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Reduced body size of insular black-tailed deer is caused by slowed development

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Adult body size correlates strongly with fitness, but mean body sizes frequently differ among conspecific populations. Ultimate, fitness-based explanations for these deviations in animals typically focus on community-level or physiological processes (e.g., competition, thermoregulation). However, proximate mechanisms underlying adaptive body size adjustments remain poorly understood. Adjustments in adult body size may result from shifts in growth-related life-history traits, such as the length of time to achieve adult body size (i.e., growth period) and how quickly the body increases in size (i.e., growth rate). Since insular populations often demonstrate dramatic shifts in adult body size, island populations represent a natural experiment by which to test the proximate mechanisms of size change. Here, using dental eruption patterns, we show that a dwarfed population of black-tailed deer (Odocoileus hemionus columbianus) experiences significant heterochronic shifts relative to mainland conspecifics. Namely, juvenile development slowed, such that teeth erupted ≥ 1 year later, but cranial growth suggested no concurrent adjustments in skeletal growth period. Thus, slowed growth rate, shown here with teeth, combined with unchanged growth period resulted in dwarfism, consistent with ultimate predictions for insular, resource-limited populations. Therefore, selection on body size may act on life-history traits that influence body size, rather than acting on body size directly.

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Fig. 1

Figure adapted from Palkovacs (2003)

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Mainland data are from Robinette et al. (1957)

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We thank T. Crowley, Jr. for generously providing access to land. L. Arnold, T. Hildebrand, B. McMillen, R. Pedersen, and Z. Wilson provided technical support in extracting and processing teeth. Accommodations in the field were provided by the Thomas B. Crowley Laboratory at the Blakely Island Field Station.


This work was supported by the Murdock Charitable Trust and a faculty research grant, through the Seattle Pacific University Center for Scholarship and Faculty Development.

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ESL, KLC, and JCL collected samples and analyzed data, CMWS directed data collection, ESL and CMWS wrote the manuscript and secured funding for the project.

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Correspondence to Eric S. Long.

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The authors declare that they have no conflict of interests.

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Communicated by Mathew Samuel Crowther.

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Long, E.S., Courtney, K.L., Lippert, J.C. et al. Reduced body size of insular black-tailed deer is caused by slowed development. Oecologia 189, 675–685 (2019).

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