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Abstract Seasonal changes in reproduction have been described for many taxa. As reproductive seasons progress, females often shift from greater energetic investment in many small offspring towards investing less total energy into fewer, better provisioned (i.e. larger) offspring. The underlying causes of this pattern have not been assessed in many systems.Two primary hypotheses have been proposed to explain these patterns. The first is an adaptive hypothesis from life‐history theory: early offspring have a survival advantage over those produced later. Accordingly, selection favours females that invest in offspring quantity early in the season and offspring quality later. The second hypothesis suggests these patterns are not intrinsic but result from passive responses to seasonal changes in the environment experienced by reproducing females (i.e. maternal environment).To disentangle the causes underlying this pattern, which has been reported in brown anole lizards (Anolis sagrei), we performed complementary field and laboratory studies. The laboratory study carefully controlled maternal environments and quantified reproductive patterns throughout the reproductive season for each female. The field study measured similar metrics from free ranging lizards across an entire reproductive season.In the laboratory, females increased relative effort per offspring as the reproductive season progressed; smaller eggs were laid earlier, larger eggs were laid later. Moreover, we observed significant among‐individual variation in seasonal changes in reproduction, which is necessary for traits to evolve via natural selection. Because these patterns consistently emerge under controlled laboratory conditions, they likely represent an intrinsic and potentially adaptive adjustment of reproductive effort as predicted by life‐history theory.The field study revealed similar trends, further suggesting that intrinsic patterns observed in the laboratory are strong enough to persist despite the environmental variability that characterizes natural habitats. The observed patterns are indicative of an adaptive seasonal shift in parental investment in response to a deteriorating offspring environment: allocating greater resources to late‐produced offspring likely enhances maternal fitness.
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The Evolution of Offspring Size: A Metabolic Scaling Perspective
Abstract Size at the start of life reflects the initial per offspring parental investment—including both the embryo and the nutrients supplied to it. Initial offspring size can vary substantially, both within and among species. Within species, increasing offspring size can enhance growth, reproduction, competitive ability, and reduce susceptibility to predation and starvation later in life, that can ultimately increase fitness. Previous work has suggested that the fitness benefits of larger offspring size may be driven by energy expenditure during development—or how offspring metabolic rate scales with offspring size. Despite the importance of early-life energy expenditure in shaping later life fitness trajectories, consideration of among-species scaling of metabolic rate at the time of birth as a potential source of general metabolic scaling patterns has been overlooked by theory. Here, we review the patterns and processes of energy expenditure at the start of life when mortality is often greatest. We compile existing data on metabolic rate and offspring size for 191 ectotherm species spanning eight phyla and use phylogenetically controlled methods to quantify among-species scaling patterns. Across a 109-fold mass range, we find that offspring metabolic rate scales hypometrically with size, with an overall scaling exponent of 0.66. This exponent varies across ontogenetic stage and feeding activity, but is consistently hypometric, including across environmental temperatures. Despite differences in parental investment, life history and habitat, large-offspring species use relatively less energy as a proportion of size, compared with small-offspring species. Greater residual energy can be used to fuel the next stages of life, particularly in low-resource environments. Based on available evidence, we conclude that, while large knowledge gaps remain, the evolution of offspring size is likely shaped by context-dependent selection acting on correlated traits, including metabolic rates maintaining hypometric scaling, which operates within broader physical constraints.
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- Award ID(s):
- 2141592
- PAR ID:
- 10402349
- Date Published:
- Journal Name:
- Integrative and Comparative Biology
- Volume:
- 62
- Issue:
- 5
- ISSN:
- 1540-7063
- Page Range / eLocation ID:
- 1492 to 1502
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/icb/icac076
