COMPARATIVE ANALYSIS OF BODY SIZE EVOLUTION WITHIN POCKET GOPHERS (RODENTIA, GEOMYIDAE)

Pocket gophers (family Geomyidae) are the dominant burrowing rodents in North America today. They are also one of the rodent groups driving species richness increase in North America around 30 million years ago. Pocket gophers are the last one of these important Oligocene-aged rodent groups whose body size evolution is yet to be analyzed. Body size is a critical property of animals associated with many ecological characteristics, notably locomotion and habitat. In this study, we explore the pattern of body size evolution within two clades of Geomyidae as a tool to constrain the processes at play in the radiation of burrowing rodents. We explore body size changes within Geomyinae, the living subfamily of pocket gophers, and within Entoptychinae, an extinct subfamily present during the Oligocene and much of the Miocene.

We use upper and lower toothrow lengths from 266 fossil specimens and body mass data from museum collections and the literature for extant specimens. We use a published phylogenetic framework of geomyines and an unpublished phylogenetic tree of entoptychines to explore the mode of evolution of body size in the two groups of rodents. The results of our analyses show that the evolution of body size in Entoptychinae follows an Ornstein Uhlenbeck model whereas body size evolution within Geomyinae follows a directional model. The ancestral entoptychine was a mid-sized rodent. Within Entoptychinae, small body size evolved at least three times across three different genera. Within Geomyinae, the genus Thomomys remained small whereas several genera (e.g., Heterogeomys, Zygogeomys) independently evolved large sizes.

We also explored body size variation within Entoptychinae. There is no significant difference in body size among the genera Entoptychus, Gregorymys, and Pleurolicus; the rare genus Ziamys is significantly smaller. Our results show that species from the western United States and Mexico are larger than those found in the Great Plains. There also appears to be a consistent pattern of association between a large and a small species living contemporaneously within a given region. Ongoing analyses will help refine the timing of the evolution of body size in Geomyidae and help shed light on its possible association with locomotion and heterochronous environmental changes across North America.