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Within-clutch egg-size variation in a subsocial bug: the positional effect hypothesis

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If there are differences in predation risk among the offspring within a clutch, parents should allocate less resources to the offspring facing higher risk. Predation risk, and thus offspring size, may depend on the spatial position of individual offspring within a clutch. To test this positional effect hypothesis, I examined egg-size (egg-mass) variation in the subsocial bug Elasmucha signoreti Scott, 1874 (Hemiptera: Acanthosomatidae). In subsocial insects, including Elasmucha, in which females guard their clutches against predators by covering the clutch with their bodies, there are large differences in survival between offspring at the centre and at the periphery of the clutch. There was considerable variation in reproductive output among females; female body size was positively correlated with egg mass but not with clutch size. Females laid significantly lighter eggs in the peripheral, and thus more vulnerable, part of the clutch. No phenotypic trade-off between egg mass and clutch size was detected. Egg mass was positively correlated with hatched first-instar nymph mass. Thus, E. signoreti females seem to allocate their resources according to the different predation risks faced by the offspring within a clutch. I suggest that the positional effect hypothesis can generally be applicable to species whose females lay eggs in clutches and that the eggs suffer different mortality rates which depend on their spatial positions within the clutch.

Lorsqu’il y a des différences dans le risque de prédation parmi les rejetons d’une même ponte, les parents devraient allouer moins de ressources aux petits qui courent un risque plus élevé. Le risque de prédation et donc la taille du rejeton peuvent donc dépendre de la position spatiale du rejeton en question dans la portée. Afin de vérifier cette hypothèse sur l’effet de la position, la variation de la taille (masse) des oeufs a été déterminée chez la punaise subsociale Elasmucha signoreti Scott, 1874 (Hemiptera: Acanthosomatidae). Chez les insectes subsociaux, tels qu’Elasmucha, chez lesquels la femelle protège sa ponte des prédateurs en la couvrant de son corps, il existe de fortes différences dans la survie des petits entre la périphérie et le centre de la ponte. Le rendement reproductif chez les femelles est très variable; il y a une corrélation positive entre la taille corporelle de la femelle et la masse des oeufs, mais pas avec l’importance de la ponte. Les femelles pondent des oeufs significativement plus légers en périphérie de la ponte, donc dans la zone la plus vulnérable. Il n’y a pas de compromis phénotypique entre la masse des oeufs et la taille de la ponte. La masse de l’oeuf est en corrélation positive avec la masse de la larve néonate de premier stade. Ainsi, les femelles d’E. signoreti semblent attribuer leurs ressources en fonction des risques de prédations courus par leurs rejetons au sein de la portée. L’hypothèse de l’effet de la position semble donc s’appliquer de façon générale aux espèces dont les femelles pondent leurs oeufs en masses et dont les oeufs subissent des taux de mortalité différents d’après leur position dans la masse d’oeufs.

Document Type: Research Article

Publication date: 2006-11-01

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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