Upon hearing a conspecific signal, animals must assess their relationship with the signaller and respond appropriately. Territorial animals usually respond more aggressively to strangers than neighbors in a phenomenon known as the “dear enemy effect”. This phenomenon likely evolved because strangers represent a threat to an animal's territory tenure and parentage, whereas neighbors only represent a threat to an animal's parentage because they already possess a territory (providing territory boundaries are established and stable). Although the dear enemy effect has been widely documented using behavioral response variables, little research has been conducted on the physiological responses of animals to neighbors versus strangers. We sought to investigate whether the dear enemy effect is observed physiologically by exposing territorial male song sparrows (Melospiza melodia) to playback simulating a neighbor or a stranger, and then collecting blood samples to measure plasma testosterone levels. We predicted that song sparrows would exhibit increased testosterone levels after exposure to stranger playback compared to neighbor playback, due to the role testosterone plays in regulating aggression. Contrary to our prediction, we found that song sparrows had higher testosterone levels after exposure to neighbor playback compared to stranger playback. We discuss several explanations for our result, notably that corticosterone may regulate the dear enemy effect in male song sparrows and this may inhibit plasma testosterone. Future studies will benefit from examining corticosterone in addition to testosterone, to better understand the hormonal underpinnings of the dear enemy effect.
We presented territorial male song sparrows with experimental playback of recordings of conspecific song at the Queen's University Biological Station (44° 34′ N, 76° 19′ W; Ontario, Canada). During April of 2016 we recorded birds and mapped territories; we spent ≥ 90 min observing each bird, over multiple days, and we logged each of his song posts using a Global Positioning System (Garmin 60CS). From May 4 to May 19, 2016 we conducted playback experiments on 22 paired male song sparrows. All birds had established territories and were paired at the time of playback (i.e., each male was observed affiliating with a female over extended periods, regularly foraging together and moving around their territory together), as determined during observations in April and early May. Four birds were banded from studies in previous years with unique combinations of colored bands and a Canadian Wildlife Service numbered band. In addition, we banded 12 of the birds we caught in 2016; these birds were banded after the post-playback blood collection, rather than before playback. We did not want to capture the birds prior to playback, exposing our subjects to a potentially stressful event and reducing our ability to capture them a second time for blood collection after playback. We had no difficulty identifying birds, regardless of whether or not they were banded, based on location, behavior, and individually distinctive songs (song sharing is low in Eastern populations of song sparrows; Hughes et al., 2007, Stewart and MacDougall-Shackleton, 2008; although see Foote and Barber, 2007). Testosterone levels did not differ among birds from different banding cohorts (i.e., banded in 2015, banded in 2016, or unbanded; Spearman's rank correlation: rho = 0.30, p = 0.18, N = 22).