Abstract

We assessed the impact of disease investigative techniques on survival of wild big brown bats (Eptesicus fuscus) as part of a larger ongoing investigation of the ecology of bat rabies in Fort Collins, Colorado.

Introduction

Our objectives were to determine if the health monitoring techniques we employed had an effect on survival, which, if true, could lead to erroneous overestimates of the impact of disease on the population at risk.

Methods

To estimate impacts of anesthesia and bleeding on short-term (14-day) survival, we randomly assigned adult female and volant juvenile bats captured at maternity roosts in buildings during summer 2002 into two groups within each age category. Techniques employed in group 1 (controls) included: capture, transport, morphologic measurements, oral swabbing for PCR, aseptic wing biopsy and dorsal subcutaneous passive integrated transponder (PIT) tag injection (with tissue glue closure), and re-release at the site of capture within 6 hours of bat collection. In group 2 (treated), inhalant anesthesia and blood sampling from vessels in the tail membrane (targeting samples at <1% of body weight) were added to the above procedures.

Results and Discussion

Anesthesia was delivered in a capsule specially designed to minimize stress and exposure of personnel to bites from bats. We recorded 876 immobilization times for bats anesthetized and bled in 2002; these averaged 9.1±5.1 (SD) minutes (range 1–71). Volumes of blood samples averaged 58±12 µl (range 13–126, n=718), not including wastage. We monitored fate of bats subsequent to release by placing PIT tag hoop readers with computer memory at roost openings of wild colonies in urban areas. Bats were passively verified as alive when they crawled across the hoops during egress and returns to the roost. Short-term (14-day) survival probabilities were calculated using subroutines within program MARK,³ an online software package that estimates numeric maximum likelihood-based survival probabilities from recapture data. In the control groups studied for 14-day survival in 2002, 86 adult females and 92 volant juveniles (females:males=47:45) were processed. In the treatment groups, 187 adult females and 87 volant juveniles (44:43) were processed and studied. These data suggested no adverse effect of anesthesia and sampling on 14-day survival (adults: χ²=22.22, d.f.=27, p>0.05; juveniles: χ²=9.72, d.f.=18, p>0.05), when compared to bats undergoing the same procedures minus anesthesia and blood sampling.

We also computed 1-year return rates of bats PIT tagged in 2001 and not anesthetized and bled, anesthetized and bled once, or anesthetized and bled twice that year. Returns were monitored at roosts with hoop readers in 2002, and confidence intervals for rates calculated on the basis of the binomial distribution. The 1-year return rates for big brown bats processed in 2001 and monitored in 2002 were comparable among adult bats not anesthetized and bled (81%, n=72, 95% CI=70–91%), adult bats anesthetized and bled once the previous year (82%, n=276, 95% CI=81–84%), and adult bats anesthetized and bled twice the previous year (84%, n=50, 95% CI=74–94%). Lack of effect was also noted in 1-year return rates of volant juvenile females not bled (55%, n=29, 95% CI=37–73%), juvenile females anesthetized and bled once the previous year (66%, n=113, 95% CI=58–75%), and juveniles anesthetized and bled twice (71%, n=17, 95% CI=49–92%). Comparability of 1-year return rates among control and treatment groups of females (males are not expected to return to maternity colonies and were not included in the analysis) suggest no long-term effects on survival. These 1-year return rates also are comparable to or exceed more crudely estimated values in the literature^1,2 for wild big brown bats that were captured and handled in various studies but without collection of multiple biologic samples, suggesting that the entire suite of sampling techniques has no marked effect on survival. These data clearly show that our techniques for anesthesia and sampling blood do not adversely bias assessment of impacts of disease on either short-term survival or 1-year return rates in wild big brown bats. Passive integrated transponder technology allowed this assessment to be made with considerable efficiency.

Acknowledgments

Project funding was provided by NSF grant # 0094959.

Literature Cited

1.  Hitchcock, H.B., R. Keen, and A. Kurta. 1984. Survival rates of Myotis leibii and Eptesicus fuscus in southeastern Ontario. J. Mammal. 65: 126–130.

2.  Tuttle, M.D. and D. Stevenson. 1982. Growth and survival of bats. In: Kunz, T.H. (ed.). Ecology of Bats. Plenum Press, New York, NY. Pp. 105–150.

3.  White, G.C. and K.P. Burnham. 1999. Program MARK—survival estimation from populations of marked animals. Bird Study. 46 (Supplement). 120–138.