Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Serum and hair testosterone concentrations do not differ in stallions between social ranks.

Abstract

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.