Bats Provide Pest Control Services to Deer

Camera traps catch bats feeding on the biting flies attracted to deer.

Posted Feb 21, 2019

Meredith Palmer, used with permission.
A bat forages on the flies attracted to a white-tailed deer.
Source: Meredith Palmer, used with permission.

There are a dozen or so species of blood-sucking flies that plague white-tailed deer. There is little a deer can do to defend themselves from these biting fly attacks.

But what is a painful nuisance to a deer is also a dinner buffet for bats. In the journal Ethology, the University of Minnesota’s Meredith Palmer and colleagues report numerous instances of insect-eating bats feeding on the swarms of flies attracted to white-tailed deer.

Palmer didn’t set out to study bats. As part of a project looking at wolves' effects on ecosystems in North America, she set out camera traps to capture images of wolves and their prey. Since the motion-activated cameras are operating 24/7, there is also a substantial amount of “bycatch”—images of all kinds of other animals caught on the camera traps.

A camera trap survey deployed over eight weeks at the Cedar Creek Ecosystem Science Reserve in Minnesota revealed repeated incidences of interactions between bats and deer. While bats are large enough to trigger the cameras by themselves, about 94% of bats captured during the camera trap survey were photographed in association with deer. In these photos, bats can be seen foraging on the large clouds of flies attracted to the deer.

“This demonstrates the power of camera trapping to reveal the hidden lives of animals,” says Palmer. “These cameras are such a great way of obtaining a novel, interesting view into animal lives that you might not necessarily get otherwise.”

This bat-deer association is a positive interaction that appears to benefit at least one, perhaps even both, partners. Bats may save foraging time and energy by feeding on the flies attracted to deer, and this feeding could reduce a deer’s pest load and supply some relief.

The relationship is reminiscent of a specific type of positive inter-species interaction called a cleaning mutualism, in which one species removes and feeds on parasites infesting the other. Think of cleaner fishes servicing parasite-ridden “clients” in tropical coral reefs or oxpecker birds gleaning ticks off hoofed mammals in the African savannas.

“In your traditional cleaning mutualism, you have one animal that’s infested with parasites and you have another animal that removes those parasites and eats them,” says Palmer. “This is slightly different in that the flies are not parasites but pests attracted to the animal.”

Meredith Palmer, used with permission.
Source: Meredith Palmer, used with permission.

There are things we cannot know from just camera trap data and a few observations, like whether the bats are eating enough flies nightly to make it beneficial to the deer or if either the bats or deer seek each other out for this purpose.

That’s one reason why Palmer and her colleagues call for more research into this and other inter-species interactions. Describing new associations helps shed light on the often surprising diversity of interconnections within ecological communities and underscores the myriad ways species within these communities are connected.

For instance, another recent study showed that white-tailed deer browsing habits shape the acoustic properties of their forest habitat, potentially affecting the vocal communication of songbirds and other species. Heavy browsing by deer can reduce the abundance and diversity of understory shrubs and trees. Since heavily forested areas tend to reverberate and scatter sound more easily than open space, birdsong might be more effectively transmitted in deer-browsed environments.

“Communities are tied together through predation and competition, which are heavily studied, but also through positive inter-species interactions such as mutualisms,” says Palmer. “I think about how we as humans are changing these ecosystems and the cascading effects of that. The more we know about the ways animals are interacting, the better we can predict how communities will change.”


Palmer, M. S., Krueger, J., & Isbell, F. (2019). Bats join the ranks of oxpeckers and cleaner fish as partners in a pest-reducing mutualism. Ethology. DOI: 10.1111/eth.12840.

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