- A new predator-prey study reveals that flying is riskier for female katydids, making them vulnerable to predation by an insectivorous bat.
- In the first ever radio telemetry study on insects in India, a canopy-dwelling katydid, was fitted with a radio transmitter and behavioural experiments were then conducted in the natural habitat of the katydid and bat.
- It revealed that females moved more often and farther than males for the purpose of mating and egg laying. This makes female katydids more vulnerable to predation.
- The study opens avenues for future research on predator-prey interactions using radio telemetry, which could enable deeper understanding of evolutionary behaviour and therefore better conservation management.
Bush crickets, also known as katydids, form an important part of the diet of the insectivorous lesser false vampire bat Megaderma spasma. A new research study by the Centre for Ecological Sciences (CES), Indian Institute of Science (IISc), Bengaluru, published in the journal Behavioral Ecology and Sociobiology, explores the possible reasons for significantly higher number of female katydids being eaten by this bat species.
Katydids (Onomarchus uninotatus) are tiny nocturnal insects. The males are smaller, about 2 grams, while the females weigh 4-6 grams. Due to their size, camouflaging abilities, and nocturnal behaviour, katydids are difficult to observe. The habitat of the Onomarchus uninotatus katydids is wild or domesticated jackfruit trees.
Male katydids typically broadcast acoustic signals to attract female katydids during the mating season. This also attracts the attention of eavesdropping predators such as insectivorous bats.
If they are on the same branch, the females vibrate their abdomens on the branch, which can help the male katydids locate them. This behaviour, called “tremulations”, is not common across all katydid species.
“In most katydid and cricket species, the males call and the females are silent. The canopy katydids we studied were interesting because the females are capable of signalling their presence to the males through tremulations,” shared Kasturi Saha, the first author of the study. In most cases, however, when the katydids are not on the same tree, the females have to fly, in response to the male’s acoustic calls.
The study found that female katydids moved 1.6 times more often and 1.8 times farther than males. Since the canopy katydids rely on camouflage to avoid predation, flying puts them at risk.
“This study is significant because it builds our knowledge of the decision-making process for the predator (the bats) and the prey (the katydids). We tried to understand if signalling or moving is riskier for the katydids and found that the female katydids fly a lot more, making themselves available for the bats,” said Harish Prakash, one of the authors of the study.
Ecologists hypothesise that animals make optimal decisions, but it is not easy to say what that constitutes. For the male katydids, signalling can expose their location to eavesdropping predators but is necessary to attract the females. The female katydids fly in response to the male calls and are offered a “nuptial gift” in the form of protein-rich food that gives the female rough energy to lay eggs, by the males, which is a possible motivation for the females to fly in response to the calls.
“From the bat’s perspective, the optimal decision would be to attack the bigger prey, the females. However, we found that the bats decided to prey on the female katydids not based on their size but because they were easier to catch,” said Prakash.
Katydids have evolved a system where the males produce species-specific calls, with a frequency, tempo, and pattern that only females of their species understand. “Some sing nonstop, others call with frequent gaps. The female can listen to the calls of other species too but she is most sensitive to the calls of her own species. This system acts as a barrier to hybridisation,” shared Chandranshu Tiwari, a bioacoustics expert who documented a diversity of katydid species for his Ph.D. Tiwari was not associated with this study.
First radio telemetry study on insects in India
Earlier in 2015, a study by the CES analysed the diet of the Megaderma spasma bats and found that female katydid remains were 1.85 times higher in number than male. The researchers wondered why female katydids, that are usually silent, are consumed in significantly higher numbers than the males, who are capable of calling.
Hanumanthan Raghuram, the first author of the 2015 study, explained, “We conducted experiments on how often the bats responded to male katydid calls and female katydid flight. The bats showed less than 50% response to katydid male song trials, but a 100% response to the flight of female katydids. These experiments thus substantiated that flying is riskier than calling for the katydids.”
With this information, the 2023 study set out to find which sex is flying more or farther. “When both sexes fly, do the bats prefer to approach the males or the females, and is one more detectible or easier to catch?” shared Kasturi Saha.
The study was conducted in two parts, the first within an enclosure built in Mala village, Udupi district of Karnataka, where the movements of the bats and katydids were observed. The researchers found that the bats approached both male and female katydids equally, though the females displayed better escape strategies.
The second part of the study observed landscape-level movements of the katydids. The katydids were fitted with a 0.15-gram radio transmitter during the day, when they are not as active.
“We had to be careful about gluing the transmitter so as to not not hinder their movement. The male katydids are smaller and easier to handle. They do not bite, nor are they harmful to humans if you hold them properly. After some control experiments, we observed that they were able to move around and mate and call without it restricting their movement,” said Saha.
The researchers then released the katydids to the same branch or tree that they caught them from. They collected movement data every 30 minutes of the katydids from 200-300 metres away. This part of the study revealed that the females were flying farther than the males.
“The female katydids have to be very cautious while finding a site to lay eggs. They have to find trees that have some moisture but in places with few predators. They typically lay their eggs in tree crevices, and the process takes 10-15 minutes during which they cannot move, so if they get attacked during this time they cannot escape. This is why female katydids are very picky when choosing a site to lay eggs and they move around a lot before they make this decision,” shared Chandranshu Tiwari.
Most radio tags fitted on the katydids were retrieved by the researchers, but some were found lying under trees. “Bats typically discard the wings and the legs, so we were sure that they would not consume the radio tags. We retrieved a couple of radio tags with bite marks. Ants were also seen carrying some dismantled radio tags,” said Saha.
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Potential for future studies
Previous studies have shown that katydid females are eaten more by bats not only during the breeding season but also during the non-breeding season. “Interestingly, the difference between male and female katydids consumed by predators is even higher during the non-breeding season. We do not know why females are at a higher risk when they are not moving around as much for mating. There are no acoustic signals broadcast by the males during this time, but it is possible that the females are moving around to lay eggs,” said Saha.
One area that requires further investigation is ascertaining the sex ratio of katydids in the study area, said study co-author Prakash. “If there are more females than males in the study area, then the bats are likely to feed on more females. Since this was a canopy dwelling katydid species, it was hard to establish the number of males and females in the system,” he added.
According to Tiwari, “This predator–prey study is the first to provide documented evidence as to why female katydids are eaten more often during the breeding season. Previous studies were based on the bat’s perspective or second-hand experience, such as examining the remains of katydids in bat roosts.”
By studying predator-prey interaction in their natural habitats, we can understand the evolutionary aspects of the calling behaviour of katydids, shared Raghuram. “We can find out how katydid songs are structured in the presence of predatory pressures. They must adopt some strategies to escape and also to survive and reproduce,” he said.
The study opens avenues for future research where both predator and prey could be studied using radio telemetry, which inform more about activity levels and times, movement patterns, habitat selection, realistic estimates of encounter probabilities as well as predation risk in the wild, said Saha.
The findings of this study could also inform future studies that could push the boundaries of predator–prey studies to find out exactly what animals are optimising, said Prakash. A different predator may have different strategies but the whole body of knowledge informed by this study comes together to advance conservation efforts. “Predator-prey studies tell us where the predators and prey are found, what they are doing, how they find food, and how they mate. Without such crucial information, there is no way one can conserve a species,” he said.
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Banner image: A female katydid. Females move 1.6 times more often and 1.8 times farther than males, which puts them at more risk of being hunted by predators. Photo by Kasturi Saha.