When it comes to mating, most species of animals have one sex that does all the work. Stereotypically, we think of the male putting in most of the effort, whether it’s with extreme ornamentation like peacocks or with extreme effort like dung beetles. But it’s not always the case, and sometimes females take the initiative. But in most cases the solicitor for sex doesn’t really vary, it’s the male, or the female, but not generally both.
Enter the butterfly: Bicyclus anynama. It’s an African species often called the “Squinting Brown Bush Butterfly”. I imagine that the genus “bicyclus” refers to the dual life cycle of the genus (though I couldn’t find any solid information on that, anyone?), but to me it just makes me think of a butterfly riding a bicycle.
(Bike like a butterfly, sting like a bee…source)
The Bush Butterfly has a life cycle that can be divided into two seasos, a dry and a wet (DS and WS). Though they are the same species, the butterflies that develop in the Wet season look very different from those that develop in the Dry season.
The top left is the underside of the WS female’s wings (ventral), while the top right is the underside of the DS female’s wings. you can see the difference is pretty drastic. Wet season ladies have hot ventral spots, while dry season ladies have a more streamlined design. But what’s NOT so drastic is the DORSAL side of the wings, the top side. The male and female dorsal sides are shown in the bottom of the figure.
You’d think what they would be after is those big flashy spots on the top left, but you’d be wrong. What the butterflies look for in a partner is those less flashy, and less variable, spots on the DORSAL side of the wings. There’s something very specific they are looking for, that white spot in the middle of the bigger “eye” shape at the top section of the wing. It’s called the “pupil” and while it doesn’t look like much to us, it’s Uv light reflective and shines very bright to a butterfly. When a butterfly flirts, it flips its wings open and shut to reveal the pupil to a prospective mate.
In the Wet season, males court the females, and the females have a weakness for those bright white spots. But both males and females HAVE the spots. So the question these scientists asked what whether those white spots were important for FEMALES as well as males. And it turns out, it’s all in the season.
While males born in the wet season court females born in the wet season, males born in the DRY season are the ones getting catered to by females. Females born in the dry season (DS) court and seek out males far more than WS females. And it turns out that it’s a good thing the females have those little eye pupils as well. Females always prefer pupils in males (no matter what the season), but with males it varies. DS males need visible pupils just like the females do, while WS are not so demanding of their lady-loves. So for a DS female who’s flirting away at a DS male (presumably in the wild she’s not likely to hit upon too many WS males), it’s good that she has those visible pupils, she’s going to need them.
But the question now is: WHY are the DS females courting in the dry season, and the WS males courting in the wet? There’s got to be more to it than spots.
Turns out it’s not just the spots. The DS males are more desirable to females for another reason. When butterflies mate, the male often gives the female some bling. But rather than the kind of bling we think of, the male provides the female with NUTRIENTS, in the form of a highly nutritious spermatophore as a nuptial gift. The males get to put some sperm in, and the females get an extra boost of energy. Now, it would make sense fo rthe DS females to be goin after the DS males if the DS males had something particularly worth having, and it seems that they do. The authors took the butterflies, allowed them to mate, and then left the females without any food, and checked to see how long they could make it. The idea behind this is that if the DS males are providing a larger nutrient nuptial gift than the WS males, the females that MATE with the DS males will make it longer than the ones that don’t.
On the left you can see how long the female butterflies lasted when they were either alone, had mated with a WS male, or mated with a DS male. And it turns out that the females (whether WS or DS themselves) lived longer when they had mated with a WS male. And it gets better. On the right, you can see that the females who mated with the DS males (either DS or WS themselves) laid more EGGS. Those DS dudes have a significant upside to them. This suggests that the DS FEMALES have more reason to be flirting to get those DS males, while WS females (who are only around WS males) have no extra incentive, and the males have the take the lead.
So it looks like DS female butterflies have more incentive to get the guy, while WS females can wait for the males to come to them. And this change is environmentally induced, and entirely dependent on whether the butterflies are born in the dry season or in the wet season.
The question now is this: WHY are the female butterflies in the dry season in more need of a nuptial gift, while in the wet season it doesn’t matter as much? They don’t address this in the paper, and Sci doesn’t know crap about butterflies, but if I had to guess, I’d guess that the dry season might be a tougher time to get nutrients, and thus the nuptial gift from the male is more necessary. But I could be wrong.
Regardless, this paper shows that the environment that an animal is born into can change their mating behaviors, and that the environment not only drives the behavior, it drives the way the butterflies LOOK as well! Give them a dry season or a wet one, and watch these butterflies change their spots.
Prudic KL, Jeon C, Cao H, & Monteiro A (2011). Developmental plasticity in sexual roles of butterfly species drives mutual sexual ornamentation. Science (New York, N.Y.), 331 (6013), 73-5 PMID: 21212355