In most animal species, including humans, the onus usually falls on the male to convince a female that he is a worthy mate. Animals have evolved elaborate courtship rituals with some of the most entertaining and colourful ones seen in birds. This youtube video, for instance, shows us the lively courtship ritual used by male birds of paradise to attract a female. However, not all animal species follow this paradigm of female-directed courting. A group of researchers from Yale University recently discovered that in a species of African butterflies, males aren’t always the “courters”. The onus of courtship falls both on the males and the females of the species, depending on the season in which they were born. Their results were published this week in Science.
The team led by Dr. Antonia Monteiro, studied a species of African butterfly better known as Bicyclus anynana. B. anynana butterflies look different depending on whether they were born in the wet season or in the dry season. Butterflies born in the wet season have more elaborate ornamentation on the bottom-sides of their wings than butterflies born in the dry season. However, the top-sides of the wings are identical irrespective of the season in which the butterfly is born. The top-side is adorned with colourful circles called eyespots, which are bigger in females than in males. Furthermore, the top-side of the wing is more important in B. anynanas courting behaviour, with the “courter” displaying the eyespots to the “courtee” by rapidly opening and closing his/her wings. When the researchers observed courting behaviour of these butterflies in the lab, they noticed that wet season males courted females more often than did dry season males. Conversely, dry season females courted males more actively than did wet season females. So courting behaviour seemed to be based on whether the butterfly was born in the wet season or in the dry season. Similarly, the choice of mate also seemed to depend upon the season of birth. The researchers noticed that wet season females were pickier than dry season females in choosing a male. Wet season females preferred males who had intact eyespots, whereas dry season females mated with males with and without intact eyespots. The opposite was true of the males. Dry season males were more selective than wet season males, again preferring only those females with intact eyespots.
But why does courting behaviour change based on season of birth? The scientists think that this perhaps has to do with the increased benefits of mating for females and the increased disadvantages of mating for the males. In fact, the lifespan of wet season and dry season females increased when they mated with dry season males than when they mated with wet season males. By mating with dry season males, the females were also able to lay more eggs. This is perhaps why in the dry season females are more aggressive in their courting, because successful mating with a male increases their longevity and their offspring. Wet season females however do not have the same advantage of mating with a wet season male and hence do not actively court the males and are pickier. Unfortunately for the dry season males, the more they mated with females, the shorter did they live. This explains why dry season males were more selective than their wet season counterparts and why they did not actively court females.
It isn’t as yet clear, however, why mating with dry season males is more beneficial to females than mating with wet season males, or why mating causes dry season males to live shorter lives. Further research will be necessary to understand these biological intricacies of butterfly mating behaviour.
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