Litter Size and Postnatal Growth Rate in the Pallid Bat, Antrozous pallidus

Abstract

Factors intrinsic to the maternity roost and factors determining maternal nutrition affect postnatal growth rates of bats in the field (Tuttle and Stevenson, 1982). For example, roost temperature significantly influences growth rates, although few field studies of ontogeny have determined the thermal environment of the maternity roost. Ectoparasite populations within a maternity roost may also influence postnatal growth. In addition, factors extrinsic to the maternity roost such as weather and food availability, which determine the nutritional status of the mother, influence postnatal growth rates. Litter size in most species of bats is one, but the number of young ranges up to five in certain species (Tuttle and Stevenson, 1982). The number of young per litter also varies within a species depending on the mother's age, geographic location, season, and general climatic conditions. Pallid bats, Antrozous pallidus, produce twins in 80% of births and single offspring in the remaining 20% (Orr, 1954). I present here data on the effects of litter size on the size of young at birth and on postnatal growth rate in pallid bats in a laboratory setting. Also, the effect on the growth rate of the remaining animal of removing one member of a pair of twins at birth is discussed. Finally, growth rates determined for A. pallidus in the laboratory will be compared to those measured in the field. Pregnant A. pallidus were collected in an attic roost near Napa, Napa Co., California, during late May 1979, 1980, 1981, and 1982 and were transported to Seattle, Washington, by air freight. Bats were housed as a group in the laboratory and were maintained on a diet of mealworms supplemented with vitamins and minerals. Cage temperature was maintained between 26-28?C with a relative humidity of 55-65%, and the animals were exposed to a 12L:12D cycle. Parturition occurred in the middle of June. Newborn bats remained with the mother until weaned. A portion of the animals born each year were used in studies of the ontogeny of microvascular function in the cutaneous circulation of the wing. Experimental manipulation of these animals had no effect on growth rate compared to control bats (F = 1.24, dMf. = 1, 40, P > 0.05); therefore, data for all animals reared in the laboratory were pooled. Growth rates were determined for 63 bats, which resulted from 33 twin and seven single births. Growth rate was determined by measuring forearm length to the nearest 0.01 mm with a vernier caliper during the period of linear growth between birth and 22 days of age. The initial measurement (day 1) was made within 12 to 18 hours after birth; newborns were not handled until 12 hours after birth to allow completion of mother-young bonding. Forearm length was then measured periodically between days 1 and 22, but no attempt was made to measure all animals at specific ages. Growth rate was determined by regressing forearm length on age (Sokal and Rohlf, 1969) after forearm length was normalized to adult forearm length. Forearm length of adults used in this study averaged 57.4 mm (n = 43, SE = 0.31 mm) and was not significantly different from year to year. Forearm length was normalized to allow comparison of growth rates among different populations of A. pallidus and with other bat species. Growth rates were expressed as percent of adult forearm length per day, and forearm lengths were expressed as percent of adult forearm length. Comparisons of growth rate (slope of regression line) and size at birth (Y-intercept of regression line) among subgroups of interest were made with regression analyses and dummy variable tests for parallelism and coincidence found in Sokal and Rohlf (1969) and Kleinbaum and Kupper (1978). Forearm length on days 1 and 22 was compared among subgroups using a one-way ANOVA (Sokal and Rohlf, 1969). Pallid bats reared in the laboratory developed on a schedule similar to that described by Orr (1954) for field- and laboratory-reared animals and by Davis (1969) for field-reared animals. The mother provided the sole source of nutrition; weaning in A. pallidus occurs at 40 to 45 days of age (Orr, 1954). Growth of newborns did not appear to be limited by nutritional status of the mother because females in this study