Why Does He Bring Her a Dead Fly Wrapped in Silk? It Buys Time

The offering takes several forms across the animal kingdom. It may be captured prey, presented whole or wrapped. It may be a substance the male produces himself, a gland secreted for the female to eat, or a package transferred with the sperm that she digests afterwards. In the most extreme cases the male's own body is consumed. What unites them is that a male transfers resources to a female around the time of mating, at a real cost to himself, and that the transfer influences whether and for how long mating proceeds.

The benefits are more calculated than the behaviour looks:

• •Acceptance by a female who might otherwise refuse
• •Time, since mating continues while she is occupied eating
• •More sperm transferred the longer that lasts
• •Reduced risk of being eaten by the female during mating
• •Resources that go into eggs he has fertilised
• •Sometimes chemicals that make her less receptive to other males

Wherever a gift buys time, selection favours making the gift cheaper without losing the effect, and several species show exactly that. Certain dance flies present the female with a silk-wrapped package, and in some species the package contains a smaller item than it appears to, an inedible seed or plant fragment, or nothing at all, wrapped elaborately enough to be accepted. The male gains the mating time while the female unwraps it. Females in turn evolve to assess packages faster or to break off mating when the contents disappoint, which is a straightforward arms race running in both directions.

The benefit to the receiving side is real in many species and absent in others, which is why the behaviour is studied as a conflict rather than as cooperation. Where the package is genuinely nutritious it feeds egg production directly, and marked substances fed to males have been traced into the eggs afterwards, so the contribution is measurable. In some moths the package carries defensive chemicals the female cannot make herself and passes to the eggs as protection. Against that, packages frequently contain substances that suppress her interest in further mating, which serves him and not her, so the same transfer is food and manipulation at once.

A few species push the logic as far as it will go. Male redback spiders somersault into the female's fangs during mating, and being eaten prolongs the transfer and makes her less likely to mate again, so the behaviour pays despite the obvious cost. Certain crickets produce a large gelatinous mass attached during mating which the female eats for hours. Male scorpionflies offer prey or a drop of saliva, and the size of the offering determines how long mating lasts almost exactly. In several moths the package is large enough to be a significant fraction of the male's body weight and measurably increases the number of eggs laid.

Handing over food or a manufactured package around mating buys acceptance, buys time and therefore buys sperm transfer, and sometimes buys the male's survival. Because the time matters more than the contents, several species wrap worthless items convincingly, and females evolve to detect it. In redback spiders the male offers his own body, which prolongs mating and reduces her remating.