This BBC report explains how a species of Peruvian poison frogs appear to have evolved the only known sexually monogamous mating system among amphibians. It seems it all comes down to feeding the young-uns.
A closely-related species, which is promiscuous, lays its eggs in large rainwater pools, which contain enough food for the growing tadpoles. But the monogamous frogs are restricted to smaller pools, where the tadpoles require special packed lunches from the parents in order to survive.
When the researchers moved tadpoles from both species into different sized pools, they found that the tadpoles grew quickly in the larger pools, which contain more nutrients, but could not survive alone in smaller ones. ... Overall, the researchers believe they have found convincing evidence of an evolutionary chain of causation: changing the breeding pool size forced the mimic poison frog to change its system of parental care, with males and females working together. That then culminated in social and genetic monogamy.
The researchers concluded that, "If the pools were bigger, the frogs wouldn't have to remain faithful, as they wouldn't be tied by their need to work together to raise their brood."
What's the connection to our marginally less slimy species, Homo sapiens?
Anthropologists studying hunter-gatherers (people who live the nomadic hand-to-mouth lifestyle of our ancestors) are nearly unanimous in pointing to the crucial importance of sharing as the central organizing principal of these societies. This so-called fierce egalitarianism results in a radically equitable distribution of anything of value to the group (food, protection, healing, child care, etc.). (This often leaves innocent young anthropologists frustrated by the native people's lack of respect for private property – in such societies, there is little notion of stuff that is unshared.) Legendary anthropologist Marshall Sahlins called this hunter-gatherer approach "the original affluent society."
My point? The pooling of resources results in a larger, richer pool for the kids. And when all the adults are more or less responsible for all the kids, individual parents aren't saddled with 100% of the responsibility for their survival. This sad, stressful situation didn't come about until the advent of agriculture, just 10,000 years ago (at most). For over 95% of our existence as a species then, human beings lived in social groups where sexual monogamy would have been unnecessary and would have actually reduced evolutionary fitness overall.
