A story about the eavesdropping bats and the risks associated with finding mates.
The animal kingdom, if observed, shows you numerous versions of mating signals - visual, auditory or chemical (or even a subtle mix of any and either). These signals indicate strong readiness for a long-awaited courtship.
Most often, it is the male that initiates these obvious and grandiose displays of colour, contrast, hypnotizing movement, mesmerizing dances or harmonious acoustics. The female then picks her favourite and moves towards them to mate.
Courtship signals, though highly beneficial in terms of roping in a mate, also make one vulnerable to predation, especially when this signalling occurs over a long distance and for a long time. This cost of predation has been seen to be carried on the shoulders of the gender that displays the signals – the males. And obviously so - which predator in their right mind would not attack a dancing, prancing, squawking meaty rainbow!
The question thus arises, is this always the case? Do signalling genders always carry a higher risk of predation? What about the other gender – the females that carry off in search of their mate? Do they get preyed upon equally or not?
So, in the hopes of determining whether signalling males are really more prone to be eaten by predators than non-signalling females, Dr Hanumanthan Raghuram of The American College, Madurai, Dr Rohini Balakrishnan of the Indian Institute of Science, Bangalore, and their associates, collaborated to study bat predation of katydids (also called bush crickets).
The first incriminating fragment of evidence
As it linguistically seems, Katydids are not a product of what “Katy did” but are rather part of a family, including crickets, locusts, and grasshoppers. This family has narrow feathery front wings for flight and large hind legs for jumping. They also produce various trilling, chirping or buzzing noises as courtship signals
Male katydids specifically, produce trilling noises and the females, producing no sounds of their own, use these signals to seek out the males and fly towards them.
Katydids are often preyed upon by birds, frogs and bats among other animals. Insectivorous bats like the Megaderma spasma, also called the Lesser False Vampire Bat, is one such significant predator. It lives in groups/roosts of 4-5 individuals, across various countries of South-East Asia like India, Sri Lanka and Indonesia. Characteristic large round ears and leaf-shaped nose help it pick up on the sounds of insects that make up a large part of its diet. Once caught, the bats bring the prey back to the roosting sites where they eat them at leisure, littering the ground with the prey’s uneaten fragments.
Using the knowledge that the M. spasma bats eavesdrop on the courtship signals between male and female katydids, the team of researchers set out to find out if it was true that the bats picked noisy males over silent females as their meal of the day.
The Lesser False Vampire Bat feasting on a katydid; The 5mx8m polythene sheet set up placed under bat roosts for the collection of falling prey fragments
(Photos by Dr Hanumanthan Raghuram)
Visiting a village Kadari, in Karnataka, they located five bat roosts. Underneath each roost, they placed polythene sheets to collect fallen fragments of the prey like wings, antennae, legs, and other relevant organs. These fragments were collected every month for an entire year to analyse bat diet, especially the species and gender of the insects eaten. The fragments were then identified, classified and categorized according to the characteristic features displayed –
All katydid forewings that had intact stridulatory structures (organs that help produce the katydid’s signals or trilling noises) were considered to be from male katydids and all katydid forewings that lacked the stridulatory structures were considered as female.
Female katydid with a pair of ovipositors (only one is visible); Male katydid with the stridulatory structure (Photos by Dr Rohini Balakrishnan)
Careful examination of all the 3714 fragments collected, confirmed that the bats ate a lot of insects – almost 95%! Out of these remnants, 453 were katydid fragments. And a total of 159 bore stridulatory structures whereas 294 did not carry any stridulatory structures.
This was a clear indication! The bats truly did consume more silent female katydids than their noisy male counterparts. Almost 1.85 times more!
The evidence piles on
Now that the researchers had something to go on – the understanding that silent females actually do make easier prey for bats than noisy male katydids – they attempted to observe this feeding behaviour in real-time.
With the help of 18 bats that were caught and released safely into a large flight tent, the researchers set up three behavioural experiments.
Left: Tent set up for the playback and flight trials; Right: M. spasma bat spotted napping inside the tent. (Photos by Dr Hanumanthan Raghuram)
In the first experiment, previously-recorded calls of three different species of male katydids were played on randomly placed speakers. These calls were loud and conspicuous courtship calls – exactly the kind of sounds the bats would eavesdrop on. There was also an inactive speaker set up in the tent to help the researchers ascertain that the bats were drawn to the sound of the speakers and not to the speakers themself. Both speakers had a katydid placed on top, serving as a reward. Thus, the active speaker represented the male katydid – producing sounds but not moving.
The second playback experiment played previously-recorded flight sounds of female katydids. The set-up remained the same – one active speaker and one inactive speaker, both with immobilized katydids on top as rewards. The idea was to understand if the conspicuous sounds of the female in flight attract the predators (instead of the sight of them moving).
The third experiment had a different setup. Here, live female katydids were tied up and dangled from the top of the tent and induced to fly (by blowing air or prodding them). In the same tent, freshly killed katydids were dangled at random distances, in the same manner, to serve as a control – ensuring that the bats were attracted to the movement of katydids and not an arbitrary immobile katydid. Thus, this trial represented the female katydids – showing movement but not producing sound – and also tested whether bats are attracted to them by sight.
The bat's responses were monitored and recorded using infrared-sensitive video cameras. In each of these experiments, every time the bat approached the speaker/insect at least once, the response was recorded as positive.
Once the data was collected and analysed, the researchers came to the following conclusions: Unsurprisingly, in the male playback trials, it did not matter which male species called, the active speakers were attacked by the bats only almost 1/3 of the time. In the trials using female flight sounds, the active speakers were attacked only two times. However, interestingly, tethered females that were induced to fly were attacked in every trial – that is, a 100% response! These results strongly suggested that although signalling male katydids attract the bats, it is actually the movement of the females that captures their attention without fail.
Tent experiment: With two bush-crickets dangled side-by-side, only the one that was induced to move was attacked by the M. spasma bat. (Video credits: Dr. H. Raghuram)
The final verdict
It is natural to assume that noisy males that call out and thus bring attention to themselves, are easier prey than silent females. It is also natural to assume that night predators like bats target auditory cues because of their extra-sensitive hearing. The case of the female katydid goes to show that nature has a multitude of ways of dealing with our “natural assumptions”.
The combination of the diet analysis and the behavioural experiments cleverly demonstrated that contrary to our expectations, the mobile female katydids face a higher risk of predation than the calling male katydids.
This surprising discovery, as determined by the authors of this paper, could probably be explained by the location of insects:
The male katydids, when calling out, utilize a camouflage strategy that involves hiding in highly cluttered and hard-to-access sites like dense brambles or leaf clutters. The bats overhear these calls from a long distance and close in on the location of the prey, waiting for it to emerge. The female katydids, having overheard the male’s calls too, fly to the male in response – a behaviour that, under the nearby bat’s watchful gaze, makes them a lot more obvious than the hidden males. And so, the bats in trying to make use of the strategy that utilizes the least amount of energy, zoom in on the movement of the females and fly down to capture them. Easy peasy!
An ingenious research experiment, and an even more ingenious bat!
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Source: Raghuram Hanumanthan, Deb Rittik, Nandi Diptarup and Balakrishnan Rohini 2015 Silent katydid females are at higher risk of bat predation than acoustically signalling katydid males proc. R. Soc. B.2822014231920142319
Link to the article: http://doi.org/10.1098/rspb.2014.2319
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Article written by: Nikita Gupta, edited by Devica Ranade Huge vote of thanks to Dr. Hanumanthan Raghuram for fact checking the article and providing us with all the required media.