Sibling rivalry takes a grim turn in the parasitoid wasp species Isodontia harmandi, as larvae commonly cannibalize their nest mates. A new study suggests the stronger larvae eat their siblings both to grab an easy meal and to reduce competition for the food provided by their mother. Shown here, an I. harmandi larva feeds on its nest mate. (Photo by Yui Imasaki, Ph.D., and Tomoji Endo, Ph.D., via PLOS ONE, CC-BY 4.0)

By Ed Ricciuti

Ed Ricciuti

While episodes of sibling rivalry among humans can sometimes be amusing, it is not so funny in the animal kingdom, where it often results in cannibalism and “siblicide.” Researchers at Japan’s Kobe College have been studying this phenomenon in the parasitoid wasp Isodontia harmandi for the past few years. In a study published last week in Environmental Entomology, they reveal how I. harmandi larvae chow down on nest mates with gruesome regularity.

Eating one’s peers, in this case, has a double benefit. The cannibals not only reduce competition from their siblings but also grab an easy meal, especially valuable when the paralyzed grasshoppers and other insects provided as food by their mother run low.

Competition for parental care or food—or both—is surprisingly rife among young animals. It may be as simple as one nestling bird monopolizing all the food brought by parents while its sibling perishes. Or, one sibling may kill and eat another; sand tiger shark pups do it while still in the womb.

The researchers—Yui Imasaki, Ph.D., who conducted the study as part of a doctoral thesis at Kobe College, and Tomoji Endo, Ph.D., professor emeritus in the Kobe College Department of Biosphere Science—confirmed routine cannibalism among larvae of the Isodontia harmandi wasp in previous research. Now they have provided a closer look at the grisly details in the context of sibling rivalries as a whole. Some rivalries aim at eliminating sibling competitors for food resources, others at using them as a resource. And in some, as in I. harmandi, it is both.

“Brood reduction,” write the scientists, “occurs routinely through larval cannibalism in I. harmandi nests.”

The results of their study “strongly imply that wasp larvae commit sibling cannibalism in the context of resource-based sibling rivalry, ensuring the double benefit of nutritional gains and reduced resource competition,” the researchers write. The two-pronged approach doubles the benefits of a successful rivalry.

Among the other findings:

• Siblings are more likely to turn cannibal when normal food runs short, especially if the decline in prey was rapid, but before it was gone.
• Females are more likely to cannibalize than males.
• Larvae cannibalized others through all stages of development.
• Bigger larvae were more likely to cannibalize and got even bigger as a result.

Sibling rivalry takes a grim turn in the parasitoid wasp species Isodontia harmandi, as larvae commonly cannibalize their nest mates. A new study suggests the stronger larvae eat their siblings both to grab an easy meal and to reduce competition for the food provided by their mother. Shown here, an I. harmandi larva feeds on its nest mate in a lab experiment setup. (Photo courtesy of Tomoji Endo, Ph.D.)

The study indicates that the growth rate of larvae that become victims often slow before they are attacked, suggesting that cannibals target individuals with less chance of surviving in the first place. It is possible that the cannibalistic larvae can assess the chances of survival of a potential victim and focus on the least fit. The clue to a larva’s survivability may be a change in chemicals called cuticle hydrocarbons, which form a waxy coat on insects that prevents desiccation and have communicative roles. Whether or not I. harmandi larvae can indeed assess fitness in their siblings, cannibalism has a positive evolutionary impact in that it eliminates the unfit from the gene pool.

Found in Japan and Korea, I. harmandi is one of the so-called “solitary wasps,” a large group whose larvae grow up not in individual brood cells of a communal nest but in a nest chamber. Females generally lay one or more eggs in or on one paralyzed host. In many such species, competition among larvae for the food results in cannibalism. The female I. harmandi opts for a different method. She looks for a pre-existing tube, such as in a cut bamboo or abandoned beetle tunnel, which she lines with grasses. The female stows prey in the nest and deposits a fertilized  egg on it. She may repeat the process a dozen or so times, finally plugging the tunnel with moss, bark or other plant material when she is done laying.

“Communal brooding (gregarious development) in Isodontia harmandi is highly unusual among solitary nesting wasps,” says Endo. The departure from the nesting norm led the scientists to assume that it suppressed larval cannibalism, which would usually occur among voracious larvae of solitary wasps.

“Our research shows that cannibalism does occur and is regulated by prey availability in this species,” says Endo. “We think that this finding can promote the understanding of sibling cannibalism as an adaptive solution to the extreme condition. Secondly, our study system allows us to visualize the process of cannibalistic interaction.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.