Infant Insectivorous Bat Rehabilitation

by Mike Fry
ABSTRACT

Despite a lack of evidence to support it, there is a common belief that infant insectivorous bats are incapable of being successfully raised and returned to the wild. Some wildlife rehabilitators have speculated that: young bats are incapable of learning to catch live food without the assistance of adult bats; captive raised bats might not be accepted into colonies of wild bats; captive raised bats would be incapable of finding shelter.

In the face of no scientific evidence to support these speculations, the author attempted to collect data that would shed light on the releasablility of captive raised insectivorous bats.

INTRODUCTION

During the spring and summer of 1993, the author networked with the Minnesota Department of Natural Resources, local nature centers, wildlife rehabilitators, pest control companies and naturalists in order to acquire infant bats in need of rehabilitation. Infant bats were then raised with limited human contact and provided with large amounts of natural food sources. When the young were old enough to fly, they were placed in a flight cage at Tamarack Nature Center in Ramsey County, Minnesota.

The cage was constructed in such a way as to allow the free movement of flying insects into and out of the enclosure. A battery powered fluorescent lantern was suspended in the center of the enclosure, attracting large quantities of flying insects in the evening. Supplemental food was gradually reduced over the summer and weights were monitored twice daily.

Radio transmitters weighing 0.6 grams were purchased from Holohill Telemetry and ATS Systems. Dummy transmitters were attached to the backs of two bats, using ANDREA Perma-Lash Adhesive.

In addition to the active rehabilitation that took place during the study, opportunities arose to explore more proactive approaches to suburban habitat preservation of maternal colonies.

PHILOSOPHY

During the course of this study, in the face of little or no scientific data concerning the release of infant bats, there were a number of assumptions made by the reasearchers. The primary assumption was that fundamental rehabilitation principles that were effective with other species of animals would also apply to infant insectivorous bats. Based on this assumption, we made the following decisions:

  1. Each individual animal would be measured against a specific set of release criteria.
  2. Whenever possible, bats would be raised with con specifics.
  3. Whenever possible, we would attempt to reunite young bats with their mothers rather than pursue rehabilitation.

In addition to these decisions, we concluded that bats were poor candidates for life-long captive care. Therefore, any bats which were deemed unreleasable were humanely euthanized.

RABIES

Due to the seriousness of potential exposure to rabies, careful consideration needs to be made prior to handling bats. This study does not propose to make any claim as to the safety of handling bats with regard to this disease. Nor does this study propose circumstances that would be safe for handling any species of bat.

In some areas of the United States, it is illegal to rehabilitate bats due to the potential for spreading this disease. So the importance of considering this aspect of bat rehabilitation can not be understated. For the purposes of this study, all persons responsible for handling bats received pre-exposure rabies vaccinations.

NON-REHABILITATION ACTIVITIES

During the season, we encountered two maternal colonies of Little Brown Bats which were in danger due to construction or demolition of the buildings in which they resided. Through the efforts of project volunteers, both construction projects were postponed to a point when young bats were capable of flying. This also provided the rare opportunity to observe some mother/infant interaction in a natural setting.

REHABILITATION

Neonate bats were housed in Styrofoam coolers with air holes punched through the sides to allow for ventilation. Soft cotton cloth was hung on the sides of the coolers and placed on the bottom. The cloth provided folds in which the young bats would hide. The material also facilitated daily cleaning of the enclosures. The ambient room temperature of the bat nursery was kept at about 90 ľf during the day with the temperature dropping to about 80 ľf in the evening.

Infant bats were initially fed small amounts of formula mixed with a rehydrating solution of 2.5% Dextrose in Lactated Ringers. Once infants appeared stable and no signs of digestive problems were observed, the amount of rehydrating solution was decreased and the amount of formula was increased. By their third day in the bat nursery, infant bats were being fed 100% infant bat formula. For a detailed description of the formula fed, see Table 1.

Since no data could be found concerning the maximum comfortable stomach capacity of bats, the stomach capacity was assumed to be similar to most other placental mammals. Therefore, feeding schedules were calculated based on a stomach capacity of 50 cc per kilogram of animal body weight. Anecdotal information received from talking with other rehabilitators suggests that infant bats may have a comfortable stomach capacity that is considerably larger than this. However the 50 cc per kilogram volume is recommended as a starting point in feeding.

Bats were weaned off of formula onto a combination of live and dead insects including crickets, mealworms and waxworms. Flies, leafhoppers, moths, mosquitoes, gnats and beetles were also provided.

During the summer, 16 bats were accepted. Big Brown Bats (Eptesicus fuscus), Little Brown Bats (Myotis lucifugus), Silver-haired Bats (Lasionyceris noctivagans), and Red Bats (Lasiurus borealis) were all seen. Lone infants as well as injured females carrying young were treated.

MOTHERS WITH YOUNG

All infant bats that were admitted with a mother were left with their mothers during the rehabilitation process. Even stressed and injured mother bats demonstrated great care in tending their young. On the basis of this observation it is recommended that young be cared for by their natural mothers for as long as possible. Further study needs to be conducted concerning the release of young with their mothers. However, given the territorial nature of most bats, as well as their willingness to care for their young under stressful circumstances, there does not appear to be any reason to remove the young prior to releasing the mother, assuming they can be returned to their colony.

PRE-RELEASE CONDITIONING

A total of four infant bats were raised over the summer which could not be returned to their original maternal colony with their mothers, two Big Brown Bats, one Silver-Haired Bat, and one Red Bat. For the purposes of this study, a flight cage measuring 8'(h) X 8'(w) X 16'(l) was constructed. When the young, motherless bats reached approximately 5 weeks of age, they were moved into the flight cage. Their Styrofoam coolers were mounted to the walls in an area of the enclosure that was protected from weather.

Once active flight patterns were observed, supplemental feeding was gradually reduced. Weights were monitored throughout the summer. Dramatic weight fluctuations were observed throughout the season. Many possible variables seemed to affect the weight of the young bats, including: temperature, bad weather, amount of supplemental feeding, the presents or absents of live insects in the enclosure, and the bats’ activity levels.

While bats were not directly observed catching live food, all of the data indicates that young bats were self-feeding on live, flying insects. Suggestive data includes:

  1. All of the bats showed weight gains, even in the absence of supplemental food.
  2. Weight gains accelerated after supplemental feeding stopped.
  3. Live insect populations inside the flight cage dropped throughout the summer.
  4. Feeding-type behavior was observed from all four bats in the flight cage.
  5. Ecolocations observed with through a bat detector indicated ‘feeding buzzes’ as bats flew in the aviary.
RELEASE CRITERIA

When measured against our release criteria, all four of the orphaned bats qualified for release to the wild. Our primary criteria were:

  1. Bats could maintain prolonged, strenuous flight.
  2. Bats were capable of self-feeding.
  3. Bats were recognizing humans as natural predators.
  4. Bats were locating and using various ‘shelters’ (other than their coolers) in the aviary.
TELEMETRY

To minimize the negative impact of a transmitter on a released bat, the recommended transmitter weight should be kept to under 5% of the animal’s body weight. Due to this weight restriction, only the two infant Big Brown Bats were large enough to carry the 0.6 gram transmitters.

Three weeks prior to their planned release, we glued dummy transmitters to the backs of the two Big Brown bats. This was to provide an opportunity for the bats to adjust to flying while carrying the additional weight. Dummy transmitters were made by filling short sections of PVC tubing with latex material to achieve the desired weight.

Commonly used glues in telemetry studies include Skin Bond (a product used for affixing colostomy bags to human patients) and eyelash glue. We used a product called ANDREA Perma-Lash Adhesive. As predicted by the literature, after a period of about 2-1/2 weeks, the dummy transmitters fell off. Unfortunately, there was considerable damage to the skin of the bats under the transmitter. Injuries resulting from the eyelash glue prevented the timely release of these two bats.

CONCLUSIONS

Based on the lack of scientific data concerning the release of infant insectivorous bats, wildlife rehabilitators have to make decisions about these animals with incomplete information. There is great opportunity for future study in this area, not only in regard to the releasability of infant bats, but also on the potential effects of glues used in telemetry studies.

While we were unsuccessful in proving or disproving the releasability of captive raised bats, we found no evidence to support speculation that proven rehabilitation techniques that work with other species of animals will not work with bats as well.

In our literature searches, we found no rehabilitation studies performed where infant bats were released and known to have died. If such a study existed, it would not prove that captive raised bats are not releasable, only that the rehabilitation techniques used were not effective with the bats in that study.

We found infant insectivorous bat rehabilitation to be challenging, time consuming, and labor intensive, but results of this study suggest that successful rehabilitation of insectivorous bats is possible.