Tilman's model predicts an asymmetric relationship, with ascending, peak, and descending parts, between plant species diversity and a resource gradient. I evaluated the applicability of Tilman's model to mammals by regressing rodent and carnivore species diversity on estimates of net aboveground primary productivity. Rodent diversity was highest at low productivity levels and declined as productivity increased. Although my data did not confirm the upslope and peak phases of Tilman's model, this may be because I did not sample from areas of extremely low productivity. Published studies from very dry desert habitats indicate that rodent diversity increases rapidly across a narrow gradient of very low productivity and peaks at moderately low levels. This suggests that rodents, which are largely herbivorous, exhibit an asymmetric productivity-diversity curve similar to that predicted by Tilman for plants.The rodent data are also consistent with the intermediate disturbance hypothesis, which predicts diversity as a peaked function of a disturbance gradient. Rainfall predictability, which may function as a disturbance, is inversely related to my plant productivity gradient. The regression curve for carnivores helps to distinguish between these two hypotheses. At a given locality carnivores and rodents should be exposed to about the same disturbance levels and according to the intermediate disturbance model should peak at about the same place. Since carnivores as a whole are about one trophic level above rodents, if plant productivity controls diversity the peak for carnivores should occur at about an order of magnitude of productivity higher than the peak for rodents. My carnivore diversity data exhibit upslope, peak, and downslope phases and are shifted to higher levels of productivity by about one order of magnitude. This provides evidence against the intermediate disturbance hypotheses and corroborates predictions of Tilman's model as applied to mammals.

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We compared sequence variation in the complete mitochondrial cytochrome-b gene with chromosomal and geographical variation for specimens of Peters' tent-making bat (Uroderma bilobatum). Three different chromosomal races have been described in this species: a 2n = 42 race from South America east of the Andes, a 2n = 44 from NW Central America and 2n = 38 from the rest of Central America and NW South America. The deepest nodes in the tree were found within the South American race (42 race), which is consistent with a longer history of this race. Average distance among races ranged from 2.5 to 2.9%, with the highest amount of intraracial variation found within the 2n = 42 race (1.7%), intermediate values within the 2n = 38 race (0.9%) and lowest within the 2n = 44 race (0.5%). Variation among chromosomal races accounted for over 55% of molecular variance, whereas variation among populations within races accounted for 6%. The 2n = 38 and 2n = 44 races hybridize in the coastal lowlands of Honduras, near the Gulf of Fonseca. Introgression between these two races is low (two introgressed individuals in 45 examined). Clinal variation across the hybrid zone for the cytochrome-b of U. bilobatum, is similar to clinal variation reported for chromosomes and isozymes of this species. Mismatch distribution analyses suggests that geographical isolation and karyological changes have interplayed in a synergistic fashion. Fixation of the alternative chromosomal rearrangements in geographical isolation and secondary contact is the most likely mechanism accounting for the hybrid zone between the 2n = 38 and 2n = 44 races. If a molecular clock is assumed, with rates ranging from 2.3 to 5.0% per million years, then isolation between these races occurred within the last million years, implying a relatively recent origin of the extant diversity in Uroderma bilobatum. None the less, the three chromosomal races probably represent three different biological species.

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