Date of Degree
Behavior and Ethology | Ornithology
Brood Parasitism, Coevolution, Species Recognition
Social parasites exploit the behavioral repertoire of their hosts for their own benefit, thereby reducing host reproductive success. Whether and how hosts respond to prevent, reduce, or eliminate the costs of parasitism requires the characterization of host cognitive algorithms in response to parasites. In this dissertation, I review the suite of the defense behaviors and decision rules of hosts targeted by avian and insect brood parasites, and present new experimental data on the detection of parasitism through the visual system of focal host species. In Chapter 1, I review extensive data already accumulated to isolate the cognitive mechanisms used by avian hosts to assess, identify, and reject foreign eggs in the nest. The two most commonly evoked candidate heuristics are the discordancy mechanism, wherein the host rejects the egg most dissimilar in appearance to the other eggs in the clutch, and the template-based mechanism, wherein the host compares an egg to an internal template of its own eggs’ appearance. When experimenters directly pitted these competing mechanisms against one another, they found dominant support of the template-based process for egg discrimination. More recent and detailed analyses, however, to tease these mechanisms apart suggested that these alternatives are not mutually exclusive, and may be simultaneously activated to work in tandem to effect egg rejection decisions. Furthermore, Chapter 1 overviews a growing body of work demonstrating, at the individual level, the extent of plasticity that exists in setting the phenotypic threshold for own-foreign discrimination by hosts.
To examine egg rejection behaviors experimentally, the potential host species investigated must meet certain requirements. These include that they be rejecters of foreign eggs to some level of predictability and consistency in the first place. The globally ubiquitous house sparrow Passer domesticus is known to engage in conspecific brood parasitism, and its invasive proliferation across many continents and biomes represents a potentially attractive system to study anti brood parasite behaviors across ecological contexts and scales. Chapter 2, therefore, assesses the generalizability of previous studies demonstrating egg rejection patterns in house sparrows in Spain and South Africa, especially following the publication of more recent data from China suggesting that this species is not an egg rejecter. Here we robustly examined house sparrow responses to experimental parasitism in the distinct regions of North America, Israel, and New Zealand, and found negligible rejection rates in all three, suggesting that the house sparrow is not a suitable global model for antiparasitic egg rejection behaviors.
In Chapter 3, the cognitive mechanisms of rejection responses characterized in chapter 1 were experimentally tested by analyzing diverse published and unpublished datasets from the great reed warbler Acrocephalus arundinaceus, a well-studied host species of a mimetic race of the obligate brood parasitic common cuckoo Cuculus canorus. Specifically, the simultaneous activation of the discordancy and template-based decision rules suggested in chapter 1 was considered to test whether multiple methods were employed at the same time in a way that they may have interfered with one another to reduce rejection accuracy. Host individuals were experimentally parasitized with painted eggs of varying colors, quantities, and uniformities. Hosts were found to be more permissive of foreign eggs, and thus more error prone, when both the proportion of foreign eggs the nest increased and the eggs in the nest became more perceptually distinct from one another. This indicates that host defenses could be compromised by causing recognition mechanisms to yield differing rejection targets, and that multiple parasitism (or repeated targeting: more than one parasitic egg laid in the host nest) can mediate this beneficial outcome for the parasite itself.
No matter which cognitive egg rejection mechanism(s) is(are) employed, most studies agreed that the primary visual cue used by hosts to distinguish foreign eggs is the overall degree of distance in color between the egg being assessed and the host’s own egg(s). Until recently, rejection decisions were attributed to the absolute (regardless of direction) perceptual distance between own vs. foreign eggs. Chapter 4 is a new original but also parallel study to Hanley et al. 2017 and 2019’s discoveries that directional difference on a continuous color gradient of avian eggshell colors may be the relevant salient recognition cue. In particular, Hanley et al. found that their hosts preferentially rejected eggs browner than their own, but not eggs more blue/green than their own, suggesting a single threshold of rejection only on one side of the natural avian color gradient, rather than multiple symmetrical thresholds of absolute distance. We examined this phenomenon focusing on the European redstart Phoenicurus phoenicurus, which in contrast to the house sparrow observed in Chapter 2, consistently demonstrated rejection of non-mimetic eggs and acceptance of mimetic eggs in prior studies, thereby providing an attractive subject for variable egg rejection rates to investigate the limits of color-based rejection threshold(s). In addition, this study assessed Hanley et al.’s hypothesis in the context of a host species parasitized by a mimetic parasite race, which none of their prior studies included. Specifically, we experimented with redstarts in Finland, where they were simultaneously under parasitic pressure from the common cuckoo, and in the Czech Republic, where no parasitic pressure was present. Using 3D printed eggs painted along a continuous color gradient of natural brown to blue/green avian eggshell background colors, we experimentally parasitized redstart nests and recorded their rejection behaviors. In support of the single threshold model, we found the redstarts, regardless of locality, preferentially rejected noticeably browner eggs but not noticeably more blue/green eggs.
Finally, in Chapter 5, I shift the lens to the insect kingdom to examine highly analogous host vs. brood parasite systems to what we have seen in birds. Though the exact antiparasitic sensory modalities and recognition mechanisms differ, the evolutionary arms race of mimicry and recognition as parasites attempt to exploit unrelated individuals for offspring care is remarkably similar, and the relative advantages of examining host-parasite interactions from this new perspective are carefully enumerated. Relative to avian brood parasitism, the study of social parasitism in insects is still patchy, even when there are strong analogies (such as parasitic larvae manipulating caretakers to receive disproportionate attention, just as many avian brood parasite chicks manipulate foster parents to receive biasedly greater share of the provisioning), yet the sensory mechanisms of the larval manipulation remain largely unknown. Making use of these multiple perspectives on host-parasite dynamics across taxa can inspire more cohesive research across taxonomic boundaries. Such work then also inspires both conceptual advancement and applied analyses, for example, in the context of an impending conservation crisis as the collapse of honeybee colonies in Africa accelerates due to the recent surge of virulence of its brood parasitic congener.
All but Chapter 4 of this dissertation represent peer-reviewed and published articles that have already appeared in print and online as Manna et al. Chapter 4 in turn, will be the basis of a new manuscript with the same first author and institutional affiliation, yet again.
Manna, Thomas J., "Cognitive Models of Defense Behaviors in Hosts of Brood Parasites" (2019). CUNY Academic Works.