Unexpected prey behaviour reveals positive effects of predators

Fathima Athar

When selecting sites for laying eggs, female Aedes mosquitoes avoid water puddles that lack predators and choose ones with a few predators instead. Researchers from the Indian Institute of Science (IISc), Bengaluru, and the Nature Conservation Foundation (NCF), Mysore, explain this puzzling behaviour of Aedes mosquitoes.

Aedes aegypti during blood meal

If you were a female Aedes mosquito, would you lay your eggs in a pool with predators or in one without? 

If you chose a pool without predators, Manvi Sharma and Kavita Isvaran will tell you that you wouldn’t do well as a mosquito!

In their recent study, Sharma and Isvaran investigate why female Aedes mosquitoes choose to lay their eggs in pools with few predators present rather than no predators at all. Sharma and Isvaran, from the Centre for Ecological Sciences, Indian Institute of Science (IISc), Bengaluru, collaborated with Vishwesha Guttal, their colleague at IISc, and Suhel Quader from the Nature Conservation Foundation (NCF), Mysore for their study. 

There is this idea in the literature (and in our heads) that prey is always wary of predators and avoids them. But in fact, animals may be attracted to low levels of predators,” says Isvaran. 

During an unrelated field trip, the authors observed that all the water puddles in the rocks did not have a similar number of mosquito larvae. They wondered if this was because of the presence of predators and designed a study to investigate if predators influence female mosquitoes’ choice of egg-laying sites. They used the predator-prey model system of Aedes mosquitoes and dragonfly nymphs. Mosquitoes are a great and underutilised model system; they allow easy combining of controlled lab experiments and field experiments,” says Isvaran.

To study how female Aedes choose their egg-laying sites, the authors set up experiments where mosquitoes were given choices of pools to lay eggs in, each with a different number of dragonfly nymphs, creating what the authors call a predator-gradient. First, the researchers observed that the female mosquitoes altogether avoided pools with the highest number of predators. This was as expected. However, surprisingly, they also avoided pools with zero predators. Instead, the female mosquitoes chose to lay their eggs in pools containing predators, albeit in lower numbers. 

The egg-laying behaviour of female Aedes mosquito is unique. The females deposit eggs on the edges of water-filled pools (unlike Anopheles mosquitoes that lay their eggs directly in the water). The eggs hatch only post rainfall when water inundates the eggs. In a few days preceding the rain, the eggs accumulate in large numbers and hatch almost simultaneously after the rains, resulting in high larval densities in the pools. 

Larval crowding and the competition for resources affect the fitness of adult mosquitoes. Thus, larval competition is a significant influencer of female egg-laying site selection. A low density of predators helps with the larval density problem. 

Experimental set-up with predator gradients.
Laboratory setup to raise larvae for the predator gradient experiment (Photo: Akshay Pattayil)

To understand the mosquitoes’ choice of egg-laying sites containing predators, the authors decided to measure the outcomes of this choice. Experimental measurements of larval survival, development time, and adult body size – factors that collectively ensure a fitter mosquito generation – were carried out in a predator-gradient. At high larval densities, the authors found that the competition for resources increased and negatively affected the offspring. However, the presence of a few predators in dense pools of larvae ensured a positive effect on the mosquito generations. 

The results were exciting, but the experiments were tedious. I spent many a night in the lab taking readings every five hours. It was similar to waking up multiple times, in the middle of the night, to feed a baby!” quips Sharma. 

We did experiments in this study that have been rarely done before, like using a combination of predator and competitor gradients to understand fitness and to measure both behaviour and the underlying trade-offs simultaneously. I think Manvi has managed to pull off something pretty amazing,” says Isvaran.

As for what’s next, Isvaran says, We want to explicitly tie traits like competition, predation, etc. with the different life stages of mosquitoes, and with their performance as vectors of disease. Studying the influence of competition on mosquito population dynamics is useful in designing better methods for mosquito control.” 

The strength of this study is its combination of field and experimental approaches, and the quantification of fitness in an exemplary manner through grand-offspring production,” says Amitabh Joshi, from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCSAR), Bengaluru. He was not involved in the study. 

Our main message is that predators can have unexpected effects on animal behaviour, and we have been able to measure these responses in our study practically. I think this is an exciting finding,” concludes Isvaran. 

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Fathima has a PhD in Life Sciences specializing in Cell and Molecular Biology. She is a freelance writer for Science Communication.