Limited association between stallion-like behavior and hormonal indicators of testicular remnants in geldings.

Overview

• This study investigated whether persistent stallion-like behavior in geldings (castrated male horses) is linked to the presence of leftover testicular tissue by analyzing hormone levels.
• The findings show that most geldings displaying stallion-like behavior do not have hormonal evidence of testicular remnants, indicating other factors may contribute to such behavior.

Background and Purpose of the Study

• Geldings are male horses that have been castrated, normally removing testicular tissue that produces hormones influencing male behavior.
• Sometimes, geldings display persistent stallion-like behavior, which can be a sign of cryptorchidism—the condition where testicular tissue remains undescended and functional.
• Identifying whether testicular remnants are present involves measuring hormones typically produced by testicular tissue, primarily testosterone and anti-Müllerian hormone (AMH).
• This study aimed to evaluate how well these hormonal indicators correlate with stallion-like behavior in supposed geldings, using a large retrospective sample.

Methodology

• Samples from 1,202 presumptive geldings were analyzed retrospectively for serum testosterone and AMH concentrations.
• Hormone levels were compared to established laboratory reference ranges:
  • Testosterone: < 60 pg/mL considered low (typical for geldings), above this suggests cryptorchidism.
  • AMH: ≤ 0.15 ng/mL typical for geldings; higher values suggest presence of testicular tissue.
• The study also investigated the relationship of these hormone levels with variables such as season and age categories within recognized cryptorchid samples.

Key Findings

• Most geldings with stallion-like behavior had testosterone (80.6%) and AMH (77.8%) levels below the cryptorchid threshold, indicating no hormonally active testicular tissue.
• A minority of samples showed discordant hormone levels:
  • 1.8% had AMH typical of geldings but elevated testosterone.
  • 4.3% had high AMH levels (suggestive of cryptorchidism) but testosterone levels consistent with no testicular tissue.
• Among confirmed cryptorchid stallions:
  • Hormone concentrations (both testosterone and AMH) did not vary across seasons.
  • Hormone levels varied with age, indicating age-dependent hormone production in cryptorchid stallions.

Implications and Conclusions

• The study demonstrates a limited association between stallion-like behavior in geldings and the presence of testicular remnants as defined by these hormonal markers.
• Most behavioral signs mimicking stallions are not due to residual functional testicular tissue that can be detected by testosterone or AMH levels.
• This highlights the complexity of equine behavior and suggests alternative causes may explain persistent stallion-like behaviors, such as learned behaviors, environmental influences, or other physiological factors.
• Veterinarians should consider that hormonal testing for testicular remnants may not fully explain behavioral issues in geldings and should explore other diagnostic and management options.