Parasites can influence many aspects of their hosts' lives, including the mates they chose. Animals may mate with less-parasitized individuals, and, by doing so, ensure that genes for parasite resistance are passed on to their offspring. A large body of work has focused on examining how parasites influence mate choice by measuring immune response and determining whether preferred mates have a stronger immune response. This assumes that a stronger immune response results in fewer parasites infecting the host. However, this assumption is rarely tested. Moreover, many studies have found conflicting results, with some reporting that preferred mates have a stronger immune response, while others report the exact opposite result. Dr. Zuk and Ms. Jacobs will clarify the role of immunity in determining parasite levels and mate choice using an experiment conducted on western bluebirds. They will use feeding trials, in which some individuals receive supplemental food, while others are left alone as controls, to examine how immune response influences parasite levels in wild birds. Supplemental food has been shown to allow individuals to mount a stronger immune response; if a strong immune response always results in lower parasite loads, then fed birds should have fewer parasites, and birds with stronger immunity and fewer parasites should be preferred as mates. The investigators will use new molecular methods to accurately measure infection with avian malaria, a common parasite of birds. This project will help researchers better understand what affects immune response and parasite levels in wild animals. Such knowledge has important consequences for human health, as many emerging diseases in humans come from animals. Results of this project will be broadly disseminated using open access journals, and the project itself will provide an opportunity to train undergraduate research assistants in basic field and research techniques.
This grant was awarded to us to study how parasite infection and immunity affect mate choice in wild birds. In many animals, including birds, we expect females to be very selective about the males with which they mate. Females should choose to mate with unparasitized males because these males have been successful at fighting off parasites, and may pass on the genes for that success to their offspring. This success may depend in part on immunity. We set out to test some of these ideas in western bluebirds (Sialia mexicana). We found some evidence that females may be selective about their mates. In particular, males with brighter blue feathers sired more of the offspring in their own nests. In bluebirds, as in many species of birds, females will mate with their social mate (the male with whom they nest), but also with other males from neighboring areas. Some of her offspring may be fathered by these neighboring males. However, females paired to brighter blue males had fewer of these extra-pair offspring. In addition, males with bright blue feathers tended to pair with females that also had bright blue feathers, implying that males may be choosy about their mates as well. When we examined immune response and parasites, we found that individuals infected with avian malaria tended to have fewer parasites in their blood if they had a strong immune response. It was possible for a bird to completely get rid of the infection, although this only happened in a few cases. Immune response did not predict how good a male was at producing offspring or how bright his feathers were. The parasites themselves, however, did matter in reproduction. Males that were not infected with avian malaria were more likely to mate and sire extra-pair offspring with females at other territories. This could be because females prefer to mate with these uninfected males or because infected males are less likely to seek out other females. As part of this project, we studied immune response and parasites in wild animals. This is an important area because wildlife diseases can have a huge impact on conservation. Avian malaria, the parasite we studied, has caused problems for bird populations in Hawaii, for example. Our findings indicate that immunity can help control the infection, but that birds rarely get rid of the parasite completely. Also, some wildlife diseases have the ability to transmit to humans, making it important to understand how parasites and their hosts interact in the wild.
