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Survival and Reproduction in Big Brown Bats

We obtained detailed information on bat survival and reproduction for incorporation into models that integrate bat population dynamics with rabies epidemiology. We relied in part on individual-based data to obtain this information by tagging over 4,000 big brown bats with uniquely coded passive integrated transponders (PIT tags).


Automatic PIT tag readers placed at entrances to roosts in buildings provided "sight-resight" records that allowed calculation of survival rates with much narrower confidence intervals than previous studies of bats that relied on banding. We are also among the first studies of North American bats to apply Cormack-Jolly-Seber approaches to survival estimation. We used Program MARK [] to calculate apparent survival, and coupled survival estimation with AIC-based models to investigate the importance of potential covariates to survival. Final, comprehensive analyses are underway, but a number of important results pertinent to techniques we employed are available [Neubaum et al. 2005, O’shea et al. 2005, Wimsatt et al. 2005]. In general, we found that survival of adults (about 0.75 to 0.80) is high for a small mammal. Survival appears to vary by roost and by season, with survival during summer lower than in winter. Mortality during summer appears to be of greater significance than mortality at hibernacula.

Hoop of a PIT reader at the
entrance of a big brown bat roost
Automatic PIT tag recorder


Reproduction is markedly seasonal in temperate zone bats, occurring once each summer. This is the case in Colorado. Each summer the reproductive rates of big brown bats in Fort Collins are high. The proportion of adult females that were captured in hand and found to be pregnant or lactating each year was 92.5 %, varying from 89% to 99 % over the five summers. There was little variation in timing of reproductive events, with most females giving birth during the same 2-3 week period each year. Litter size is one, but twinning occurs. We employed mammography to ascertain litter size non-invasively, and found that 10.7 % (12 of 112) of pregnant females had twin pups. Age of first reproduction of females is not uniform. It is typically 1 year, but nearly 30% of females did not reproduce until age 2 or older (based on in-hand examinations of 508 known age or known-minimum age PIT-tagged females).

Bat with single fetusBat with twin fetuses

Young big brown bats first emerge and start to fly in mid to late July, when ratios of adult females to young captured at emergence at roosts are about 1:1 (figure 1). The appearance of naïve young around Fort Collins coincides with the period of greatest number of reports of possible public exposures to bats received by Larimer County in their rabies surveillance system (figure 2).

Figure 1. Percentage of big brown bats that were adults or juveniles caught at roosts from 2001—2005. Figure 2. Total number of big brown bats turned into the Larimer County Humane Society from 1997—2003.
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