A Mating Debacle: To Call or to Respond

Just how deadly is it to signal for a mate in the animal kingdom? We know that several taxa use a variety of signals (eg. visual, chemical, and acoustic) to attract potential mates. Think of it like lighting a fire or flashing a light on a deserted island (throw in some colorful flag waving as well!); while this could attract a rescue team, it may also attract some unwanted guests (a confused bear perhaps, perhaps not to a fire, but you get the point). The same principles apply here. One sex is usually stationary and sends out a signal which acts as a beacon for individuals of the other sex. If it works, great, you now have a happy union! But this comes with a risk to the signaling sex; the risk of attracting predators. Couple this with the fact that sending out signals can be quite energetically expensive and you have a double whammy. Common examples of male signaling come from frogs, crickets, and katydids (insects in the family Tettigoniidae, also called bush crickets). Several studies have focused on the dangers males face due to the rather conspicuous signals they send out. However there is a less explored, equally important problem here, which is the dangers females face as they go out in search of signaling males. Does this increased mobility make them vulnerable to predation? How do these risks and costs square up with those incurred by males? These are obviously extremely challenging questions, owing in part to the immense variety and diversity found in the living world. This paper - “ Silent katydid females are at higher risk of bat predation than acoustically signaling katydid males” (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 282: 20142319.), published by researchers at the Indian Institute of Science and The American College, Madurai, attempts to shed some light on these questions.

The researchers used a bat (Megaderma spasma) and katydid model system. One aspect of the study was diet analysis of 5 roosts of M. spasma found within a 10km radius of the Kadari village in Karnataka. The results of the diet analysis are shown in Figure 1, taken directly from the paper.

Katydids and crickets belong to Order Orthoptera, confirming that they constitute a significant part of the bats diet. It is quite important to appreciate the challenges associated with the collection of this data. The roosts were monitored for an entire year, and different prey species were identified based on their remains beneath the roosts.

Another dimension of the study was behavioural testing of the bats in response to pre-recorded male calls played on speakers and tethered katydid females (live, simulated female katydids flying in the wild). Calls from different katydid species which are sympatric (located in the same geographical area) as the chosen bat species were used. The tethered katydid females predominantly belonged to Mecopoda sp. The intricacy of the actual experiments and the various procedures followed/ controls used can be best appreciated by reading the manuscript itself. For example, each bat was subjected to 5 trials per night, including 3 playbacks of male calls from different katydid species, one playback of pre-recorded flight sounds of tethered Mecopoda sp. females, and one trial involving a live tethered female. One would not want the order of these trials to affect experimental results, hence the trials were carried out in a random order as dictated by a random number generator. The results of these experiments are summarised in Figure 2, taken directly from the paper.

What immediately stands out is that all tested bats (18) responded to tethered katydid females that initiated flight (in several cases, tethered females failed to initiate flight, details in paper). In fact, the diet analysis also revealed a greater fraction of female katydid remains than male katydid. The authors go on to point out that the situation most closely simulated in their experiments is that of either a calling male or flying female in close proximity of a bat predator. They also note that both were easily accessible to the predator. The results suggest that a flying female katydid in the near vicinity of a Megaderma spasma predator faces a higher predation threat than a calling male which is also in the near vicinity. One hypothesis that the authors propose to explain this observation is that males often call from rather inaccessible areas, such as within dense brambles, making flying females a relatively easier target. The paper also discusses potential implications of this predation pressure on the evolution of katydid signals.

You are all encouraged to check out this interesting and thought-provoking article!

            Figure 1. Diet Analysis of M. spasma represented in a pie chart.

              Figure 2. Results of the behavioural testing experiments.