Patterns of parental care in invertebrates

The tremendous diversity of social behaviour among the invertebrates is an asset and a challenge. There is richness in both the number of phylogenetic lineages that have evolved extended parental care and the forms of care provided by parents. Fifty families of insects in more than a dozen orders have evolved parental care (Costa 2006), and sixteen separate lineages of arthropods exhibit paternal care (Tallamy 2001; Nazareth and Machado 2009). There are three principal avenues for progress in evolutionary biology: new theory to test, new technology for measurement, and new subject matter. This last avenue is still wide open for students of parental care in invertebrates. Among birds and mammals, extended parental care is universal (Chapter 4) and most forms of care have been well studied. On the other hand, new species and new forms of care in the invertebrates are being discovered yearly, permitting a creative interplay between inductive and deductive approaches. The non-eusocial invertebrates are where we will find the subject matter for understanding the origins of parental care, transitions between types of care, and manipulable systems for testing theories. Broad phylogenetic comparisons will permit testing of the ecological factors that favour different social solutions, and also whether convergent parental behaviour is built upon convergent physiological mechanisms. More narrow comparisons will reveal how closely related species can take widely divergent social paths. For studies of single species, invertebrates offer many pragmatic advantages over vertebrates. First, most invertebrates live in a very different sensory world than humans. While this imposes barriers to our understanding, it also allows manipulations, such as observation under red light, that have minimal effect on experimental subjects. Second, the smaller size and shorter lives of many invertebrates make it practical to follow large numbers of subjects over their lifetime, often in the laboratory. Third, many invertebrates are also suitable subjects for selection experiments and genetic analysis, which will eventually allow an understanding of how interactions between genes and developmental environments produce the tremendous variation in parental care and social behaviour. According to broad definitions of parental care, care includes all parental traits that enhance offspring fitness (Chapter 1). For the purpose of discussing the origins and transitions of extended care in the present chapter, consideration is confined primarily to post-fertilization traits that increase offspring fitness, beyond the temporary housing and passage of the fertilized egg within the female. Viviparity and ovoviviparity will therefore be discussed, but the amount of yolk in an egg and selection of an oviposition site will not. After an overview of the forms of parental care, the origins, transitions, and loss of parental care will be discussed, including male versus female care. Lastly, the microbiology of care is salient for appreciating the complexity of social invertebrates and needs to be integrated into our understanding of parental care. The physiology of care is not treated here because this topic was covered in a prior review (Trumbo 2002).