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Nurture vs. Not

You're standing by a pond. It's spring, and a female Wood Duck is leading a flotilla of newborn ducklings across the water. The ducklings mimic their mother, dabbling at the glassy surface and figuring out what's good to eat by trial and error. Though flightless, these ducklings are already great swimmers and surprisingly nimble on land.

Once the ducks pass, a Red-winged Blackbird catches your eye. It's at its nest. You can hear the insistent begging of newly hatched young. The adult blackbird feeds the youngsters, then flies to the edge of the pond to catch insects before returning to the nest for more feeding. It's a ritual that the blackbird and its mate will perform countless times until their offspring fledge and can hunt for themselves.

Yet the ducklings and the blackbird chicks were born on the same day. The reason the two species have such different approaches to development and dependency is that ducklings, as the scientists say, are precocial and blackbird chicks are altricial.

Precocial animals are born well developed and can accompany their parent away from the place of birth. The eggs of precocial birds are usually large, contain about 35 percent yolk, and represent a huge investment of their mothers stored energy reserves.

Many ground-nesting birds are precocial. Killdeer, for example, lay their eggs in open gravelly places where the eggs are nearly invisible. As soon as the young hatch, though, they must be able to move and seek shelter if threatened by a predator.

The most extreme example of precocial birds is the moundbuilders of Australia. These birds lay huge eggs in mounds of organic matter that generate the heat required for incubation via composting, and the young hatch fully feathered and ready to fly.

All songbirds, by contrast, are altricial. Their nests are usually well concealed or situated in places where predators won't have easy access to the defenseless young. In the case of blackbirds, their eggs are relatively small and contain only 20 percent yolk. Thus most of the nutrition that fuels growth comes from insects fed to the hatchlings during the weeks before their first flight. And these birds hatch without protecting feathers; even their eyes are closed.

The terms apply to other groups of animals, too. All reptiles are precocial. Mammals, however, demonstrate extremes of both systems. Rodents are altricial. Most hoofed mammals are precocial. A North American Pronghorn at four days of age can outrun a human.

Speaking of humans, we're technically precocial -- our eyes are open at birth, and there's hair present -- but it takes years before a human child is truly independent.

 

 

 

 

 

2007 eNature.com