August 29, 2012
Male-only Childcare The Norm For The Marine Whelk
Watch the Video: A Snail Sex Story
April Flowers for redOrbit.com - Your Universe Online
Researchers at the University of California, Davis, have classified S.macrospira as belonging to a small club of reproductive outliers characterized by male-only childcare. Throw in extensive promiscuity and sibling cannibalism, and the species has one of the most extreme life histories in the animal kingdom.
This tiny snail lives in the tidal mudflats off Baja California, and the S. macrospira family secrets can be found in an online study in the journal Ecology Letters.
On average, the study finds, 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 a 1973 issue of The Festivus, an amateur shell-collectors newsletter. Grosberg started studying the snails when he brought some back from a shell collecting trip and realized that only the male snails had egg capsules on their shells.
When S. macrospira snails mate, the female glues capsules containing hundreds of eggs each to the male's shell. The male's shell acts as a substitute rock, since the habitat of S. macrospira contains few surfaces on which to glue eggs.
Because the male moves in and out with the tide, unlike a stationary rock, the egg capsules are protected from the extremes of heat and drying they might face otherwise.
Each capsule may contain up to 250 eggs, and the male's shell becomes covered in dozens of them. Hatching takes approximately a month, and as the babies emerge they begin to eat each other with only a handful of survivors from each capsule living to crawl away.
Stephanie Kamel, a postdoctoral researcher, carried out a DNA analysis of brood capsules to determine the eggs' parentage. She found that, on average, the males sire just about 24 percent of the offspring on their backs.
"The promiscuity in the female snails is extraordinary," Kamel said, noting that some females mate with as many as a dozen different males.
In the typical animal reproductive scenario, females invest more resources in an egg than a male does in a sperm, so mothers have a stronger interest in providing child care. Males may mate with multiple partners to increase their chances of siring offspring, but they typically make less of an investment in caring for those young. When fathers do get involved, it's nearly always because they are assured that all or most of the offspring are their own. For example, male seahorses carry developing young in a pouch, but all of the babies are genetically their own.
One explanation for the unusual mating habits of the S. macrospira could be that caring for the offspring doesn't cost the male snails much. Grosberg refuted this theory by tethering male snails to a post sunk in the sand. The snails he followed showed that the capsules do impose a significant burden reflected in weight loss of the male.
It may be that carrying the egg capsules simply represents the best of limited options for the males, Grosberg said, since it's impossible for them to mate without the female attaching an egg capsule to their backs. Another theory is that carrying egg capsules may be a way for a male to show a female that he's good parent material.
"If he wants to get any action, he has to pay the price," Grosberg said.
Grosberg's research stems from his fascination with the conflicts that occur between parents, between siblings, and between parents and offspring as they each try to get resources and maximize their success in breeding. He notes that these conflicts are apparent from simple animals like the snail to human beings.
"Everything that intrigues me about family life happens in these snails," he said.
At the same time, no animal has gone as far as humans in evolving increasing cooperation between relatives, tribes and larger and larger (and less closely related) groups over time.
"We're good at seeing other forms of reward," Grosberg said.