I knew that monogamy was rare in mammals, but I didn’t know how rare until I started researching my novel, Odd Bird.
Less than 5 per cent of the 4,000 mammal species are monogamous. Our closest monogamous relatives are marmosets and tamarins. Others include the red fox, a few rodents and the giant otter.
But approximately 90 per cent of birds are monogamous. Why are we more like birds than mammals?
The benefits of monogamy
Birds are monogamous because their young are tiny, helpless and immature (or altricial) and require loads of parental care.
For an indication of just how altricial most nestlings are, look to a typical pied flycatcher nest. The pied flycatcher is a widely studied passerine (and the ‘star’ species of my novel). Young pied flycatchers are blind and weigh just 1.5g on hatching. However, they gain an astounding 1-1.5g per day over the next 10 days. That’s a nine-fold increase in mass in just 10 days.
Achieving this intensity of feeding would be horrendously difficult for a single mum, particularly since she must also spend a lot of time warming the brood. And so cooperation and teamwork, aka monogamy, are essential.
For a more tangible sense of the demands on pied flycatcher parents, consider this: each will deliver 300g of food per hour to the nest. They do that for 19 hours a day for 15 days. By my calculations they will each carry 6,000 times their own bodyweight to the nest. Staggering, isn’t it?
Of course, human infants are altricial too. Fortunately – especially for those of you sporting mammary glands – babies don’t grow nine-fold in 10 days. They are undoubtedly very cute, but let’s be honest, they are rubbish. “The human new-born is little more than an embryo,” according to anthropologist Helen Fisher.
It’s all because big brains can’t pass through small birth canals. Consequently, human babies have small brains at birth, meaning they are super altricial. So, like the pied flycatcher parents, we are monogamous too.
Is monogamy made to last?
In birds, monogamy comes in variety of different styles.
For one thing, pair-bonds differ markedly with respect to duration. Most monogamous birds pair only for a single breeding season. In these species, the whole courtship and mating process starts afresh every year. Other species (a minority) pair for several seasons and even for life.
Most people know that swans mate for life. But there are other examples too, including cranes, vultures, eagles, geese and albatrosses. And of course, scientists have pondered why.
These species are all long-lived. We believe that lifelong pair-bonds are beneficial because they allow pairs to optimise their effectiveness as parents, resulting in more surviving young. This hypothesis has been studied and validated in kittiwakes (coastal gulls).
Read more about mating behaviour:
Maybe humans are most like long-lived birds. Yet, according to the charity Relate, 42 per cent of UK marriages end in divorce. Data from the ONS suggest that divorce usually occurs within the first few years of marriage (the mode is five years). Could it be, as some have suggested, that our pair-bonds are intended to endure just long enough to rear one infant through the most intensive and demanding years?
I think it’s more complicated than that.
‘Divorce’ is also quite common between kittiwake pairs. Approximately half of pairs divorce if they fail to produce young in their first season together. They are three times less likely to divorce if that first season is successful. The analogy isn’t a perfect one, but it does seem that when the pair-bond isn’t working as it might, kittiwakes and humans both favour an early exit.
Fidelity in monogamous relationships
Monogamy, I’m afraid, doesn’t usually mean ‘forsaking all others’.
Most monogamous species are ‘socially monogamous’, which simply means that they pair in order to raise their young successfully. Sexual or genetic monogamy is an extreme form, and it is rare in nature.
When scientists look for evidence of extra-pair paternity (EPP), i.e. cuckoldry, in birds, they almost always find it. Levels vary substantially, however. In kittiwakes EPP is very rare. In pied flycatchers 10-15 per cent of young are fathered by an extra-pair male. Indigo buntings are definitely not to be trusted, because EPP occurs at a rate of greater than 20 per cent.
Before I reveal what the data tell us about human EPP, let’s pause for a moment to consider our testicles. Adjusting for body weight, chimpanzees have huge testicles compared to us. This is because a female chimpanzee in oestrus may copulate with a dozen males and when, simply put, you are up against 11 amorous competitors, more sperm means more paternity.
Gorillas by contrast have tiny testicles, because the male has exclusive access to the females in his harem.
Human testicles, it turns out, are intermediate in size (no matter what your friend says about his). For this reason some have postulated that there was moderate sperm competition in ancient human societies. And when you find out that 40 per cent of married men and 25 per cent of married women in America admit having had an adulterous affair, you can be forgiven for fearing the worst.
More on human relationships:
- Why don’t humans have a mating season?
- What happens in my body when I fall in love?
- Is the ‘seven-year itch’ a real thing?
However, levels of EPP in contemporary human populations have been shown to be, surprisingly (to me at least), just one to two per cent. Reacting to these findings, some suggested that the level of EPP was significantly higher historically, because of lack of effective contraception and other contributory factors. However, analyses going back 500 years have concluded that cuckoldry occurred at low levels then too.
So we really are very like kittiwakes, though less squawky.
I believe the similarities between courtship and mating behaviour in birds and humans extend beyond monogamy. For example, in Odd Bird I also explore the concept of female choice.
In both humans and birds, females tend to be ‘choosier’ because for them copulating with the wrong mate can be very costly in terms of genetic legacy. They need to be confident that potential mates have good genes and the ability to provide for the family.
That’s why many a hopeful young man has taken his date out for dinner (demonstrating his ability to provide) and then dancing (demonstrating his physical prowess and genes). Similarly, a male great grey shrike painstakingly builds a cache of rodent, bird and amphibian corpses in order to impress a mate but, she still won’t consent to copulate until she has seen him dance.
For me, the parallels between the human and avian worlds are striking and instructive. And yes, of course our behaviours are more nuanced, complicated and plastic. But I still believe we are Odd Birds.
Lee Farnsworth’s novel Odd Bird (£8.99, Farrago) is out now.