Female members of one fish species show strong responses in a primitive part of her brain when she watches her chosen male lose in a fight. Humans might also possess this overwhelming evolutionary imperative to find the toughest, most combat-ready mates.
The African cichlid fish has a particular part of its brain that fires up when the female has chosen a male as its preferred mate. If that chosen male then later gets into a fight with another male, a more ancient part of the brain fires up, sending strong signals of anxiety to the rest of the brain. In the fish, this newfound concern is generally enough to make the female find a new mate, and a more subtle form of this may well go on in human brains as well.
Researcher Julie Desjardins explains:
"It is the same as if a woman were dating a boxer and saw her potential mate get the crap beat out of him really badly. She may not consciously say to herself, 'Oh, I'm not attracted to this guy anymore because he's a loser,' but her feelings might change anyhow."
Her colleague Russ Fernald says the team is fairly sure that these responses occur not just in the fish but humans and other animals as well. These brain areas are found in all vertebrates, he says, and they all act in more or less the same way.
Desjardins says humans could have this response to actions other than simply being knocked out. More complex or indirect forms of losing a fight, like say losing a game of pickup basketball or even getting passed up for a promotion at work, could also trigger this anxiety response. And although a female doubting her defeated male is the one with the most evolutionary background in the rest of the animal kingdom, it's perfectly possible all other possible combinations of potential mates could have this reaction - those just aren't as well understood yet.
That said, this shouldn't be taken as proof that you lost your beloved because you came up short in a game of Monopoly. Desjardins is quick to point out that humans obviously have reasoning abilities far, far in advance of fish, and the twinges of doubt they might feel after seeing their potential mate defeated can be far more easily dismissed than the almost instinctual aversion fish feel in the same situation.
Still, Desjardins suggests that love really might have far more evolutionary undertones than we ever suspected:
You may not know immediately why you are attracted to a certain person, for example. But it is these sorts of unconscious internal reflexes that we have that are shared with all vertebrates, including fish, that make us feel one way or another before we've even had time to think about it."