Serum and hair testosterone concentrations do not differ in stallions between social ranks.
In many species, male social rank has been found to correlate with testosterone concentration ([T2]), with more dominant males having higher circulating [T2]. However, the influence of [T2] on social ranking has not been evaluated yet in feral horse herds. The purpose of this study was to compare [T2] and cortisol concentration ([CORT]) in serum and hair in stallions from a variety of different social ranks. We hypothesized that dominant stallions would have higher [T2] and submissive stallions would have higher [CORT]. We also hypothesized that there would be a correlation between serum and hair [T2] and [CORT]. Stallions used in this study were part of the feral horse herd managed on the Confederated Tribes of Warm Springs Reservation (n=15). After gathering stallions for the purpose of castration, behavioral evaluations were conducted on each horse in a small and large pen to determine how they interact with other stallions. A 5-point behavior score was created with 1=highly submissive and 5=highly dominant. Samples were collected when each stallion was anesthetized. A jugular venous blood sample was collected, allowed to clot, and serum was stored at -20°C. A mane hair sample was pulled out by the root. [T2] and [CORT] were extracted from the hair as previously described. Briefly, 100±20 mg of hair was weighed, minced into 3-4 mm pieces, sonicated in methanol (2 mL) at 20°C for 30 minutes, and incubated overnight at 50°C in a water bath with gentle shaking. The methanol was pipetted off into a new glass vial and evaporated to dryness under nitrogen. The samples were then reconstituted with 125 µL of assay buffer (#80-0170, Assay Designs, Inc., Ann Arbor, MI). Chemiluminescence (Immulite 1000, Siemens Healthcare Diagnostics, Tarrytown, NY) was used to measure [T2] and [CORT] from serum and extracted hair samples. The behavior score collected from both pens was averaged and then categorized as either submissive (<2.5), neutral (2.5-3.5), dominant (>3.5). The behavior score category was compared using an ANOVA for serum and hair [T2] and [CORT]. In addition, a Pearson correlation analyses was performed to determine if there was an association between serum and hair [T2] or [CORT]. Significance was defined as p<0.05. There were no significant differences between hair (ng/100 mg) and serum (ng/mL) [T2] or [CORT] in submissive, neutral, or dominant stallions (mean±standard deviation reported in the table below). There was also no correlation between serum and hair [T2] or [CORT] (R2=0.0257, and 0.0025, respectively). While it was unexpected that [T2] and [CORT] did not differ between social ranks, this may reflect co-dependencies that exist within bachelor bands and warrant further research.