Mothers modify eggs into shields to protect offspring from parasitism

Abstract

Eggs are an immobile, vulnerable stage of development and their success often depends on the oviposition decisions of the mother. Studies show that female animals, and sometimes males, may invest parental resources in order to increase the survival of their offspring. Here, we describe a unique form of parental investment in offspring survival. The seed beetle Mimosestes amicus may lay eggs singly, or may cover eggs with additional egg(s). This egg stacking serves to significantly reduce the mortality of the protected egg from parasitism by the parasitic wasp, Uscana semifumipennis. The smaller top eggs serve only as protective shields; they are inviable, and wasps that develop in them suffer negative fitness consequences. Further, we found egg stacking to be inducible; M. amicus increase the number of stacks they lay when parasitoids are present. However, stacking invokes a cost. When wasps are absent, beetles lay more single eggs, and produce more offspring, highlighting the adaptive value of this extraordinary example of behavioural plasticity in parental investment.

Egg and time limitation mediate an egg protection strategy

“…These results suggest that the incidence and level of stacking have not been shaped by the same evolutionary dynamics. Although we did not vary parasitism risk continuously in this experiment, we have found in our previous study that beetles lay larger stacks as parasitism risk increases, and larger stacks confer better protection in environments where wasps are able to attack bottom eggs (Deas & Hunter, 2012). In environments of high parasitism risk, there is an advantage to having higher potential fecundity, and we should see selection for larger female body size.…”

“…When exposed to egg parasitism cues, females respond by superimposing one to three protective eggs on top of a viable egg, thus reducing or preventing its access to the egg parasitoid, Uscana semifumipennis. In contrast to other studies where laying multiple eggs per oviposition is triggered by changes in host availability or quality, egg stacking behaviour is triggered by a parasitism cue whenever it occurs, regardless of previous host experience (Deas & Hunter, 2012). This beetle system thus provides an opportunity for a novel test of general theory in which physiological state can be manipulated independently of the cue that triggers the behaviour.…”

“…Each egg in a stack is laid before a female moves on to lay her next stack or single egg. Upper eggs in a stack protect the bottom egg from parasitism by U. semifumipennis (Deas & Hunter, 2012). They do not produce beetle progeny even in the absence of parasitoids, but may support the development of a parasitoid.…”

“…We used the principles of egg and time limitation theory to generate predictions on how host availability ultimately affects the frequency and level of protective oviposition behaviour. Then, we tested the effects of unlimited host access and host deprivation over time on the protective 'egg stacking' behaviour in the seed beetle, Mimosestes amicus (Deas & Hunter, 2012). When exposed to egg parasitism cues, females respond by superimposing one to three protective eggs on top of a viable egg, thus reducing or preventing its access to the egg parasitoid, Uscana semifumipennis.…”

Multiple Lines of Egg Defense in a Neotropical Arachnid with Temporary Brood Desertion

“…), (4) producing physical barriers that hinder predator access to their clutch, such as burrows (e.g., vipers: Greene et al. ) or a layer of non‐viable eggs (e.g., beetles: Deas & Hunter ), or (5) making the embryo itself less vulnerable to attacks, such as a hard layer covering the chorion (e.g., beetles: Ang et al. ).…”