Changing reproductive rates in a neotropical forest rodent, Proechimys semispinosus

1. Theory predicts that if optimal reproductive effort is adjusted with changes in density, then an equilibrium should result in which any change in density will result in a change in reproductive effort as the system returns to equilibrium (Schaffer & Tamarin 1973). If reproduction lags behind density, then a clockwise trajectory through reproduction-density space will result. Seasonality may alter the equilibrium point by affecting resource abundance. Many temperate rodents appear to adjust reproductive effort with changes in density to optimize individual fitness, thereby conforming to predictions. 2. Populations of Neotropical rodents living in seasonal environments have not been studied for sufficient duration to examine relationships among resource abundance, density and reproductive activity. Populations of Proechimys semispinosus (Tomes 1860) (Central American spiny rat) on four small(<3 ha) islands in Panama were sampled by monthly live-trapping for 4 years to examine changes in density and reproductive effort. Fruiting trees were also censused to examine the relationship between density and resource abundance. 3. Density and reproductive effort (number of young born per adult female) varied significantly overtime and among populations. Length of each annual breeding season and age and body mass at first reproduction in females also varied. 4. Phase plots of young born per adult female and breeding season length vs. total density both progressed clockwise through reproduction-density space in all four populations. 5. Breeding-season density was positively related to density of fruiting trees and lianas. 6. We suggest that resource abundance was a major determinant of breeding-season densities, and reproductive effort may have been adjusted in response to changes in density. Thus, resource abundance was a major force driving population trajectories through reproduction-density space.