Maternal care and interactions within and between families : how the environment and chemical communication shape family life in a social insect

The evolution of parental care represents an important step in the evolution of sociality and is widespread across different species and different taxa. Parental care is a trait that shows a wide diversity regarding duration and forms of care within and between species and is an important field of research both in evolutionary biology and behavioral ecology. However, its importance in these fields has only been recognized relatively recently. Environmental effects influence condition and individuals will choose distinct behavioral strategies to maximize their fitness. Condition can be communicated through chemicals and such condition-dependent cues can be used by conspecifics to adjust their own behavior. The focus of my dissertation was the investigation of environmental effects and chemical signaling on maternal care and within and between family interactions. I used the European earwig (Forficula auricularia) as a model system for my experimental work.
As an introductory chapter, I have written a review article about the evolution of parental care in insects (Chapter 1). Here, I summarized present hypotheses about the roles of ecology and life history from the literature and combined them with new suggestions regarding the influence of social interactions on parental care.
In my first experiment (Chapter 2), I investigated the influence of maternal nutritional condition on mother-offspring interactions. Female condition was manipulated through a high-food and a low-food treatment. I could show that the period and amount of maternal food provisioning was dependent on the condition of the female. Females in poor condition provided food to fewer nymphs and for a shorter period of time compared to females in good condition. Offspring attendance remained at a constantly high level independent of female condition and was maintained by both the female and the nymphs, suggesting strong benefits of living in a (family) group.
In my second experiment (Chapter 3), I investigated effects of nymph condition and food availability on brood mixing in F. auricularia. Females provide care for foreign nymphs that join their brood. This however increases brood size and thus competition between the offspring. Previous work showed that cannibalism is directed primarily against unrelated nymphs under conditions of low food availability. My results showed that the brood mixing dynamics are influenced by the condition of the nymphs, but are independent of the food availability in the environment. The overall degree of brood mixing was high, suggesting again benefits of living in groups.
Furthermore, my third experiment (Chapter 4) tested the presence of family specific cuticular hydrocarbon profiles. Insects mainly use cuticular hydrocarbons as means of communication and individual recognition. Previous results from our group show that there are significant negative effects of inbreeding, which makes kin recognition important in this species. I could show that cuticular hydrocarbon profiles are indeed family specific and that the earwigs have the potential to use them to discriminate kin from non-kin. This allows to direct social behaviors to the appropriate individuals and to avoid inbreeding and the associated fitness losses.
In a fourth experiment (Chapter 5), I tested the effect of maternal condition-dependent cues on nymph selfishness and survival. In species where parental care is provided, offspring could use cues of parental condition to adjust their begging behavior and their selfishness by varying the degree of sibling competition. My results show that maternal condition cues influence offspring survival depending on the time of breeding, and further suggest that offspring use these maternal condition-dependent cues to adjust their degree of selfishness, which changes during the breeding season. Together with former evidence on maternal sensitivities to condition-dependent nymph chemical cues, my study shows context-dependent reciprocal information exchange about condition between earwig mothers and their offspring, mediated by cuticular hydrocarbons.
My last experiment (Chapter 6) investigated the effects of high and low food availability during the juvenile and early adult development and its influences on development, maternal care and egg production. Females of the European earwig represent two distinct phenotypes. One type produces only one clutch, the other type produces two clutches during its life. Previous experiments have shown that the phenotype is not purely inherited genetically, but likely to be condition-dependent. My results revealed that especially restriction in the late juvenile development has negative effects on development and the probability of second clutch production. Environmental conditions experienced early in an individual’s lifetime can have detrimental effects once individuals become adult and need to be considered to understand individual variation in reproductive success and life-history trade-offs.
All in all my results show that behavioral strategies are driven by the condition of the individuals involved. I demonstrated that condition is reflected in the cuticular hydrocarbon profiles of the earwigs. Such condition-dependent chemical cues allow individuals to adjust their behavior according to their own state and to the state of interacting conspecifics. I showed how the environment and chemical communication shape family life in a social insect and revealed how this affects not only maternal care and social interactions within and between families, but also reproductive success of individuals. Thus, my work shows how individual condition affects not only parent-offspring interactions, but also major life history traits like reproductive success.