Earwax cortisol in bovines: biological and seasonal influences

Accurate stress assessment in animals requires reliable biomarkers, yet the impact of intrinsic (sex and age) and extrinsic (seasonal) factors on earwax cortisol concentrations (ECCs) has not been thoroughly studied. Therefore, the effects of sex, age and seasonal climate on ECC were investigated in Hanwoo cattle (Bos taurus coreanae).

Monthly earwax samples were collected from the left ear canal of 22 individual cattle. These comprised 7 males and 15 females as a cohort of 19 animals in March 2022 with 3 additional calves recruited through the study period up to August 2022. Their overall mean age was 32.7 ± 19.6 months. Earwax cortisol was extracted using methanol and quantified by a validated enzyme immunoassay. Based on the temperature-humidity index (THI), conditions were classified as cold stress (THI < 38), no stress (THI 38-72) or heat stress (THI > 72). Months were grouped into cool and warm seasons.

Earwax cortisol concentrations were higher in females than males (30.3 vs 20.6 pg/mg; P-value = 0.0015) and differed by THI category (P-value = 0.0074), peaking in females when the THI was 77 (July). Females older than 25 months had higher ECC than age-matched males (31.4 vs 15.4 pg/mg; P-value = 0.0036). Season alone did not affect ECC (P-value = 0.4943), although females maintained higher ECCs than males in the cool season (P-value = 0.0350). In females, the ECC was positively associated with age (β = 0.113 pg/mg/month by linear regression; P-value = 0.0121), but not in males.

Sex is the primary influence on ECC, age and environmental conditions being lesser ones. Both intrinsic biological and extrinsic environmental factors should be considered when interpreting chronic stress in bovines.