Egg-laying parasitism: Ecological shifts broaden host selection

The ecological instabilities affecting the world today will shape the evolution of interspecies relationships. For egg-laying parasites, this translates into a diversification of host choices. Climate change, territorial competition, and human presence all contribute to growing ecological uncertainty. In response, animals are adapting, but these adjustments can affect the dynamics of interactions between species and shape the course of evolution. This is particularly true for symbiotic species, which depend on relationships with others. Logically, the more unstable, variable, and unpredictable the ecological environment is, the less a parasitic species needs to depend on a single host species. Diversifying partners opens the door to more life options. Although this relationship is consistent, it has so far been difficult to establish empirically because environmental factors are not the only ones influencing the co-adaptation of species. However, American researchers have turned to the model of avian brood parasitism to provide a particularly relevant framework for understanding how uncertainty—driven by environmental variability and the phenology of symbionts—influences the evolution of interspecies interactions. Egg-laying parasites are birds that lay their eggs in the nests of other species and leave the rearing of their young to the host parents. They have evolved independently multiple times within the avian phylogeny and, depending on the species, target hosts with varying degrees of specificity. Some are more generalist, such as the brown-headed cowbird, which parasitizes some 300 host species, while others are more selective, such as the yellow-throated cuckoo, which parasitizes only a single host species. From an evolutionary perspective, species that have diverged more recently generally specialize in fewer host species, suggesting that new parasitic lineages arise through specialized interactions with a single host or a few hosts.

Parasitic virulence, the degree of co-adaptation (egg mimicry), the host’s reproductive strategy, and reproductive capacity are all factors to consider in understanding these choices.

Characterizing the factors that determine host diversity among egg-laying parasites has enabled researchers to better understand resource use—and thus the effect of resource instability—associated with host-parasite coevolution in avian systems. Phylogenetic, behavioral, and geographic data have thus shown that increasing uncertainty in thermoregulation and parental investment in young parasites are positively associated with host richness and diversity. Ecological uncertainty thus actively shapes the specialization of complex interspecies relationships. In the current context of global change, with climate patterns becoming increasingly variable and unpredictable, it is becoming important to understand the impact on co-evolutionary systems. Many parasites, predators, and symbionts—already threatened with extinction—are already facing ecological variability.