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Title: An Integrative Investigation of Convergent Bipedal Locomotion in Desert Rodents
Abstract: Bipedalism is commonly assumed to be an adaptive convergence because it has evolved independently three times in small desert rodents. However, the functional benefits of bipedality in this ecosystem have long been unclear. In this thesis I integrate phylogenetics, functional morphology, biomechanics, information theory, and behavior to investigate whether and how bipedality in- creases fitness in desert ecosystems, and whether bipedal rodents convergently evolved to have the role in their respective ecosystems.
Based on the diversity of extant dipodid rodents, I begin by statistically reconstructing the pattern of morphological evolution in jerboas. I find that the strongest indicator of bipedality is metatarsal to humerus length, and that changes in this ratio are associated with increased rates of speciation, supporting a punctuated equilibrium pattern of evolution in this clade. Furthermore, the distinct patterns of morphological evolution suggest that a complex suite of genetic and developmental mechanisms governs the acquisition of bipedality in jerboas.
I then use an inverse dynamics approach to characterize the biomechanics of bipedal hopping in a derived jerboa. I find that the dynamics of jerboa hopping are generated predominantly by muscular contractions, rather than tendon–based elastic energy storage and return between strides. Therefore hopping in small rodents favors rapid production and absorption of forces, rather than sustained bouts of steady–state locomotion.
By reviewing predator–prey studies in biomechanics, ecology, and evolution I hypothesize that ricochetal locomotion enhances the ability of small rodents to evade predators that hunt via ballistic interception. I then develop Information Theoretic techniques to measure the unpredictability of escape trajectories in sympatric bipedal and quadrupedal rodents. As expected, bipedal rodents use significantly more unpredictable escape trajectories, likely enhancing predator evasion ability and enabling foraging in exposed areas with higher predation risk. I then found that bipedal rodents exhibit a stronger preference for exploring open spaces, matching previously established foraging patterns. These findings suggest that the evolution of bipedality enables spatial resource partitioning to limit interspecific competition in desert rodents.
Based on the functional studies in my thesis, I evaluate ecological models to predict the occurrence of convergent bipedal rodents in Myomorpha. I show that diet specialization and aridity are insufficient to predict the locomotor morphology of these rodents and then develop novel hypotheses for the convergent evolution of bipedalism in desert rodents.
My thesis investigates the functional consequences of morphological evolution in the context of evolutionary ecology. By considering the interconnectedness of ecology, behavior, and evolution, studies in biomechanics can be designed to inform each of these fields. This interdisciplinary approach is necessary to study the adaptive nature of behavioral traits that are governed by myriad genetic, developmental, and environmental factors.