Tierra de Oro Laboratory

Physiology and Ecology in the Pinyon-Juniper Savanna

Tag: predation

  • Spiders, Bats, and Mystery Poop

    Who has been eating our beetles?

    Mimicry is in the eyes of the predator

    Some beetles that do not produce defensive stink are thought to be mimics of those that do. For example, Eleodes species can shoot streams of noxious chemicals when threatened, whereas Philolithus and Stenomorpha, which have very similar size and appearance, do not. In this way, the non-stinky beetles may be protected from predators who have learned their lessons during previous meetings with Eleodes. This form of mimicry was first proposed about 150 years ago by Henry Bates, and is referred to as Batesian mimicry.

    Eleodes obscura, showing smooth pronotum (thorax) and moderately ridged elytra. This species, which defends itself using noxious chemicals, is considered to be the model for the two undefended species to the right. The red spot on the left elytron is an identifying paint mark.
    Philolithus elatus, at same scale as E. obscura. Patterning on elytra is very similar to E. obscura in this region, but raised edges of pronotum are distinctive.
    Stenomorpha rimata. Overall shape and size resemble E. obscura, but pronotum and patterning of elytra are distinguishable after a close look.

    Because we humans perceive the world largely through our eyes, we tend to think of mimicry in terms of visual appearance. The examples shown in textbooks (see below) tend to be butterflies or snakes with matching colorful patterns. These are expected to be especially effective against birds or monkeys that have excellent color vision.

    The coral snake posseses a potent venom and is dangerous to humans. Note that red and yellow sections are in contact (“red touch yellow, kill a fellow.”) Credit: Todd Pierson. CC BY-NC 3.0
    Harmless scarlet kingsnake, a Batesian mimic of the coral snake. Credit: Glenn Bartolotti , CC BY-SA 3.0,

    A close look at the snakes above will show that they are relatively easy to distinguish in photographs. Mimicry does not need to be perfect, because a mistake in the field could cost an unwary predator or hiker their life. As a human, you could memorize a rhyme that allows you to distinguish them, or you could simply avoid bright red, black, and yellow snakes.

    What about the beetle mimics? Should we think about vision, or other senses? If a predator has poor vision or hunts in the dark, then what is the purpose of visual similarity?

    Who Eats Local Beetles?

    One of my goals for the 2026 field season was to find out who actually eats the beetles. Each of the beetles shown at the top of the page would yield 1/2 to 1 gram of water, protein, and fat, so it is worth some effort to subdue the defended ones, figure out who is undefended, or both.

    Previous work by a number of authors has shown that large darkling beetles are consumed by a wide variety of predators. Depending on the location and beetle species, predators include mice, skunks, coyotes, bats, ringtails, jays, scorpions, and there are likely to be more.

    Below is a summary of what I have figured out so far. As always, there is more to come, but the evidence for the ones shown below is quite solid.

    Waiting in Ambush: Black Widow

    Prey caught in web. Bottom right is Eleodes obscura, upper left and out of the focal plane, is Embaphion contusum, 9/3/26.
    The culprit, a western black widow, repairing and extending her web at night. 9/9/26

    When I came across some beetles wrapped up in sloppy webbing, I was not sure who had killed them. After collecting the beetles and moving the prickly pear pad below them, I saw a large spider with a red spot on its abdomen in a hole below. Then it all made sense. Black widows spin notoriously messy webs, and are perfect predators of any large beetle that comes by. When caught in the web, Eleodes‘ defensive chemicals will be sprayed ineffectually until they are used up, and the spider can then immobilize and consume the beetle at her leisure.

    This spider also trapped an Embaphion contusum (also chemically defended) and a large scarab beetle. One would expect that undefended Tenebrionid beetles, such as Philolithus and Stenomorpha, would easily fall prey to such a spider. Her web is not selective, and she is able to subdue E. obscura, one of the largest and best-defended beetles.

    Chomped by Bats

    Pallid bats are known to eat Stenomorpha, and the resident group has dropped bits and pieces of large insects such as Jerusalem crickets and beetles. For more about the bats themselves, have a look at this post.

    In August, one of the bats was particularly messy, leaving most of the front half of a small Stenomorpha, possibly S. obovata.

    Head and thorax of a half-eaten small Stenomorpha species. Collected 8/14/26.
    Partial elytra and middle legs of S. rimata. Collected 9/17/26

    Now that fall is upon us, Stenomorpha rimata has become the most common Tenebrionid in the neighborhood. I was lucky enough to find some half-eaten, but readily identifiable bits of S. rimata under the roost last week.

    At this point, we don’t know if the bats are focusing on undefended beetles, or whether they would eat Eleodes if given the chance. It is clear, however, that pallid bats are willing and able to eat large Tenebrionid beetles such as Stenomorpha.

    Work in Progress: Beetles in Scat

    During the summer and fall, one can readily find scat containing beetle remains. Predators often swallow them whole, making identification much easier.

    A sampling of head capsules recovered from scat collected on 9/4/26. Three Tenebrionid heads are on the left, some of the Carabid (ground beetles) in the middle, and some very large ant heads (and a small one) on the right. The scale is in millimeters. 9/26/26

    Analysis is in the early stages, but there is lots to find. The photo above shows some of the head capsules that were in one scat. There were many Carabid beetles of a range of sizes, plenty of big and small ants, and a few good-sized Tenebrionids. Based on an abdomen that was also in the collection, at least one of the Tenebrionids was Eleodes obscura.

    There is much more to find, but we can say for sure that somebody, probably a mammal, is eating a lot of beetles. Importantly, they are including Eleodes in their diets. With persistence and luck, we should be able to figure out who it is as we dig deeper into the samples.

    The predators we have identified so far, spiders and bats, are not visual predators, so the outward resemblance of the mimics to Eleodes is probably not that useful. Moreover, it looks like at least some predators are happy to eat Eleodes, further reducing the possible benefit of mimicry. Nonetheless, we may find that birds such as jays and ravens are hunting the beetles, and we may yet identify some selective advantage to the visual resemblance shown by the mimics.

  • Predators: Tiny and Large

    We are hardly in the wilderness, but there is plenty of danger if you are small enough.

    Fly on daisy. The flower is about 2 cm across, and the fly seems to be upside-down. 8/25/26

    Wandering around the garden this morning, I saw a tiny fly sitting on a blackfoot daisy flower. It stayed put, so I took some photos for identification.

    Looking more closely, it became clear that the fly was remaining on the flower because a crab spider had grabbed it.

    Closer view, showing spider holding fly more clearly.

    As is often the case, the spider matched the color of the flower closely, which both allowed it to capture the fly and to avoid detection by the many potential predators that would like to eat it.

    Another view of the spider holding the fly.

    A few more photos showing the drama happening at the millimeter to centimeter scale.

    Side view of spider holding fly.

    On a larger scale, I have been trying to figure out who is eating the local beetles, by tethering a few and setting up a trail camera. So far, nobody has tried to eat the beetles, but I did get a nice view of another resident walking through.

    It went through the meadow at about 7:20, which is when we often see bunnies back there. Interesting to think that the cats are probably watching us many of the nights we are walking around behind the yard.

  • Bats Snacking On The Porch

    Pallid Bats Roosting Under Over Our Noses

    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).

    Uric acid stain on wall. 7/31/26

    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.

    Bat scat below a roost. 7/31/26.

    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.

    Jerusalem cricket jaws. 7/31/26

    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.

    Pallid bat trying to ignore the photographer . 7/27/26

    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.

    Coming in for a landing. 7/30/26
    Bat group greeting next arrival. 8/1/26

    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.

    Leg of a Tenebrionid beetle under bat roost. 7/31/26

    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.

    References

    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.

  • The Natural World: Winners and Losers

    Here at the lab, we are are lucky to have a close up view of nature. It’s a constant parade, with beetles emerging from their winter homes, ants swarming to start new colonies, winter birds leaving, summer birds arriving, and much more.

    Carpenter ants swarming. A major worker (front left) guards the nest entrance while a winged male gets ready for his brief duty of finding a mate. 5/9/26

    Four of five of the eggs in Terry’s nest box have hatched, and the owlets are growing rapidly. Of course, for each meal the owlets enjoy, a mouse, bird, or insect has a very bad day.

    Dead mouse being delivered to growing owlets. 5/726

    Snake in a Box

    We have been tempted to add another bluebird box that has a camera so that we can watch the process of growth and fledging. I have been hesitating, largely because some broods do not survive and I am not sure that I want to watch that happen. My attitude was reinforced this week, when the female bluebird did not go into the nest box, and there was a something strange and pink sticking out of the hole.

    When I opened the box, there was a large pink coachwhip snake inside. It had made a meal of the brood, even though the nest is on a pole about four feet off the ground.

    Large coachwhip occupying bluebird box, having consumed most recent brood. 5/10/26

    Looks like it will be necessary to add some flashing around the pole to make it harder for snakes to reach the box.

    Lame Coyote

    There seems to be a large crop of rabbits this year, which has led to plenty of coyote sightings. I see them at least a few times a week on morning surveys, and they show up regularly on the wildlife cams.

    Terry was lucky enough to spot one finishing its rabbit meal, and fast enough to get a video. Another coyote, with an injured front leg, tries to share, but the first coyote is having none of it.

    Coyote having breakfast, courtesy of Terry Morgan. 5/9/26

    It is a harsh life out there, and a coyote that can’t hunt will not be able to eat. Animals can heal, so it may be back on all four feet and able to hunt again.