Variation and Frequency of Occurrence of the Baculum in a Population of Mexican Free-Tailed Bats

Abstract

Bacula occurring in 51 approximately adult male bats (Tadarida brasiliensis) were prepared, studied and measured. The frequency of occurrence in the population was 78 per cent but was 100 per cent in old bats, indicating that osteogenesis occurs about the time of maturity. The baculum is short (0.53-0.85 mm., mean .637, N 39) and usually spindle-shaped in dorsal view, tapering to acuminate termini proximally and distally. In lateral view the bone is usually sharply curved upward. No significant differences were found between the mean bacular lengths of two age classes; probably little if any growth occurs after the baculum forms. It is highly variable (CV for 39 bats having bacula, 10.6) in comparison to variation of other mammalian structures or to the length of cranium in T. brasiliensis (r is not significant in analysis of the paired variables). High variation may result from a relatively unimportant functional length; from natural selection operating on either or probably both sexes; from lack of integration with other skeletal parts; from a factor associated with late osteogenesis; or from combinations of these. If variation of the baculum in mammals in general is high, as it appears to be in this bat, the great diversity of the structure in Mammalia is not so surprising. A capacity for rapid evolution of the the baculum, governed by natural selection, may be provided by high variation with genetic basis, and may be important in speciation because of the potential or correlative development of mechanisms of reproductive isolation. Among species and higher categories the baculum, or mammalian penis-bone, is greatly differentiated (Burt, 1960; and others). However, its individual, ontogenetic, and geographic variations have been seldom investigated. The extent and nature of individual variation of the baculum may be related to its being so diverse. Such variation that has genetic basis may be acted upon by natural selection. We studied individual and ontogenetic variation in the baculum of Mexican free-tailed bats (Tadarida brasiliensis mexicana) taken 2628 July 1964 from a cave at Cofre de Perote, 9200 feet elevation, Veracruz, Mexico. The baculum is minute in bats and is extremely small in Tadarida brasiliensis according to Krutzsch and Vaughan (1955), who found only one in ten bats dissected. We have obtained information concerning the frequency of occurrence of the baculumrn in T. brasiliensis, of ontogeny of the baculum, and of its size, shape, and variation.