Philosopher Thomas Nagel wrote in a paper he published in 1974, ‘What is it like to be a bat?, that it is impossible to fully know a creature like a bat. But there are ways in which we can gain insights into them. Take the example of the insectivorous bat, Megaderma spasma, which roosts in old, abandoned, and relatively undisturbed buildings during the day in Udupi District, Karnataka. Come twilight, the bats leave their roost to find food, and only return late at night or early in the morning, occasionally with a bush cricket in their mouth.

What is it like to be a bat in an agricultural area?

Where does the bat spend its night searching for food in an area in which plantations and remnant forests are interspersed with each other? How does the bat find its prey, the bush cricket? These are some of the exciting research questions we asked in our recent research publications.

Lesser False Vampire Bat (Megaderma spasma) in its roost.

A typical bat roost in Udupi District.

To observe where the bats go, we glued radio transmitters on their backs and tracked them across nights. Three human observers with receivers and antennas would simultaneously record the direction of the transmitter’s signal and pinpoint the location of the bat. We then used satellite images in which you can visually tell apart a forest patch from a plantation to prepare a habitat map of the area. With these two pieces of information, we were able to say where on the habitat map the bats’ locations were and concluded that the bats were more likely to be found in forests than plantations. But why do these bats prefer forests over plantations?

A satellite image of Kadari Village and its habitats. Courtesy: Harish Prakash.

The agricultural landscape, with a forest patch (left) and a rubber plantation (right).

There could be two reasons for the bats’ preference for forest patches. One reason could be evolutionary adaptation – the broad and short wings of the Lesser False Vampire Bat are ideally suited for manoeuvring in areas with dense vegetation; true to these claims, we found that forest patches have denser vegetation than plantations.

An areca plantation, which is less dense than a forest patch.

Another reason for the preference could be ecology – the differences in the numbers of insect prey in forests and plantations. To test this, we deployed insect traps in the area and collected insects. Contrary to what we expected, we found no difference between the types of insect orders or the number of insects caught, in forest and plantations. i.e. the number of lepidopterans (insect order of moths and butterflies) caught in forests and plantations were similar. But a bush cricket prey species of the bat, Mecopoda, is found in greater numbers in forests than plantations, suggesting that forests have more of the bats’ favourite prey than plantations and hence responsible for the bats’ preference for forest patches.

The bush cricket numbers in forests raise new research questions: why does the bush cricket (Mecopoda) prefer forest patches? One could speculate that food resources could drive the bush cricket’s preference just like it did for its bat predator. Or it could be an entirely different reason. For example, in most plantations, there is a constant clearing of understorey vegetation; bush crickets might not tolerate such disturbance and hence take to the relatively undisturbed forest patches. But these different speculations need to be tested in the future to get answers. For now, all we can conclude is that the bat and its prey, the bush cricket, overlap spatially in the forest.

Bush cricket, Mecopoda sp.

So how do bats hunt crickets hidden inside these dense forests? In most cricket species, the male crickets call, and the females respond by flying towards the males to mate. But unintentionally, these calls attract bats as well. Besides the well-known phenomenon of echolocation that bats use to catch prey, some bats are known to eavesdrop on prey calls to locate them. If that is the case, then the bats should be eating more male crickets that call, right? But strangely, when researchers examined the wings of crickets discarded by bats in their roosts, they found more female wing-remains than male. To solve this mystery, researchers carried out experiments and found out that bats respond to cricket calls one in three times, but approached flying crickets on all occasions. The bats seemed more tuned to a flying cricket than a calling cricket. This may explain why female crickets flying to mate with males might be more at risk.

A male cricket calling is at risk from bats; a flying female, more so. But what happens when you have multiple male crickets calling in a group? Does the risk of attracting eavesdropping bats increase? For example, if you are one among ten, the chance of being eaten is one-tenth; being part of a group dilutes your chance of being eaten. But are there any other advantages to calling in a group?

We tested whether calling in a group benefits bush crickets against their bat predators. In a choice experiment, we found that bats are more attracted to three speakers playing cricket calls than a single speaker. Does this mean calling together in a group is more noticeable and hence riskier? Yes. But that is just one part of the story. We also tested how long it takes for bats to capture a calling cricket when it is alone compared to when it is in a group. Bats took considerably longer to capture a cricket calling in a group of three than when the cricket was calling alone. This delay in capture time allows the cricket to stop calling, hide, and escape being eaten. So, though bush crickets calling in a group attract bats, the bats’ inefficiency in capturing prey in groups can benefit crickets.

An experimental set-up to understand how bat and bush cricket interactions play out.

Why do bats take more time to capture prey calling in a group? One possible reason could be a confusion effect: it is hard for a predator to target and capture one individual prey when it is among many. This is akin to humans facing a ‘paradox of choice’ when confronted with too many options of breakfast cereals or peanut butter to choose from in a supermarket. Previous studies had looked at this only in visual predators (like monkeys, geckos, and fish). Our study provides the first evidence for an auditory confusion effect in eavesdropping predators.

These findings reveal how science can unravel some of the mysteries surrounding bats that Thomas Nagel mentioned were impossible to know. Many such unknowns still exist and remain to be uncovered through careful observations and experiments. The remnant forest patches in agricultural areas, where all this drama between the bat predator and its bush cricket prey play out, need to be preserved. Conserving this valuable resource will not only help in the persistence of bats and bush crickets in the landscape, but also in the maintenance of the overall biodiversity and the ecosystem services the forest provides. Both of which are crucial for our future survival.

 

Acknowledgment:

I thank Madhura Amdekar, Nitika Sharma, Seshadri K. S., Pooja Nayak, and Rohini Balakrishnan for their valuable suggestions to improve the text. 

References:

  1. Prakash, H., Greif, S., Yovel, Y., Balakrishnan, R., 2021. Acoustically eavesdropping bat predators take longer to capture katydid prey signalling in aggregation. J. Exp. Biol. 224. https://doi.org/10.1242/jeb.233262
  2. Prakash, H., Saha, K., Sahu, S., Balakrishnan, R., 2021. Ecological drivers of selection for remnant forest habitats by an insectivorous bat in a tropical, human-modified landscape. For. Ecol. Manage. 496, 119451. https://doi.org/10.1016/j.foreco.2021.119451
  3. Raghuram, H., Deb, R., Nandi, D., Balakrishnan, R., 2015. Silent katydid females are at higher risk of bat predation than acoustically signalling katydid males. Proc. R. Soc. B Biol. Sci. 282, 20142319. https://doi.org/10.1098/rspb.2014.2319