There is a possible adaptive upside to a loss of sexual interest. Males that run around looking for sex aren’t very good fathers. It may be that the chemical changes that accompany bonding help us pay attention to the job at hand: supporting babies. Also, when male guppies live with one female for a long time, they focus much less on sex and spend much more energy and attention trying to find food. As a result, they grow bigger and stronger than guppies who encounter a constantly changing cast of females. Those males put more effort into copulating and less into foraging for food, proving once
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