Tall young females get ahead: size-specific fecundity in wild kangaroos suggests a steep trade-off with growth
When resources are limited, organisms face allocation conflicts. Indeterminate growth creates a persistent conflict with reproduction, as growth may enhance future reproduction, but diverts resources from current reproduction. Little is known about allocation trade-offs in mammals with indeterminate growth. We studied growth and reproduction in adult female eastern grey kangaroos (Macropus giganteus), an iteroparous mammal with indeterminate growth. Allocation trajectories varied with age and size: for 4-year-old females, fecundity increased from 30 to 82% from shortest to average-sized individuals. Older females had high fecundity regardless of size. The smallest females grew 30% more annually than average-sized females, but females that reached average size at an older age had lower growth rates. Environmental conditions affected allocation to size and reproduction. Rainy springs increased fecundity from 61 to 84% for females that had previously reproduced, but rainy winters reduced leg growth. Females in better relative condition grew 40% more than average, whereas most young of females below average relative condition failed to survive to 10 months of age. These results highlight an age-specific trade-off between growth and reproduction. Tall young females benefit from a smaller trade-off between somatic growth and early fecundity than shorter females of the same age, but older females appear to favor reproduction over growth regardless of size. Our study highlights how individual heterogeneity determines trade-offs between life-history components. We speculate that cohort effects affect age-specific reproductive success in this long-lived mammal.
KeywordsAllocation Indeterminate growth Life history Macropus giganteus Reproductive success
We thank all students, collaborators, and assistants who helped us monitor kangaroos at Wilsons Promontory National Park, in particular Allison MacKay and Dave Forsyth. We acknowledge Parks Victoria staff for logistical support. Handling of animals was authorized by the University of Melbourne Animal Ethics Committee (0810628.1, 0911512.1, and 1312902.1), the Victorian Department of Sustainability and Environment (1004582, 1005558, and 100007062), and by the Université de Sherbrooke Animal Care Committee (MFB2012-02), affiliated with the Canadian Council on Animal Care. This study was supported by the National Geographic Society, the Natural Sciences and Engineering Research Council of Canada, the Fonds de Recherche Québécois en Nature et Technologies, and the Ministère de l’Éducation, du Loisir et du Sport du Québec.
Author contribution statement
All conducted fieldwork and collected data; LQ analyzed and interpreted the data, and wrote the manuscript; LQ and MFB designed and executed the study; WJK, GC, and MFB provided editorial advice during the writing of the manuscript.
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