Pity the male of the marine whelk（峨螺）, Solenosteira macrospira. He does all the work of raising the young, from egg-laying to hatching -- even though few of the baby snails are his own. The surprising new finding by researchers at the University of California, Davis, puts S. macrospira in a small club of reproductive outliers characterized by male-only child care. Throw in extensive promiscuity（乱交，混战） and sibling cannibalism, and the species has one of the most extreme life histories in the animal kingdom.

The family secrets of the snail, which lives in tidal mudflats off Baja California, are reported online in a study in the journal Ecology Letters.

In the study, UC Davis researchers report that, on average, only one in four of the hundreds of eggs that a male S. macrospira carries around on his back belong to him. Some carry the offspring of as many as 25 other males.

Such extreme cases provide the raw material on which natural selection can work and shed light on more "mainstream" species, said study author Rick Grosberg, a professor of evolution and ecology at UC Davis.

"It opens our eyes to viewing other kinds of behavior not as weird or harmful but as normal," he said.

The snails were first described in an amateur shell-collectors newsletter, The Festivus, in 1973. Grosberg started studying the animals in 1994, when he brought some back from a collecting trip and realized that only male snails had egg capsules on their shells.

When the snails mate, the female glues capsules containing hundreds of eggs each to the male's shell.

The male's shell likely acts as a substitute（代用品） rock, since the snails' habitat offers few surfaces on which to glue eggs, said co-author Stephanie Kamel, a postdoctoral researcher in Grosberg's lab.

Moving in and out with the tide on dad's (or stepdad's) back also protects the egg capsules from the extremes of heat and drying they might face if left on a stationary rock.

A male's shell may become covered in dozens of capsules, each containing up to 250 eggs. As the eggs hatch, a process that takes about a month, some of the baby snails devour the rest of their littermates. Typically only a handful of hatchlings survives the fratricide to emerge from a capsule and crawl away.

Kamel carried out DNA analysis of brood capsules to determine the eggs' parentage. On average, she found that the male snails had sired just 24 percent of the offspring on their backs. Many had sired far less.

"The promiscuity in the female snails is extraordinary," Kamel said, noting that some females mate with as many as a dozen different males.