Playing the numbers game

THE mother of the octuplets born recently in the US has her hands full.

For once, the media circus may actually be a help; she and her babies have celebrity billing. That should bring advertising contracts and financial support. Science will also benefit; psychologists and child development researchers will be banging on her door.

In the animal kingdom, where multiple births are the norm, mothers get no special treatment. Animals have large broods for insurance reasons; a predator may take a youngster or two but is unlikely to destroy an entire brood. Likewise, food shortages hit the weaker offspring first, leaving the scarce resources for stronger siblings. Then there’s the prospect of winning the biological Lotto; if conditions are good and several young survive to breeding age, the genes of the lucky grandparents will be spread far and wide in a new dynasty.

The stoat has a litter of five to 12. A house mouse produces four to eight pups and may do so up to ten times in a year. But family size is not written in stone. Arctic foxes, for example, breed erratically. Where they live, the winters are dark and vicious, life is short and few foxes reach the grand old age of five. Feeding on rodents and young birds during the brief Arctic summer, a mother will have just a few cubs, or none at all, most years. But, in the occasional “lemming year”, food is abundant. Making hay while the sun shines, a fox might produce up to 20 babies. It’s a big family but tiny compared to that of some creatures; the giant puffball, the football-sized fungus found in Irish pastures, produces a staggering 7,000,000,000,000 spores. If you want immortality for your seed and breed, having octuplets might seem the way to go, all other things being equal. But other things are seldom equal and, the American octuplets apart, there are no free lunches. Parents have as many babies as their resources will allow. Brood size is dictated by circumstances.

A swallow lays four or five eggs in as many days and may have three clutches during the season. An Irish bat, however, working the night shift while the swallow sleeps, has only one baby each year. Bats may be the largest order of mammals, but they are very poor breeders. The bat lifestyle doesn’t lend itself to having babies; pregnant bats find it hard to catch insects on the wing. If an expectant mother had several embryos on board, she would be too heavy and sluggish to catch anything at all. The swallow solves this problem by carrying only one egg at a time. Tomorrow’s egg will only develop when today’s has been laid.

But not all birds have large clutches. Pigeons, being strict vegetarians, produce milk in their crops with which to feed their young. They can’t produce or carry large amounts of milk so they, like the bats, can raise only one or two chicks at a time.

Weight is not a constraint for flightless birds, so they produce lots of eggs. The ostrich, with a clutch of up to eight, is an exception to the rule that large creatures have small broods. Big things develop slowly and the gestation periods are long. Elephants, rhinos and hippos have one baby at a time. Deer, cows and horses occasionally have twins but a single baby is the norm. Hoofed creatures must be agile enough to outrun predators; a baby zebra can stand up and follow its mother just after being born. Producing a precocious baby stretches a mother’s resources; the foetus must be kept on board until fully kitted out with limbs, senses and insulating fur.

Ducks and farmyard chickens have a similar breeding strategy. Their young can see, hear, call and follow their mother when only a day old. But such luxuries come at a price; the egg of a precocious baby is big and heavy, a drain on the mother’s resources. Incubation also takes longer. A blackbird hatches her clutch in 13 days, whereas a mallard sits for 28. Blackbird chicks are on the wing 14 days after hatching, while the latch-key mallard youngsters, fending for themselves, take up to 60 days to fledge.