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A damselfly’s colours have more to them than adding to the looks

Read time: 4 mins
11 Jun 2020
Agriocnemis pygmaea [Image credits: Shantanu Joshi]

In nature, colours are everywhere! Insects, birds, and mammals—all of them have vibrant colours and patterns on their bodies, which not only make them look pretty but play a role in behaviour and survival. Some use their kaleidoscopic looks to attract mates and stand out from their background; others prefer a dull look to blend into their surroundings and hide from predators. In some insects, like a few damselflies, males and females are differently coloured. Since the males mostly initiate mating in these insects, this difference in colours helps them to find a mate. A new study has now explored how colours impact the mating behaviour of a species of damselfly, Agriocnemis pygmaea.

In the study, published in the journal Ecological Entomology, Shantanu Joshi and Prof Deepa Agashe from the National Centre for Biological Sciences (NCBS), Bengaluru, have experimentally analysed the role of colours on the mating behaviour of Agriocnemis pygmaea.

A. pygmaea is a small damselfly found around the globe and breeds throughout the year,” says Shantanu Joshi, who studies the ecology and behaviour of damselflies. “There are two female morphs—individuals of the same species with different colours—in these damselflies. There is a red heteromorph, and a blue andromorph, which looks like the male of the species, making it a perfect candidate for the study,” he adds.

The researchers set out to confirm if females of A. pygmaea have discrete colour morphs in the wild, by capturing a few individuals, marking them, and releasing them. They found that while males did not change their colour, females indeed transitioned from red to blue over time, with an intermediate colour during the transition. In many organisms, the colour forms are due to different alleles. However, it does not seem to be the case in this damselfly.

“Here, the colour forms seem to be a product of an ontogenetic or age-related colour change,” explains Mr Joshi.

If the colour change is indeed age-related, would the blue colour in females indicate maturity? To confirm this, the researchers dissected the females and investigated their egg-production. Blue-coloured andromorphs were found to have significantly more and larger mature eggs in comparison to the red heteromorphs, confirming the hypothesis.

The study also found that males, in the field and the lab, preferred mature blue females or those with the intermediate colour, to mate. Interestingly, when males were given a choice between a male and a young female, they mostly tried to mate with the male. However, this male-male mating interaction decreased in the presence of a mature female. Odonates, which are a group of insects that include dragonflies and damselflies, are known to have a remarkable visual ability. The findings of this study show the relevance of colour in choosing a mate in these flies.

“Since females show this colour variation and not males, we can hypothesise that the selection pressure on females is to avoid unnecessary matings by the male”, explains Mr Joshi. “This also helps males conserve energy.”

Finally, they examined the rate of survival of the different female morphs. They hypothesised that the blue-coloured andromorphs, which are older, would spend more time reproducing and are likely to have a lower survival rate. Although in the study it appears that heteromorphs appeared to face higher predation probability, they had higher survival rates compared to the andromorphs. This could be because they are young and less invested in reproduction.

The findings of the study, which is the first to examine the effect of colours in mating behaviour in damselflies, show how ontogenic colours act as an indicator of sexual maturity. They shed light on how colours have evolved in these odonates. But, there may be more to learn of these pretty flies.

“Phenomenon such ontogenetic colour changes occur due to complex interplay of genetics and behaviour, which we are yet to understand completely” concludes Mr Joshi.

This article has been run past the researchers, whose work is covered, to ensure accuracy.