A research group led by Associate Professor Shin-ichi Kudo of the Graduate School of Education at Naruto University of Education, together with Graduate Student Taichi Nakahira (at the time of research) of the Research Faculty of Agriculture at Hokkaido University and Graduate Student Hirotaka Masamoto of the Graduate School of Systems Life Sciences at Kyushu University, announced that they experimentally demonstrated that hatched nymphs of the burrower bug discriminate between trophic eggs and viable eggs during cannibalism. The findings were published in the international journal Ecological Entomology of the Royal Entomological Society on September 1.
In some animals that engage in cannibalism, female parents are known to produce trophic eggs, which are special non-viable eggs that serve as food for their offspring.
Female parents of the burrower bug (order Hemiptera, class Insecta) build nests in depressions on the ground under fallen leaves, laying viable eggs in ball-shaped clusters and continuing to deposit trophic eggs on the surface of the egg mass until the viable eggs hatch. Females remain with the egg mass and provide parental care until the nymphs reach the second instar. Trophic eggs are slightly smaller than viable eggs and differ morphologically by lacking the micropyle, the sperm entry point. Hatched nymphs consume all the trophic eggs within about one day, after which they grow by feeding on seeds of host plants such as henbit that the female parent brings from outside the nest.
Previous research has shown that the hatching timing of this bug's eggs varies from egg to egg, with some cannibalism reported, although infrequent.
Theories explaining the evolution of cannibalism predict that offspring should selectively cannibalize trophic eggs while avoiding viable eggs that would become siblings, but there had been no empirical research demonstrating this.
Therefore, in this study, experiments were conducted by simultaneously providing hatched nymphs with two types of eggs among trophic eggs, viable eggs near hatching, and viable eggs with delayed development through cold treatment, investigating which eggs the nymphs would cannibalize. All viable eggs and trophic eggs used were from the same female.
As a result, when the nymphs were offered a combination of viable eggs near hatching and trophic eggs, the trophic eggs were selectively cannibalized. When they were offered viable eggs near hatching and developmentally delayed viable eggs (with embryos unclear to the naked eye), the developmentally delayed viable eggs were selectively cannibalized.
Furthermore, when offered a combination of trophic eggs and developmentally delayed viable eggs, there was a tendency for the developmentally delayed viable eggs to be cannibalized. Changing the spatial arrangement of eggs did not affect the selection.
These results indicated that viable eggs are potentially more suitable as food than trophic eggs. It is also indicated through a series of selection experiments that it is not the difference in external morphology between trophic eggs and viable eggs, but rather the presence of fully developed embryos just before hatching that holds the key to cannibalism avoidance.
Recently, numerous research findings have demonstrated that family members in stink bugs communicate through vibrations. For example, females of Adomerus rotundus, a closely related species to the burrower bug used in this research, have been shown to promote synchronous hatching by transmitting vibrations to egg masses just before hatching. It is believed that increased synchrony in hatching and subsequent behavior helps avoid cannibalism among siblings.
"Want to elucidate the evolution of parent-offspring relationships"
Kudo commented: "The behavior of parents and offspring in this species is as complex as that of birds. Parents and offspring appear to engage in strategic interactions to maximize their own interests. This sometimes manifests as cooperation and sometimes as conflict. It is an ideal subject for investigating the evolution of parent-offspring relationships, and I hope to reveal its full picture through future research."
Journal Information
Publication: Ecological Entomology
Title: Differential egg cannibalism by hatchlings in a burrower bug: Trophic and viable eggs
DOI: 10.1111/een.70009
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