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Morphological Adaptations in Crania of Four Species of Cricetid Rodent: Variation Through Time and Along Elevational Gradients

  • Author(s): Holmes, Michael William
  • Advisor(s): Lacey, Eileen A
  • Bowie, Rauri CK
  • et al.
Abstract

Determining how species respond to prolonged environmental change is critical to understanding both their evolutionary biology and their conservation needs. In general, organisms can respond to changing environmental conditions by moving, by adapting in situ, or by going locally or globally extinct. Morphological changes – whether plastic or adaptive – are one way that species may respond in situ to local environmental change. Cranial morphology is an important feature of vertebrate anatomy because this modular structure acts as a medium with the environment, housing the special sensory organs, containing part of the central nervous system, and acting as part of the masticatory apparatus. By examining cranial morphological change through time, we can gain insight into how species are adapting to changes in their environment.

In this dissertation, I examine cranial morphological adaptation in several species of cricetid rodent. In chapter 2, I examine cranial morphological adaptation in a widespread species. Peromyscus maniculatus is a habitat and dietary generalist found throughout North America. This species is ideal for studying adaptive change because it is found in multiple ecosystems and has not exhibited elevational range shifts in response to modern climate change. To assess potential responses of P. maniculatus to changing conditions in the Sierra Nevada Mountains of California, I quantified cranial variation in museum specimens of this species collected approximately 100 years apart. Specifically, I examined how cranial morphology varies in three populations of this geographically widespread, ecological generalist over elevation and time. My analyses indicate that cranial morphology does not differ with elevation within either modern or historical samples but does vary between time periods, suggesting that in situ responses to environmental change have occurred. Contrary to predictions based on Bergman’s rule, I found no consistent relationship between body size and either elevation or time, suggesting that morphological differences detected between historic and modern specimens are specific to factors influencing cranial structure. Collectively, these analyses demonstrate the potential importance of in situ changes in morphology as a response to changing environmental conditions.

In chapter 3, I expand my morphological study into four other species of cricetid rodent. These species, Reithrodontomys megalotis, Microtus californicus, Neotoma macrotis, and N. fuscipes are all found along broad elevational gradients in California’s Sierra Nevada and Cascade Mountains. These three species range from strict ecological specialists (N. macrotis and N. fuscipes) to ecological generalists (R. megalotis). I use geometric morphometrics to compare within-species variation across three transects over nearly a century. I show that counter to my hypothesis, there is no predictive morphological responses based on geography and ecology. Specialists and generalists show relatively similar rates of evolutionary change but no consistent pattern emerges in geographic location. Responses are heterogeneous even within taxa.

Finally, in chapter 4 I examine morphological variation over an elevational gradient. I test whether R. megalotis, M. californicus, and N. macrotis adhere to Bergmann’s. This ecogeographic postulate suggests that in colder environments such as high elevation, that endothermic organisms will tend to be larger. This theory is based on thermoregulatory physiology. I show that both M. californicus and N. macrotis exhibit negative correlations between elevation and body size. This is likely a plastic response due to shorter growing seasons and thus reduced food availability at higher elevations.

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