Partly, it’s because the strength of their colors reflect how fit they are. The patterns depend on proteins in the wings, and as caterpillars, these butterflies feed mostly on protein-poor food like cabbage and kale. Only the most successful individuals can eat enough to create bright colors later on in life. And these same males also offer females more enticing gifts.
A cabbage white’s ejaculate is very different from a human’s. Rather than a blob of white gunk, it’s a complex solid package called a spermatophore, which consists of a hard outer shell, soft nutritious innards, and a ball of sperm at the base. The male deposits this into a pouch within the female reproductive tract called the bursa copulatrix. Once inside, the sperm swim off into a second pouch—the female will later use these to fertilize her eggs. Meanwhile, she starts to break down the outer shell of the spermatophore to absorb the nutrients within. So, the spermatophore acts as a nuptial gift—a way for the male to nourish the mother of his future offspring, long after he flies away.
As gifts go, the spermatophore is a substantial one. On average, each packet makes up an astonishing 13 percent of the male’s body weight. “Scientists who work on ejaculates will often show up to meetings with props,” says Morehouse. “I’ve never had it in me to bring a five-gallon bucket along. But that’s what we’re talking about [if you scale it up to human size]. It’s a water-cooler-sized ejaculate.”
These gargantuan gifts are so nutritious that females use the proteins within them to make roughly half the eggs that they eventually lay. That’s why they pick the most colorful males—these are the ones that offer the largest spermatophores.
If males are ejaculating 13 percent of their body weight with each fling, that surely limits how often they can have sex in one lifetime. No one knows the exact number, but it’s at least two or three. And if they’re running low on resources, they start producing smaller spermatophores—and breaking down their flight muscles and internal organs. Elderly males actually digest their own innards to fund the construction of their giant ejaculates.
It is far easier, by contrast, to work out how often the females have sex. That’s because females can’t fully digest the outer layers of the spermatophores, even after they have sucked all the nutrients out. Each female carries around the legacy of all her past sexual encounters, as withered husks sitting inside her genitals. If you dissect her, you can just count them. That’s how Morehouse knows that most females mate two or three times, but some manage up to six.
From a female’s perspective, it’s best to mate as often as possible, since each encounter brings a new packet of nutrients. But that’s disastrous for a male, since the sperm of a subsequent suitor could displace his own. These contrasting incentives set up an intense battle of the sexes, in which females try to mate often, and males try to stop them. “It’s this tug-of-war over who’s in control of when the female can mate again,” says Morehouse. But it’s not a physical contest. It’s an invisible one, which plays out inside the female’s body through the chemistry of the spermatophore.