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

Abstract

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.

Within-Clutch Variation in Yolk Testosterone as an Adaptive Maternal Effect to Modulate Avian Sibling Competition: Evidence from a Comparative Study

“…For example, studies on montane lizards show complex interactions between maternal hormones, egg size, and sex on prenatal development rates that cause variation in hatching times and thus confer developmental advantages on select offspring (Radder and Shine 2007). Subsocial bugs covary egg size and spatial position within the clutch so that peripheral (insurance) eggs that are exposed to higher predation risk are smaller (Kudo 2006). In plants, the potential for maternal effects in enhancing growth rates, accelerating emergence, or increasing seed weight as an avenue toward conferring an early size advantage to seedlings has been implied (e.g., Stanton 1984a) but not directly studied in the context of sibling rivalry.…”

Coping with environmental uncertainty: dynamic bet hedging as a maternal effect

“…Mothers may increase egg size variability in order to increase arithmetic mean fitness. For example, in stinkbugs, Kudo (2001) found subsocial species to lay smaller eggs around the periphery of the clutch in response to the positional risk of egg predation, whereas asocial species produce offspring of similar size (Kudo 2001(Kudo , 2006. In this example, producing offspring of different sizes increases the number of surviving offspring in a constant and predictable environment.…”

The Relationship Between Maternal Phenotype and Offspring Quality: Do Older Mothers Really Produce the Best Offspring?

“…If offspring do not disperse immediately following release from mothers (both from the mother and from each other), then differences in fecundity can result in differences in the environment offspring experience (Einum and Fleming 2002). For example, differences in maternal fecundity can affect the offspring's oxygen availability and risk of predation-both of which are likely to affect the optimal size of offspring (Strathmann and Chaffee 1984, Strathmann 1995, Hendry et al 2001, Hendry and Day 2003, Kudo 2006. Most importantly, when offspring are not immediately highly dispersive, differences in maternal fecundity are likely to result in differences in the level of intraspecific (sibling) competition, and this may have strong effects on optimal offspring size (Parker and Begon 1986).…”