If you are moaning about how difficult it is to get that special someone to like you, spare a thought for the natural world where stepping out in flamboyance might mean risking your life. Through a creative series of experiments, a group of ecologists from the Indian Institute of Science, Bengaluru, has investigated whether male and female tree-crickets face an equal risk of getting attacked by predators while searching for the perfect mate.
The forest nightlife is abuzz with the chirp of crickets. The shrill sound we hear is generated by male crickets rubbing their forewings together as a signal to attract females. Meanwhile, the females prance around from leaf to leaf in response to the male chirps, hoping to find the perfect mate. Both sexes, thus, take an equal hand in courtship.
Also lurking around this stage are Lynx spiders, predators of the tree cricket. The sensitive hairs on the spider’s body can perceive the male cricket’s chirping and the female cricket’s movement, potentially turning this ceremony of orthopteran romance into tragedy.
The sex-specific hypothesis
For many organisms, the act of searching for a mate comes at the cost of being exposed to predators. The ‘sex-specific hypothesis’ posits that if the two sexes experience different levels of threat to their mortality during courtship, then the sex facing lower risks might play a more active role. Researchers led by Rohini Balakrishnan at the Centre for Ecological Sciences at the Indian Institute of Science (IISc), Bengaluru, have corroborated this hypothesis by putting it to test in a species of tree cricket.
The question of which sex invests more time and effort in searching for a mate can be influenced by several factors. Viewing it as a cost vs benefits game, males enjoy greater benefits of reproductive success as they mate with multiple females to potentially produce greater numbers of offspring. Indeed, for most animals, it is the males who are active in the process of mate searching.
When it comes to the costs, the sex-specific cost hypothesis suggests that females are less active in mate searching since they face higher risks during this process, due to predation, for instance. However, this has seldom been tested in comparison with male activity. As Torsekar notes, “While the benefits in mate searching have been studied extensively for both males and females, the costs were heavily studied in favour of males.”
It is in this regard that the tree cricket (Oecanthus henryi, shortened O.henryi, no known relation to the American author) presents itself as an interesting model organism since here both sexes seem to play an equally important role in mate searching. Hence, according to the sex-specific cost hypothesis, the risk of being hunted by a predator may also be the same for both sexes.
Torsekar’s team set out to ask if male and female crickets face similar costs in terms of getting hunted by their predator, the Lynx spider (Peucetia viridans) during mate searching. More fundamentally, does the threat of getting predated act as a significant deterrent in the process of finding a mate?
Three stages of the predator-prey interaction
The researchers sought to answer these questions by systematically quantifying how likely it is that a male or female cricket actively searching for a mate gets attacked by a spider. This was carried out in three sequential, but independent stages.
The first stage of this predator-prey interaction, called ‘co-occurrence’, requires the cricket and the spider to be located in the vicinity of each other. To determine this, the researchers extensively sampled wild populations of tree crickets and lynx spiders in the fields of rural Karnataka and noted instances wherein individuals of both species co-existed on a single bush.
The second and third stages of the study were performed at IISc. The team staged these experiments in a mosquito-mesh enclosure about the size of two minivans parked next to each other. The interior of the enclosure was carefully designed to mimic the natural environment usually inhabited by the cricket and the spider.
In the second stage, the researchers pitted a hungry Lynx spider against a tree cricket by placing them at different locations on the same bush inside the enclosure. The cricket belonged to one of two categories: a male cricket who signalled for a mate, or a female cricket who responded to a pre-recorded male chirp played out through a loudspeaker. In each case, the researchers carefully monitored both individuals to see if the spider caught a whiff of the cricket’s mate-searching activity and managed to locate it. As a control, the experiment was also carried out with non-signalling male and non-responding female crickets. This second stage counted the number of instances which led to the third stage: a final showdown.
In the third and final stage, the same enclosure hosted direct head-to-head encounters between the spider and cricket placed within striking distance of each other. The researchers noted how many times the spider attacked successfully without the cricket managing to evade. Overall predation risk was determined by combining probabilities from the above three stages.
This study is yet another example showing that studying insects in the wild is the key to understanding numerous aspects of their biology.
- Tom Tregenza, University of Exeter
The researchers found no statistical difference in the predation risk between the two sexes. This is in accordance with the sex-specific cost hypothesis – since both sexes contribute equally to mate searching, the risk from predation is likely the same for both.
Tom Tregenza, a Professor of Evolutionary Ecology at the University of Exeter who was not involved in this study, says, “When you look at the situation in the wild, you find that the male role of staying still and calling for females is just as dangerous as the female role of searching for males. This study is yet another example showing that studying insects in the wild is the key to understanding numerous aspects of their biology.”
Surprisingly, the obtained probabilities also revealed that the actual predation risk was very low for both males and females, whether they were active in mate searching or not. This suggests that predation might not act as a significant cost for these crickets in the context of mate searching. The fact that female crickets actively seek out signalling males may not necessarily be because they face lower risks. Instead, it may be motivated by other factors like the benefits obtained through a pairing, such as getting resources and protection from the male.Observations in the dark (Photo: Viraj Torsekar)
Over the course of this study which lasted more than two years and formed a part of his PhD dissertation, Torsekar highlights several experiences that shaped his graduate life. “The initial part of my work was completely in the field. I ended up interacting with a lot of locals which helped me learn a new language and really experience the culture. For setting up the field experiments at IISc, I was helped by undergraduate students from local colleges in Bengaluru. I learnt how to be a mentor while giving them a flavour of research and graduate life,” he says. Such opportunities to play a part and contribute to research at a young age can provide invaluable experiences for aspiring scientists.
Viraj Torsekar, the first author of this study was a finalist at the Euraxess Science Slam India 2018 competition. You can find his video entry describing this research here.
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This is a companion discussion topic for the original entry at https://indiabioscience.org/news/2019/a-date-worth-dying-for