Night of the Bargain Hunter: some bats pick prey based on the cost of the hunt

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While they’re less likely to occupy Wall Street than a barn upstate, bats are as concerned as we are about the economy. Their economy revolves around energy instead of money, though, and a problem on the balance sheet can be a matter of life and death. If they spend more energy catching a meal than that meal provides, they’ve created an energy deficit that impacts their health, growth and survival.

There’s no need to worry about bats during a recession, though. New research from biologists in Germany suggests that bats are actually pretty savvy shoppers and are able to use echolocation to make economic decisions while hunting and stay in the black, calorically speaking.

Bats gather information about their environment and prey by making clicking calls and then analyzing the echo patterns that bounce back at them. The conventional wisdom, backed by research, was that bats selected their prey based on biased information. That is, some prey had more conspicuous echo patterns because they were bigger or moved more and were over-represented in a bat’s perception. In the last few decades, though, that way of thinking has started to change, thanks to guano. Biologists began noting here and there that, in some colonies’ guano, there were more large insects than could be expected by chance and biased selection. In 2006, biologists from Frostburg State University in Maryland found that the proportion of beetles in the guano of one colony they were studying was greater than it should have been relative to the beetles’ abundance in the area. This all suggested to biologists that bats might be able to actively select their prey instead of just grabbing the most obvious things flitting about.

Since then, biologists have learned that some bats are indeed very sophisticated hunters. Greater horseshoe bats (Rhinolophus ferrumequinum) are known to be able to accurately discriminate between different types of insects based on information in the echoes of their fluttering wings (what’s more, they can change the frequencies of their echo calls to compensate for the of their own flight speed). More recently, Klemen KoseljHans-Ulrich Schnitzler and Björn M. Siemers from the University of Tubingen and the Max Planck Institute for Ornithology in Germany suggested that horseshoe bats are not only picking their prey selectively, but making those decisions based on energy economics.

Thinking about those piles of guano, Koselj wondered if the prey discrepancies other researchers had found could be explained by the bats hunting prey that optimize their energy profitability: They picked the certain insects not because they were obvious or especially tasty, but because they knew that some bugs gave them a better meal for their effort.

To find out, he’d need to watch them while they hunted, instead of just going through their waste. Koselj and his team captured half a dozen greater horseshoe bats and let them loose in a lab to hunt. To fully control the echo information the bats got, Koselj had them prey not on bugs, but computer-controlled propellers. The faux bugs came in different sizes and rotated at different speeds to produce echoes that resembled natural insects, some bigger and more energetically profitable than others. For a successful attack on a small propeller, the bats were rewarded with the wing of a mealworm and attacks on big ones netted them large, whole mealworms.

As the bats hunted, Koselj varied the conditions of the bug market, changing the frequency of small and big bugs and the amount of time between bug appearances to simulate different conditions of bug density and abundance. Throughout the hunts, Koselj paid close attention to the bat’s prey-selection decisions – catch or skip – in the context of abundance. Then, he then compared the results with standard models of prey choice to figure out if the bats were choosing prey based on energy costs and benefits. If they were, Koselj figured, they’d take the bigger, more profitable prey when they could get it and ignore the small ones, and be less picky when big bugs were scarce.

All six bats adjusted their prey selection to the different wait times between appearances of the large bugs, attacking both large and small bugs when the big ones didn’t show up as often and rejecting small ones and taking more of the big ones when they were more frequently available. One even rejected the small prey outright and took only the large prey when it showed up frequently enough. This selectivity didn’t appear to be just a matter of sensory bias, either. Often, when the small propeller started rotating, the bats would turn toward it, jerk their wings and get ready to take off, but then stand their ground, suggesting that they noticed the small prey and decided to skip it and wait for something worth the flight.

The bats, it seems, are savvy consumers, choosing their prey based on specific echo information associated with energy profitability and on that prey’s availability, estimated from how frequently they encounter it. This gives them a leg up on many other animals that also eat insects. Experiments with fish, amphibians, and birds have shown that these animals often reject the most profitable prey, an uneconomic decision that suggest they’re picking meals based on happenstance. As for other bats, which represent about a quarter of all mammals on earth, Koselj thinks that more species might be able to match the horseshoe’s decision-making, but that there might be a performance gap between bats that use short frequency-modulated (FM) calls and those, like the horseshoe bat, that use long continuous frequency ones.

Meanwhile, another German team has discovered that tropical bats balance their energy costs and gains with slower metabolisms.

Image: “Greater horseshoe bat, Rhinolophus ferrumequinum Schreb. 4/5 natural size,” by Friedrich Specht, via Wikimedia Commons.

References: Koselj K, Schnitzler HU, & Siemers BM (2011). Horseshoe bats make adaptive prey-selection decisions, informed by echo cues. Proceedings. Biological sciences / The Royal Society, 278 (1721), 3034-41 PMID: 21367788

Jones, G. (1990). Prey Selection by the Greater Horseshoe Bat (Rhinolophus ferrumequinum): Optimal Foraging by Echolocation? The Journal of Animal Ecology, 59 (2) DOI: 10.2307/4882

Salvatore J. Agosta, David Morton, & Kellie M. Kuhn (2003). Feeding ecology of the bat Eptesicus fuscus: ‘preferred’ prey abundance as one factor influencing prey selection and diet breadth
Journal of Zoology , 260 (2), 169-177 : 10.1017/S0952836903003601

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