The Elements of Evolution (66) Monogamy

Monogamy

The emergence of pair-bonding in humans was a major evolutionary transition, which dramatically altered the trajectory of our species. ~Russian evolutionary biologist Sergey Gavrilets

Although common in birds, social monogamy is rare among mammals. 90% of birds live in pairs, whereas less than 3% of mammals do. The demands of mammalian motherhood – internal gestation and lactation – make it advantageous for males to seek other mating opportunities.

In mammals, social monogamy evolved when females were largely solitary, and the ranges of roaming male overlapped those of several females. Female solitude partly descended from diet.

Pair-bonding arises in species that diet on high-quality foodstuffs which are not abundant. Fruit is the main part of the diet in 91% of socially monogamous primates, whereas only 28% of solitary primate species rely upon fruit. Foods with low nutritional content are a much larger part of solitary species’ diets than monogamous primates.

When breeding females are intolerant of each other and female density is low, males cannot guard more than one breeding female. This logistical pickle produces monogamy.

Where females are widely dispersed, the best strategy for a male is to stick with one female, defend her, and make sure that he sires all her offspring. In short, a male’s best strategy is to be monogamous. ~ English zoologist Tim Clutton-Brock

Primates are unusual among mammals in that monogamy evolved independently in all major clades. In primates, larger brains produced a more prolonged infancy. This meant that mothers were infertile for longer durations.

Killing a female’s babies brings her back into fertility. There are only few ways to evolve a prevention to this reproductively self-defeating dilemma.

Female chimps mate with many of the males in their group to confuse paternity. This spares them from infanticide.

The descent of hominins – with their enlarging brains, cunning, and often violent nature – rendered infanticide an evolutionary dilemma, especially with the lessening of tightly-knit social bonds. The adaptive solution was pair-bonding.

You do not get monogamy unless you already have infanticide, and you do not get a switch to paternal care if you don’t already have monogamy. ~ English anthropologist Christopher Opie

There is no biological imperative to monogamy in humans, even as men with small testes are more inclined to parenting. Instead, mate bonding is a cultural norm.

Only 17% of human cultures are strictly monogamous. The others involve a mix, with most people practicing monogamy, either heterosexual or homosexual, while a minority practice polygamy or otherwise. Even in those cultures with monogamy, extra-pair copulation is common.

The human mating system is extremely flexible. ~ Canadian anthropologist Bernard Chapais

Over 90% of bird species breed as monogamous pairs. In 90% of those birds, extra-pair paternity is common. Though the numbers are likely lower, similar dynamics applied to hominins.

From social insects as well as birds, cooperation is unlikely to evolve within extended families unless there is also genetic monogamy. A different form of cooperation – between unrelated individuals – may actually be promoted by promiscuity. ~ English zoologist Ben Sheldon & American zoologist Marc Mangel

Mothers are devoted to their offspring. A father’s commitment to paternal care depends upon paternity. Extra-pair mating by females creates an incentive for males to cooperate with neighbors. Infidelity likely was instrumental in extending male hominin sociality.

Where maternity certainty makes females care for offspring at home, paternity uncertainty and a potential for offspring in several broods make males invest in communal benefits and public goods. ~ Norwegian zoologists Sigrunn Eliassen & Christian Jørgensen