Doctoral Dissertations

Ya-Fu Lee

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Gery F. McCracken, Thomas G. Hallam

Committee Members

Arthur C. Echternacht, Charles D. Pless, Susan E. Riechert

Abstract

Foraging patterns and the food habits of insectivorous bats may evolve in response to a variety of intrinsic (e.g., energetic demands, nutrient requirements, and morphological or physiological constraints in acquiring and consuming food) and extrinsic factors (e.g., the distribution and abundance of insect prey, and interactions with other organisms). This study investigates the foraging behavior and ecology of Mexican free-tailed bats, Tadarida brasiliensis mexicana (Saussure, 1860) (Molossidae) residing in large colonies, with emphases on their dietary breadth and variation, their foraging activity, and their resource use patterns.

Food habits and dietary variation of insectivorous Mexican free-tailed bats were investigated at three large maternity colonies located in south central Texas. Food habits of bats were determined by analyzing fecal samples collected from individuals. Diets of bats from the two nightly feeding bouts, across seasonal phases, and from bats of different sex, age, and reproductive status were compared. Mexican free-tailed bats fed on a variety of insects, including at least 12 orders and 38 families. The size of coleopterans eaten were mostly from ca. 4 to 9 mm in length. Overall, lepidopterans, coleopterans, and hemipterans were the three most abundant prey in the diet. Homopterans, hymenopterans, and dipterans were less abundant, and other insects, including ephemeropterans, neuropterans, odonatans, orthopterans, plecopterans, and trichopterans were present in small amounts.

The diet of bats varied in the relative importance of insect orders in samples collected at midnight versus at dawn, and along seasonal phases. In late May-mid July, coleopterans and hemipterans were in higher proportions in the midnight, but lepidopterans were more abundant at dawn. In mid June-mid July, lepidopterans increased and hemipterans decreased in samples collected at midnight. In late July-August, the proportions of lepidopterans in diets decreased both at midnight and at dawn. The diet also varied among bats of different age, sex, and reproductive status. Pregnant females ate less lepidopterans, but more coleopterans and hemipterans than other adults. Juveniles ate less lepidopterans and neuropterans but higher proportions of coleopterans and homopterans than adults. In addition, reproductive females showed a more diverse diet than males, and juveniles had a more diverse diet than adults.

The foraging activity and resource use of Mexican free-tailed bats was investigated in a large maternity colony in Uvalde County, Texas. Foraging activity of bats was studied by recording their echolocation calls at sites along a sampling transect, which represented three different habitats; towns, cropland, and ranches. The proportion of time in which signals were received and the number of feeding buzzes per unit time were quantified. Insect availability and the relative abundance of different insect orders in the three habitats were assessed using light traps. The food habits of the bats were determined from fecal samples, and were compared to the relative abundance in mass of insect orders.

Mexican free-tailed bats had a higher foraging activity in town habitats than in cropland and ranches. In cropland and ranch habitats, feeding buzzes per unit time were less frequent in the pre-dawn collecting session than in the evening session, corresponding to the pattern of insect abundance in these habitats. In town habitats, however, insect abundance was similar in morning and evening sessions, and the foraging activity of bats did not differ between these two sessions. The relative importance of the common prey orders both in relative frequency of occurrence and relative volume in the diet of the bats were compatible with the relative importance of these respective insect orders in relative frequency of occurrence and in relative mass in trap collections. Graphic comparisons showed a shift of proportions of different prey in the diet between midnight and dawn, in response to the change of abundance of these insects in light traps. In addition, there was a positive correlation between the diet of bats and the light trap collections at the ordinal level, in 9 of 12 comparisons when Orthoptera was included, and in all 12 comparisons if Orthoptera were removed.

The diverse diet and patterns of foraging activity and resource use of Mexican free-tailed bats have implications for insect pest management and conservation. The temporal and seasonal variation of moth consumption in the diet of the bats showed a clear correlation with the availability of large migratory populations of agriculturally injurious moths. The evaluation at the ordinal and family levels, considering the diversity of insect prey of the bats, also suggests that the insect-eating by the bats might be agriculturally beneficial. Together with other lines of evidence, the data suggest that Mexican free-tailed bats have a potentially significant contribution in regulating agricultural insect pests. The decline of Mexican free-tailed bat populations, and their seasonal and patchy distribution, warrant conservation concerns and efforts.

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