We still have a lot to learn about the local wildlife, even the species that live near the lab. For example, bats have been partying on the property for years without us knowing.

Let’s start with a few observations.

First, we have seen streaks of uric acid on the walls of some of the portals (the local name for covered patios).

At first we thought these were from some sort of bird, maybe swallows. The marks made more sense when we learned they were from bats.

We realized they were still active roosts when we had another epiphany: the poop under the roosts was from bats rather than rodents. It seems obvious in hindsight, but there are so many deermice, woodrats, etc., that it took a while to make the bat connection.

The bats’ diet provides another piece in a beetle ecology puzzle. Local Tenebrionid beetles seem to respond differently to predators compared to the same species found elsewhere. For example, undefended beetles do not seem to closely mimic chemically defended beetles. This might make more sense if we knew who was eating them. We have found plenty of carnivore scat, presumably from skunks and/or raccoons, that contains beetle parts and shows that nocturnal omnivores are eating Tenebrionids.

Enter the bats. Unlike other bat species, pallid bats (Antrozous pallidus) feed on ground-dwelling insects, and others have shown that pallid bats readily eat Tenebrionid beetles. Rather than using echolocation to find the insects, they hunt using noises generated by the insects’ movements, and reserve echolocation to avoid obstacles (Bell, 1982). At the portal, discarded jaws and legs of Jerusalem crickets indicate that the insects (about the same size as large Tenebrionids) are popular menu items.

Among Tenebrionid prey, they have been identified feeding on Stenomorpha based on bits of exoskeleton found near roosts (O’Shea and Vaughan, 1977) and tracks in the soil next to bowls in which beetles had been left as bait (Slobodchikoff et al., 1987).

As it turned out, the roosts in our portal were sites where pallid bats eat and socialize at night. We were lucky that one decided to spend a day in the portal, so we could make a positive identification.

To get a better idea of their activity patterns, I set out a couple of trail cameras to catch them at night. After a few nights working on the sensitivity and finding the right locations, we were surprised to discover that it was quite the little group.

The size of the group was surprising. In the photo above, there are at least five bats on the walls and one more arriving. That explains the relatively large nightly deposits of poop and insect parts.

The video below gives a better idea of how much activity there is on a given night. The file is a bit large and may load slowly.

The biology of pallid bats is quite interesting. Your classic insectivorous bat uses echolocation to hunt flying insects and dodge obstacles. In practice, they basically scream and then wait for echoes to return from potential prey.

Pallid bats are considered “gleaners” or “whispering bats.” They listen for rustling to find prey on the ground. They still use echolocation to avoid running into things, but can be much quieter about it. Because some insects have evolved to hear and avoid bat calls, being quiet may improve the hunting success of the gleaning bats.

Pallid bats make use of two roosts at different times of day. During daylight hours, they roost in out of the way locations to stay cool and avoid disturbance while they sleep. At night, they make use of roosts to rest and to eat larger insects that they have caught. Our bats are using the portal as a night roost, and their daytime roost remains a mystery.

One of them left a hindleg of a Tenebrionid, possibly Philolithus, after a meal, so we can say with some confidence that pallid bats are confirmed beetle predators.

Bell, G. P. “Behavioral and Ecological Aspects of Gleaning by a Desert Insectivorous Bat Antrozous Pallidus (Chiroptera: Vespertilionidae).” Behavioral Ecology and Sociobiology 10, no. 3 (1982): 217–23. https://doi.org/10.1007/BF00299688.

Harris, J. “Pallid Bat.” In California’s Wildlife., 3 vols. California Department of Fish and Game, 1990.

O’Shea, T. J., and T. A. Vaughan. “Nocturnal and Seasonal Activities of the Pallid Bat, Antrozous Pallidus.” Journal of Mammalogy 58, no. 3 (1977): 269–84. https://doi.org/10.2307/1379326.

Slobodchikoff, C. N., T. A. Vaughan, and R. M. Warner. “How Prey Defenses Affect a Predator’s Net Energetic Profit.” Journal of Mammalogy 68, no. 3 (1987): 668–71. https://doi.org/10.2307/1381600.