Behavioural changes such as lethargy, often referred to as ‘sickness behaviours’, are generalized responses to infection (Hart 1988
) that may serve as informative cues for avoidance of conspecifics infected with a broad range of pathogens. However, in this study, healthy male house finches responded to a sickness behaviour—the reduced potential to win—by increasing time spent feeding near diseased individuals: the more likely the healthy conspecific in a given trial was to win aggressive interactions with the focal bird, the stronger the feeding preference towards the less competitive, diseased conspecific. These results suggest that healthy males seek to avoid costly social defeats, which have been shown to dynamically suppress immune responses in this species (Hawley 2006
), by foraging in the vicinity of individuals with the lowest potential for winning aggressive interactions. This study is the first demonstration of individuals using visual cues (i.e. behaviour) to alter their behaviour towards diseased conspecifics, although the additional use of chemosensory cues (Hagelin & Jones 2007
) cannot be excluded by our experimental design.
The sickness behaviour of infected male house finches may act as an ‘evolutionary trap’ by presenting a historically beneficial cue (i.e. reduced aggression) in the context of a new environment (Schlaepfer et al. 2002
), in this case, a recently emerged pathogen. Before the emergence of MG in house finches, the risk of exposure to pathogens carried by non-aggressive individuals may have been outweighed by the benefits of access to resources, whereas the behavioural cue is now likely to carry fitness costs (Faustino et al. 2004
). Since associations between this host and the pathogen have been of relatively short evolutionary duration (approx. nine finch generations), sufficient time for the selection of avoidance of infected conspecifics may not yet have passed.
Why did we detect a feeding preference for diseased conspecifics only in male house finches? Our results indicate that this difference was not driven by focal bird behaviour: focal males and females responded similarly to differences in aggression, since the interaction between sex and differences in wins on feeding preference was non-significant. Therefore, in mixed-sex flocks, which are the rule during non-breeding in house finches, we expect females to be just as likely as males to prefer feeding in the proximity of infected, non-aggressive males. Instead, the detected sex-specific feeding preference resulted from behavioural differences in the conspecific house finches: infected male conspecifics were significantly less likely to win aggressive interactions than healthy male conspecifics, driving male focal birds to prefer feeding in their vicinity. Female conspecifics, however, showed no change in the potential to win with MG infection. It remains unknown why the potential to win interactions differed between healthy and diseased males and females in our study. Previous studies suggest that the suppression of sickness behaviours depends on life-history trade-offs and can, for instance, vary with body condition (Owen-Ashley et al. 2006
). Since female house finches tend to be dominant in aggressive interactions (Brown & Brown 1988
), the motivation for infected females to maintain social status may be higher than for infected males. Alternatively, the physiological costs of infection and/or social defeat may be higher for male house finches.
Infection-induced behavioural changes are prevalent among host–pathogen systems (Moore 2002
) and can alter the ability of a disease to invade and persist in a population (Lloyd-Smith et al. 2004
). The population-level consequences for the observed behavioural changes towards infected house finches are likely to be critical for MG dynamics. Transmission experiments indicate that MG deposited on a feeder is only infectious for short (12 h or less) timescales (Dhondt et al. 2007
). Therefore, house finches that associate in close proximity with infectious individuals while feeding may be most likely to contact viable MG and contribute to the continued persistence of MG epidemics in wild populations. The extent to which infection-induced behavioural changes such as those observed here drive transmission dynamics and epidemic persistence in host–pathogen systems is an area ripe for further exploration.