Onyx Cave
Bats of Onyx Cave

Onyx Cave is used by a maternity colony of fringed myotis (Myotis thysanodes), one of the three species of long-eared myotis in Arizona. The bat gets its name from the fringe of little hairs along the bottom edge of its tail membrane. The species is patchily distributed throughout its range from British Columbia, Canada through the western U.S. to Chiapas, Mexico. Although it lives in desert scrub up to coniferous forest, it is most commonly found at middle-elevations in habitats such as oak woodland (Onyx Cave), pinyon-juniper, and ponderosa pine. Bats roost in tightly packed groups and in addition to living in caves, also use mines, tree snags, rock crevices, and buildings. The fringed myotis eats mainly beetles and moths, but also other insects. It flies relatively slowly and forages close to vegetation, so is very maneuverable.

The bats in Onyx are females that form a maternity colony to have their young. From our monitoring at Onyx, we know the mothers begin arriving in May. Colony size appears to be about 60-70, plus young of the year. Young are born between late June and mid- July. Each bat gives birth to one young. Newborn bats are pink and hairless; hair begins growing after about a week. Newborns are placed in a group separate from their mothers, who come periodically to nurse them. Young bats can fly well by the time they are 3 weeks old. There also may be a small number of males that likely roost separately from the females - researchers captured and released an adult male in June. In October, bats begin to leave for their hibernation site and are gone before mid-November. We don't know how far they travel or where they go; there are few known hibernacula of any bat species in southern Arizona.

The more we learn about the bats in Onyx, the better we'll be able to ensure the colony's persistence. Fringed myotis have been known to live 11 years, although most probably don't reach that age. Because reproduction is low and juvenile mortality is naturally high, it's important to prevent additional sources of stress on female bats or cause increased juvenile mortality. The colony naturally may move to different locations within the cave; bats seek out optimum temperatures as their physiological needs change during pregnancy, lactation, and post-lactation.

Adult females return year after year to the same maternity colony. Female young of the year also return to the cave; male young of the year disperse. Assuming that each adult female gives birth, and there is a 1:1 ratio of female to male young, you might wonder why adult female colony size doesn't increase by 50% each year. There are several reasons why this doesn't happen. The most important is that many young do not survive their first year of life. In addition, there is always some normal adult mortality, and perhaps not every female that returns to the cave in the spring is pregnant. All of these factors contribute to a stable or slowly increasing maternity colony. If there is above normal mortality of young or adults, colony size can decrease. Closing the cave to visitors during the most crucial period for females and young prevents possible additional mortality due to inadvertent disturbance by cavers. To prevent unintentional disturbance to the colony, Onyx Cave is closed between June 1 and October 31.

Login       Logout