Washington: In a new study, researchers have revealed that, on average, only one in four of the hundreds of eggs that a male snail carries around on his back belong to him.
Some Solenosteira macrospira carry the offsprings 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 behaviour 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 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, 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 survive 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.
They do so because typically in the animal kingdom, females invest more resources in an egg than a male does in a sperm, so mothers have a stronger interest in providing parental care.
Males may mate with multiple partners to increase their chances of siring offspring, but typically make less investment in caring for those young.
When dads do get involved, it’s nearly always because they are assured that all or most of the offspring are their own. Male sea horses, for example, carry developing young in a pouch — but all are their own genetic offspring.
The study has been published online in the journal Ecology Letters.