Abstract
BackgroundBody size variation has played a central role in biogeographical research, however, most studies have aimed to describe trends rather than search for underlying mechanisms. In order to provide a more comprehensive understanding of the causes of intra-specific body size variation in ectotherms, we evaluated eight hypotheses proposed in the literature to account for geographical body size variation using the Darwin’s frog (Rhinoderma darwinii), an anuran species widely distributed in the temperate forests of South America. Each of the evaluated hypotheses predicted a specific relationship between body size and environmental variables. The level of support for each of these hypotheses was assessed using an information-theoretic approach and based on data from 1015 adult frogs obtained from 14 sites across the entire distributional range of the species.ResultsThere was strong evidence favouring a single model comprising temperature seasonality as the predictor variable. Larger body sizes were found in areas of greater seasonality, giving support to the “starvation resistance” hypothesis. Considering the known role of temperature on ectothermic metabolism, however, we formulated a new, non-exclusive hypothesis, termed “hibernation hypothesis”: greater seasonality is expected to drive larger body size, since metabolic rate is reduced further and longer during colder, longer winters, leading to decreased energy depletion during hibernation, improved survival and increased longevity (and hence growth). Supporting this, a higher post-hibernation body condition in animals from areas of greater seasonality was found.ConclusionsDespite largely recognized effects of temperature on metabolic rate in ectotherms, its importance in determining body size in a gradient of seasonality has been largely overlooked so far. Based on our results, we present and discuss an alternative mechanism, the “hibernation hypothesis”, underlying geographical body size variation, which can be helpful to improve our understanding of biogeographical patterns in ectotherms.
Highlights
Body size variation has played a central role in biogeographical research, most studies have aimed to describe trends rather than search for underlying mechanisms
In this study we evaluated eight hypotheses currently proposed in the literature to account for intra-specific body size variation of ectotherms. We did this using body size data obtained from across the entire distributional range of an anuran species, the Darwin’s frog (Rhinoderma darwinii Duméril and Bibron 1841). These hypotheses were: (1) Starvation resistance: larger body sizes are adaptive to more seasonal environments where individuals spend a long time in inactivity because larger individuals have higher resistance to starvation [7, 12,13,14]
Body size All frogs were captured by hand and their snout-vent length (SVL) measured to the nearest 0.1 mm (0.5 mm for Crump’s data)
Summary
Body size variation has played a central role in biogeographical research, most studies have aimed to describe trends rather than search for underlying mechanisms. In this study we evaluated eight hypotheses currently proposed in the literature to account for intra-specific body size variation of ectotherms We did this using body size data obtained from across the entire distributional range of an anuran species, the Darwin’s frog (Rhinoderma darwinii Duméril and Bibron 1841). These hypotheses were: (1) Starvation resistance: larger body sizes are adaptive to more seasonal environments where individuals spend a long time in inactivity (e.g. hibernating) because larger individuals have higher resistance to starvation (i.e. energy stores increase with size faster than metabolic rate) [7, 12,13,14]. Relaxing selective pressure on surface/mass ratio in warm environments permits individuals attains larger sizes, increasing other size-related benefits [5]. (5) Growing season length hypothesis: larger body sizes are expected in less seasonal environments where conditions and resources allow a longer period of growth [10]. (6) Primary productivity: this hypothesis predicts that areas with higher primary productivity are associated with an elevated food supply and larger body sizes are reached [9, 16]. (7) Water availability: larger body sizes in amphibians are adaptive to drier environments because a lower surface/mass ratio reduces the loss of water [18, 19]. (8) Converse water availability: larger body sizes in amphibians are associated with wetter climates because activity in this group is associated with high water availability [9]
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