Sci-Tech

Decoding the colours on a butterfly

Butterflies are under intense natural selection to be very good mimics.

Butterflies are under intense natural selection to be very good mimics.  

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The lower surface of the wing shows better mimicry than the upper

For the wildlife enthusiast, the bright colours, rings, spots and stripes on the wing of the butterfly represent the pinnacle of nature’s beauty. However, how does a predator — a bird or a larger insect — view the wing colouration and the rings? Does it tilt the balance in favour of the butterfly in this millennia-old battle of survival in the evolutionary war that is described by biologist Richard Dawkins as the ‘Greatest Show on Earth?’

A study by scientists from the National Centre for Biological Sciences (NCBS), Bengaluru and the National University of Singapore (NUS) investigate how well butterfly mimics resemble their ‘models’, and finds the complexities that drive the evolution of mimicry.

Researchers studied over 200 specimens from the lush Western Ghats forests of ‘unpalatable’ butterflies that serve as models to ‘palatable’ butterflies. For instance, the Danaus chrysippus (plain tiger or African monarch butterfly) protects itself from predators by injecting unpalatable alkaloids from plants during the larval stage. This protection from predators allows it to fly “leisurely” without threat of the predator. It isn’t surprising then that the ‘palatable’ Danaid Eggfly ( Hypolimnas misippus) has evolutionarily learnt to mimic the look of the plain tiger in an effort to fool predators into avoiding eating it.

The predator’s point of view was obtained through the visual modelling of the avian colour vision (which differs from the colours and patterns perceived by humans).

The study, published in the latest edition of the journal Evolution, quantitatively shows that female butterflies were better mimics than their male counterparts. Krushnamegh Kunte from NCBS explains: “Female butterflies carry heavy loads of eggs, which impairs their escape flight when they are attacked by birds and other predators. Because of this risk of predation, female butterflies are under intense natural selection to be very good mimics.”

For male butterflies, the quest for sexual advantage — colours that attract mates — seems to have affected its ability to mimic, and in many species the males look completely different from females. Do their offspring — which inherit genes of mimicry as well as colours of sexual advantage — see their offspring struggle to mimic?

Surprisingly, no. The study finds that evolutionary needs trump any genetic dilution. Another surprising result thrown up is that the lower surface of the wing shows better mimicry than the upper surface which is seen in a butterfly’s flight path. This, says Mr. Kunte, could be because the butterfly is more vulnerable at rest (when its folded wings displays the lower surface) than at flight.

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Printable version | Apr 22, 2019 10:08:39 AM | https://www.thehindu.com/sci-tech/decoding-the-colours-on-a-butterfly/article7855586.ece

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