AUTHORS: Ludek Bartos, Tomas Peterka, Bruno Esattore, Radim Kotrba, Jan Pluhacek, Francisco Ceacero, Martina Komarkova, Adam Dusek, and Jitka Bartosova – Department of Ethology, Institute of Animal Science
ABSTRACT: In a previous study on red deer, changing the social environment of adult red deer males resulted in a change of the relationship between dominance rank and testosterone (T) as well as cortisol (C) concentrations despite the fact that the rank position of the adults itself did not change [1]. In the present study we added an aspect of non-agonistic relationships among the red deer males. We predicted that some males will keep company with others for a long time, while others will not; those keeping together will be either “non-Fighters” (not attacking others) or “Fighters” (competing frequently); this will modify C and T.
Nineteen tame red deer males were kept in six enclosures, covering an area of approximately 4 ha. Each enclosure (of about 0.7 ha) contained a shelter, a water reservoir, and a mud pool for wallowing. During the observation period (from 1st May to 27th August 2014) all enclosures were interconnected by two permanently opened gates in each enclosure. The animals were identified with colored, numbered collars and equipped with GPS collars.
Based on GPS data, we applied cluster analysis and divided the pairwise average distances between males into two groups: “Associates” (number of pairs, mean ± SE; n=174, 25.53 ± 1.68 m) and “Non-associates” (n=58, 175.62 ± 4.13 m). Applying the same cluster procedure, pairwise average agonistic interactions were divided into “Fighters” (n=96, 13.65 ± 0.46 interactions) and “Non-Fighters” (n=133, 3.50 ± 0.17 interactions). Percentage of pair relationships with Non-Fighters was 69.47% in Associates and 59.26% in Non-associates. Proportion of Non-Fighters for an individual male ranged from 27.27% to 100%.
Associates had higher mean C concentrations than Non-associates (LSMEANs ± SE, Associates 130.02 ± 1.35 ng/ml, Non-associates 124.07 ± 1.14 ng/ml). However, in both, Associates and Non-associates, C concentrations were decreasing with increasing proportion of Non-Fighters. (Other fixed effects were statistically eliminated in generalized linear mixed model.)
Associates did not differ in age, size, or aggressiveness from those who preferred less tight relationships with others (Non-associates). Both types did not differ in cortisol levels but Non-associates (LSMEANs SE, 0.42 ± 0.01 ± 0.91 ng/ml) had slightly higher T concentrations than Associates (0.39 ± 0.01 ng/ml). The males who tended to keep in proximity with others (Associates) benefitted from more peaceful relationships with a higher proportion of Non-Fighters which appeared a highly significant modifier of C and T. Increasing proportion on Non-Fighters decreased C in Associates as well as Non-Associates. On the other hand, proportion of Non-Fighters had different effect on T concentrations in Associates and Non-associates. Increasing proportion of Non-Fighters did not affect T concentration in Associates (P = 0.54), but decreased T in Non-associates (P < 0.01).
Influence of T on C: There was apparent dual effect according to low or high C concentrations (Table 2). In C High group, with increasing T concentrations, the concentrations of C were also increasing (P < 0.0001). On the contrary, in Cortisol Low group it was just opposite; C concentrations were decreasing with increasing T (P < 0.0001). With increasing body weight C decreased in Associates while increased in Non-Associates (at the end of observation).
Non-associates searched for the Fighters (the inverted value of Non-Fighters) more frequently than did Associates. Thus the Non-associates avoided stable social relationships and were more challenging than the Associates.
Associates and Non-Associates did not differ in the number of agonistic interactions per se. However, in accordance with the Challenge hypothesis [2] unlike in Associates, T was more pronounced to be related to aggression in the Non-associates who, living apart from others, were exposed to social instability and formation of dominance relationships when approaching other males. Any agonistic encounter thus could be more challenging first of all for Non-associates who initiated the encounter. The effects of the proportion of Non-Fighters and age therefore appeared influential in modifying T in Non-associates but not in Associates. With increasing proportion of Non-Fighters, T in Non-associates decreased. Thus the presence of close social partners had buffering effect not only for C as anticipated [3-5] but also for T in challenging situations.
Our results thus suggest that red deer males may intentionally modify their C and T concentrations, which are involved in antler growth [6] and they may theoretically increase their future reproductive success [7].
