The habitats of the Bull Creek Nature Preserve support an interesting variety of mammals. Although none of the mammals are specific only to the Central Texas region, they are an integral part of the Preserve ecosystem and food web. For instance, the guano (excrement) produced by bats such as the Mexican free-tailed bat serves as a nutrient source for cave fauna. Several mammals—including carnivores that keep prey in check and large herbivores such as the white-tailed deer that shape habitat—function as keystone species in the Preserve ecosystem. A keystone species is a species that influences and interacts with numerous other species in the community. Elimination (or overabundance) of a keystone species can have profound effects on other species and the overall structure of the ecosystem. The problem of overabundance is further discussed in the White Tailed Deer section.

The mammals of the Preserve are active during different times of the day, depending on the habits and survival techniques each species has developed. For instance, skunks and raccoons are nocturnal, meaning they engage in feeding and other activities at night. Adapted to living in the dark, these species have light-sensitive eyes and darkly-colored fur to camouflage themselves from predators. Contrarily, other species such as squirrels are diurnal, meaning they are active during the day. Diurnal mammals have eyes that are adapted to bright light and are often colored to blend in with their surrounding habitat. The white-tailed deer and the eastern cottontail are crepuscular species, meaning they are active in the early morning at dawn and in the late evening at dusk. This is an ideal time for these species to feed because the light is too bright for nocturnal predators and too dim for daytime predators. The nine-banded armadillo has adapted to its lack of fur by changing from nocturnal to diurnal depending on the season. During the summer, the armadillo is active during the late evening and night. However, during winter, the armadillo will venture out in the warm hours of the afternoon.

Since many of the mammals on the Preserve are nocturnal, it is difficult to find species while hiking along the trail. Depending on the time of day, keep an eye out for fox squirrels in the trees as well as other diurnal and crepuscular species. Given the difficulty of actually seeing mammals on the Preserve, one of the best ways to identify and learn about the behavior and habitat of mammals is by looking for signs such as tracks and scats (droppings). The signs left by mammals can often tell an interesting story. Was the animal fleeing from a predator? Does the scat contain seeds and berries, or perhaps fur and feathers? Also, look for burrows and possible nesting areas along the trail. For instance, opossums and raccoons will often use hollow trees or logs as dens. Armadillos den in burrows along creek banks as well as in the natural crevices and openings of the limestone.