House finch responses to Mycoplasma gallisepticum infection do not vary with experimentally increased aggression

Abstract

Aggression can alter infectious disease dynamics through two, non-exclusive mechanisms: 1) increasing direct contact among hosts and 2) altering hosts' physiological response to pathogens. Here we examined the latter mechanism in a social songbird by manipulating intraspecific aggression in the absence of direct physical contact. We asked whether the extent of aggression an individual experiences alters glucocorticoid levels, androgen levels, and individual responses to infection in an ecologically relevant disease model: house finches (Haemorhous mexicanus) infected with Mycoplasma gallisepticum (MG). Wild-caught male finches were housed in one of three settings, designed to produce increasing levels of aggression: 1) alone, with no neighbor ("no neighbor"), 2) next to a sham-implanted stimulus male ("sham neighbor"), or 3) next to a testosterone-implanted stimulus male ("testosterone neighbor"). Following one week of social treatment, focal males were experimentally infected with MG, which causes severe conjunctivitis and induces sickness behaviors such as lethargy and anorexia. While social treatment increased aggression as predicted, there were no differences among groups in baseline corticosterone levels, total circulating androgens, or responses to infection. Across all focal individuals regardless of social treatment, pre-infection baseline corticosterone levels were negatively associated with the severity of conjunctivitis and sickness behaviors, suggesting that corticosterone may dampen inflammatory responses in this host-pathogen system. However, because corticosterone levels differed based upon population of origin, caution must be taken in interpreting this result. Taken together, these results suggest that in captivity, although aggression does not alter individual responses to MG, corticosterone may play a role in this disease.

Changes in corticosterone concentrations and behavior during Mycoplasma gallisepticum infection in house finches (Haemorhous mexicanus)

“…Intriguingly, we found that pre-infection CORT concentrations interacted with sex in our study: male house finches with higher pre-infection CORT concentrations showed lower overall disease severity and pathogen load in response to MG infection, whereas female pre-infection CORT concentrations did not significantly predict individual variation in either metric. Our results are consistent with those of Adelman et al (2015), who found that higher pre-infection baseline CORT concentrations were predictive of lower disease severity among male house finches. It is intriguing that we did not detect a significant relationship between pre-infection CORT and disease response within female house finches, despite having a larger sample size for this group.…”

“…Finally, we examined whether CORT concentrations prior to infection were predictive of individual variation in disease outcome, as has been detected in prior work on male house finches (Adelman et al, 2015). Intriguingly, we found that pre-infection CORT concentrations interacted with sex in our study: male house finches with higher pre-infection CORT concentrations showed lower overall disease severity and pathogen load in response to MG infection, whereas female pre-infection CORT concentrations did not significantly predict individual variation in either metric.…”

“…Furthermore, although not significant, the parameter estimates for females were in the opposite direction of that detected for males, with higher pre-infection CORT concentrations associated with higher disease severity and pathogen load. Adelman et al (2015) only included male house finches (n=22), and thus our study is the only one to date to examine how pre-infection CORT concentrations predict host responses to MG in female versus male house finches. However, further study is needed with larger, and more balanced sample sizes, to examine the potential for sex-specific influences of pre-infection CORT concentrations on disease responses to MG.…”

“…Work on free-living house finches found that birds with clinical signs of Mycoplasmal conjunctivitis had higher stress-induced CORT concentrations than uninfected birds, but could not determine whether the increased CORT concentrations were a cause or consequence of infection (Lindstrom et al, 2005). Recent work by Adelman et al (2015) on male house finches found that pre-infection baseline CORT concentrations were negatively associated with the extent of conjunctival swelling and sickness behavior during infection, but the detected association between CORT and inflammation may have been confounded by population differences, making interpretation difficult. Together these studies suggest that CORT concentrations may be linked to individual variation in MG response in house finches.…”

Exploratory behavior is linked to stress physiology and social network centrality in free-living house finches (Haemorhous mexicanus)

“…From these videos we observed 175 instances in which one house finch arrived at a feeding port within 2s of another departing. On all 175 occasions, the arriving finch exhibited aggression, defined as "a rapid movement…made directly at the other bird" (Adelman et al, 2015a).…”