Parasites can have profound effects on host behavioral type and behavioral correlations. Pre-infection, behavior can influence the risk of becoming infected by parasites. For example, individuals who are more active may increase their risk of encountering parasites, putting that individual at a greater risk of becoming infected relative to a less active individual. When parasites have strong negative implications for host fitness, particular behavioral traits or behavioral correlations which decrease risk of infection may be under strong selection. Post-infection, sickness behavior or parasite manipulation of host behavior can modify numerous aspects of average host behavior and correlations between behaviors. Reviews on the effects of parasites on host personality traits can be found here and here. Information on the study systems the Sih Lab uses to look for interactions between parasites and host behavior are described below.
California killifish and their behaviorally manipulative trematode parasites
California killifish are one of the most abundant fish species in the estuaries of Southern California and Baja California. These fish are host to Euhaplorchis californiensis (EUHA) and Renicola buchanani (RENB), two behaviorally manipulative trematode parasites. EUHA lives on the killifish’s brain, while RENB lives in the liver. Infection with RENB and EUHA is associated with an increase in the frequency of conspicuous behaviors in California killifish. These conspicuous behaviors include flashing the killifish’s silvery belly up towards the sun, quick jerks forward, and body contortions. Conspicuous behaviors not only draw the attention of human observers, but also appear to draw the attention of predatory birds. Relative to uninfected killifish, infected killifish are 10-30 times more likely to be consumed by predatory birds, which are the definitive host for both parasites! This is a classic example of parasite behavioral manipulation, in which parasites modify the behavior of their hosts in order to increase their probability of being transmitted to the next host in the parasite’s life cycle. While the mechanisms through which manipulation occurs are still unclear, changes in neurotransmitter activity following infection with EUHA have been documented. These neurotransmitters are associated with behavioral traits and behavioral correlations in other fish species, and so we expect that infection with EUHA may modify numerous aspects of host behavior. A video documenting this manipulation can be found here.
Stress and sex hormones may also play an important role in behavioral interactions between parasites and host behavior, as they both influence host behavior (which could affect the probability of encountering a parasite) and influence immune system functioning (which could increase or decrease the risk of infection given an encounter with a parasite). Additionally, either parasite could be manipulating the production of hormones, with subsequent implications for host behavior. In collaboration with the Ecological Parasitology Lab at the University of California Santa Barbara and the Earley Lab at the University of Alabama, we are exploring how killifish behavioral traits and hormones influence an individual’s risk of becoming infected by EUHA and RENB as well as how behavior and hormones change in the killifish following infection.