Age Structure and Sex Ratio in Populations of Reithrodontomys

Age structure of populations of Reithrodontomys megalotis longicaudus, Reithrodontomys raviventris raviventris, and Reithrodontomys raviventris halicoetes was investigated through analysis of skulls aged by tooth wear. Age structure is similar among the three taxa. Age classes are not clearly delimited. There is no mass die-off of older adults during a specific period of the year. A long breeding period provides for recruitment into the population over an extended period of time. Sex ratios greatly favor males except in the youngest and oldest of the six age classes recognized. Selection for more males may be advantageous in small mammal populations with organizational types such as are found in these Reithrodontomys.