Increased activity correlates with reduced ability to mount immune defenses to endotoxin in zebra finches.

When suffering from infection, animals experience behavioral and physiological alterations that potentiate the immune system's ability to fight pathogens. The behavioral component of this response, termed "sickness behavior," is characterized by an overall reduction in physical activity. A growing number of reports demonstrate substantial flexibility in these sickness behaviors, which can be partially overcome in response to mates, intruders and parental duties. Since it is hypothesized that adopting sickness behaviors frees energetic resources for mounting an immune response, we tested whether diminished immune responses coincided with reduced sickness behaviors by housing male zebra finches (Taeniopygia guttata) in social conditions that alter their behavioral response to an endotoxin. To facilitate our data collection, we developed and built a miniaturized sensor capable of detecting changes in dorsoventral acceleration and categorizing them as different behaviors when attached to the finches. We found that the immune defenses (quantified as haptoglobin-like activity, ability to change body temperature and bacterial killing capacity) increased as a function of increased time spent resting. The findings indicate that when animals are sick attenuation of sickness behaviors may exact costs, such as reduced immune function. The extent of these costs depends on how relevant the affected components of immunity are for fighting a specific infection.