sex-specific macronutrient foraging strategies in a highly successful marine predator: the Australasian gannet
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The foraging challenge for predators is to find and capture food with adequate levels of energy and nutrients. Marine predators require particularly sophisticated foraging strategies that enable them to balance self- and offspring-feeding, and also in many circumstances simultaneously consider the nutritional constraints of their partners. Here we combined the use of dietary analysis, proximate composition and nutritional geometry (right-angled mixture triangle nutritional models) to examine the macronutrient preferences of Australasian gannets (Morus serrator) at Farewell Spit gannetry in New Zealand. Our results showed intra- and inter-specific variation in the protein, lipid and water composition of prey captured by our sample of 111 Australasian gannets. In addition, we observed significant differences in the Australasian gannets’ nutritional niche between seasons. We provide evidence of sex-specific macronutrient foraging strategies in a successful marine predator in the wild. We have shown that in spite of fluctuations in the nutritional composition of foods available to Australasian gannets, males consistently capture prey with higher protein-to-lipid ratios and lower lipid-to-water ratios than females. These results aid to better understand the evolutionary relationship between macronutrient selection and sex-specific traits in wild animals. They also suggest an incentive for these predators to combine individually imbalanced but nutritionally complementary foods to achieve dietary balance, further highlighting the likelihood that prey selection is guided by the balance of macronutrients, rather than energy alone.
We thank J. Melville, S. Dwyer, D. Boulton, P. Jones and S. Clements for their assistance during the sample collection. Special thanks to A. Teixeira-Pinto for assistance in early stages of the statistical analysis and C. Lea (Massey University) for assisting on the organization of the logistics for the analysis of the samples. We also thank to the anonymous referees and V. H. Paiva for useful comments that have enhanced the manuscript. Aspects of this work were funded by Faculty of Veterinary Science DVC compact fund (The University of Sydney). The Department of Conservation, Golden Bay (New Zealand) kindly allowed use of their house at Farewell Spit, and transport was provided by Paddy Gillooly of Farewell Spit Ecotours (New Zealand). This study was conducted under permits from the University of Sydney Animal Ethics committee (N00/7-2013/3/6016), Massey University Animal Ethics committee (13/65) and the New Zealand Department of Conservation (35189-FAU). D. R. is an Adjunct Professor in the New Zealand Institute for Advanced Study, Massey University.
G. E. M.-C., E. B., W. C., D. M., R. S., K. B., C. P. and M. O. collected the data. A. M. S., G. E. M.-C., E. B., C. P., A. H. T., W. C., B. W., K. B. and D. R. analysed the data. G. E. M.-C., A. M. S., E. B., A. H. T., D. M., C. P., M. O., B. W., K. A. S. and D. R., wrote and edited the manuscript. G. E. M.-C., D. R. and K. A. S. designed the study.
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