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Siblings of this wasp species eat each other as larvae

The “siblicide” study sheds light on why insects and other organisms kill their siblings during development.

(CN) — Cain and Abel. Mary Queen of Scots and Elizabeth. Liam and Noel Gallagher. Sibling rivalries are as old as anything.

But some forms of siblicide are more brutal than others. Consider a species of wasp known as isodontia harmandi. Their larvae are raised in a “communal brood cell,” a sort of pod that all the larvae share. Their mother brings them food, which they also share. Eventually, they spin cocoons, and metamorphose into wasps.

Except, not all the wasps make it that far. According to a new paper, published Wednesday in the peer-reviewed and open-access journal PLOS One, the wasp larvae “routinely” eat each other.

The researchers argue that the sibling cannibalism exists to solve a very real problem: too many babies, not enough food.

“Sibling cannibalism may efficiently regulate brood size in communal brood cells under prey shortage,” write the paper's authors, Yui Imasaki and Tomoji Endo, professors at Kobe College in Nishinomiya, Japan.

This phenomenon of siblicide has been observed in other organisms. A 1990 American Scientist paper titled “Avian Siblicide” found that nestling baby birds sometime kill off their siblings, and suggested this may be a “common adaptive strategy” that could hold some benefit for both the “surviving offspring and the parents.”

“Parents tend to produce more offspring than they can rear,” Imasaki and Endo wrote. “This problem of parental overproduction in a nursery is often solved by an intuitively paradoxical phenomenon: brood reduction through various forms of fatal sibling rivalry, such as abortion, obligate or facultative siblicide, filial cannibalism and sibling cannibalism.”

Sibling cannibalism is rare in birds, but is “relatively common among insects and amphibians.” The question has always been why.

"Our finding is neither the first description of brood reduction nor that of sibling cannibalism," said Professor Endo in an email. "There are many examples for both phenomena among various organisms. But there are few examples of species which brood size is routinely reduced by sibling cannibalism under food resource shortage. Our finding provide such an example."

Endo and Imasaki spent five years collecting isodontia nests in various stages of development. They found that the rate at which the larvae survived was “affected positively by total prey weight” — that is, the weight of the food they were given to eat — “and negatively by brood size,” that is, how many siblings were sharing a nest. By watching time-lapse video of the brood cells, the researchers concluded that three-quarters of all larvae deaths could be attributed to sibling cannibalism.

“Cannibals were usually larger than the victims,” they write. “Victims were often hatchling remaining on prey to which they were attached.”

They add: “After the cannibals touched the victim’s body with their mouthparts, they ate the victims without overt aggressive interactions. In one case, a victim and its prey item that it was feeding were shared by two or three larvae, and cannibalistic consumption was initiated by one of them.”

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