In many animals, males give females a gift in the hope of getting to mate with her. This might sound overly simplistic, but this really can be how it happens. For example, kingfishers will catch fish to give to females as part of their courtship.
However, having a system like this means that there is room for cheating. Why bother going to a lot of effort to find a genuine gift, if you can find a cheaper knock-off that you can trick a female into accepting?
Indeed, many male animals do indeed use worthless gifts instead of genuine ones. These sneaky males have been known to re-use gifts that they already gave to one female, or even offer females inedible items that are disguised as food gifts.
Researchers recently looked at this quirky behaviour in the gift-giving spider, (Paratrechalea ornata). The males of this species catch prey and wrap it up for a female in silk. He will then dance towards her on her web, vibrating it with his forelegs before he presents the gift to her while taking a particular pose. If the female spider decides that she's impressed enough with all of this, then she will grasp the gift and allow the male to mount her and insert sperm via a special leg (a pedipalp). During mating, the male will switch between inserting his pedipalp into her and facing her, grasping the gift.
The scientists collected 53 male spiders from the wild who were found to be carrying gifts for females. Amazingly, 70% of these males were holding worthless gifts such as prey leftovers, presumably after having eaten the prey themselves. Indeed, other spiders kept in captivity have been seen to eat a large proportion of their 'gifts', before (perhaps sheepishly?) wrapping up the remains for a lucky lady. These males holding prey carcasses rather than anything a female would actually want to eat were themselves in much poorer condition than males that carried actual prey items. This implies that those males that carried real gifts for females had eaten enough and were in good enough health to be able to afford to carry a heavy food item to give to a female without needing to eat it themselves.
The researchers then brought all of these spiders (both the ones holding worthless gifts and what I like to think of as the more earnest males carrying genuine gifts) into the lab. The male spiders were then given either a real gift to give to a female spider (a big juicy housefly), a worthless gift (an insect skeleton), or no gift at all.
Females were equally likely to mate with males who carried real gifts as those who carried leftovers from their dinners. This is not surprising, given that since the gift was cleverly wrapped by the male, the female may not have been able to tell what the package contained before 'agreeing' to mate with her male suitor. Instead, females much preferred to mate with males in good body condition rather than skinny, underfed males. Therefore, those males that ate their gifts were actually the 'smart' ones (whether they knew this or not) as it was better to be in good shape having had a full meal, but courting the female with the equivalent of an empty box of chocolates than to court her looking wan but holding the equivalent of a roast dinner. The well-fed males not only had a higher chance of mating a female, but females also let them mate with them sooner and for a longer period of time.
More surprisingly, females also didn't seem to care about whether males even carried a gift at all. They were actually as likely to mate with males without a gift as males with a gift (as long as their bodies were in good condition). However, all the females in this case were virgins, and so perhaps weren't fully experienced with the ways of the spidery world. To see whether the same results would hold with non-virgin females (what male spiders usually encounter out in their natural environment), the scientists repeated the experiment using females that had previously mated with males who gave them genuine gifts. When these females were then either presented with a male carrying a worthless gift, or a male without a gift, they were much more likely to mate with the male giving her a worthless gift than no gift at all.
The results of this experiment raise the question of why these little 8-legged animals go through the whole charade of giving gifts at all, given that females seem to care a lot more about the condition of her potential mate's body than what gift he's giving, or even if he has a gift at all.
It is possible that although gifts were, at one point in evolutionary history, important to females and they directly cared about their quality (i.e. the gift actually being real food), now they are kind of an evolutionary left-over, a signal of something else. It is possible that nowadays the female doesn't actually care about what's in this gift itself, but instead the gift acts as a signal, with only the best, strongest and most well-fed males carrying gifts. To make a human example, maybe a person being courted might be flattered by being given flowers. They might not care about what kind of flowers they are, but the fact that someone has gone to the effort to buy them flowers is indicative of other things (thoughtfulness and suchlike). Returning to spiders, by mating only with males that carry gifts (regardless of whether these are real or worthless gifts), the female spider is making it more likely that her sons will be gift-carriers and thus have higher mating success, meaning more of her genes get into her grandchildren's generation.
Nonetheless, females that actually get real gifts from males will be better off than females that only get worthless gifts, as they have that extra meal to munch on while mating. However, it seems likely that because all objects come gift-wrapped that the females can't tell the difference between good and worthless gifts. Mating happens pretty quickly in this species, so by the time the female has accepted the gift from the male, signalled that he can mate with her and started opening the gift, he's already finished transferring his sperm and is off, leaving her clutching the empty carcass of a dead fly.
In my point of view, it seems like the males seem to be winning this evolutionary battle between the sexes, deceiving some females into accepting worthless gifts while others get nutritive meals. However, it is possible that females have the final word, as they may be able to store sperm from multiple males and only use the sperm from the best-looking males carrying the best gifts.
Scorpionfly picture: taken from LeBas, N. R., & Hockham, L. R. (2005). An invasion of cheats: the evolution of worthless nuptial gifts. Current biology, 15(1), 64-67.
Gift-giving spider photos: Mariana Trillo
Photo of genuine and worthless gifts: M. J. Albo, taken from Albo et al. (2014)
Albo, M. J., Melo-González, V., Carballo, M., Baldenegro, F., Trillo, M. C., & Costa, F. G. (2014). Evolution of worthless gifts is favoured by male condition and prey access in spiders. Animal Behaviour, 92, 25-31. (primary reference used)
LeBas, N. R., & Hockham, L. R. (2005). An invasion of cheats: the evolution of worthless nuptial gifts. Current biology, 15(1), 64-67.