Abnormal prolactin secretion is associated with changes in metabolic health biomarkers in acyclic female African elephants (Loxodonta africana)

Circulating prolactin aberrations (chronically high or low concentrations) have been shown to have strong associations with ovarian dysfunction in African female elephants. However, despite the well-established pleiotropic role that prolactin plays in human health, comprehensive metabolic effects from chronically elevated or suppressed prolactin have yet to be investigated in this species. In humans, in addition to amenorrhea, hyperprolactinemic women have shown increased risks for accelerated atherosclerosis, hyperandrogenemia, lowered metabolism, and insulin resistance. The aim of the study was to determine if elephants with long-term abnormal prolactin secretion were also experiencing alterations in metabolic biomarkers. One year of serum samples, collected biweekly, were obtained from female African elephants (n = 74) housed in North American zoos and categorized based on ovarian cyclicity and prolactin status: normal cycling, normal prolactin (NORMAL, n = 25), acyclic, high prolactin (HIGH, n = 25), or acyclic, low prolactin (LOW, n = 24). Each sample was further analyzed for measures of metabolic function, glucose metabolism, and cardiovascular health. Overall, thyroid hormones were significantly lower in both HIGH (TSH, T3, T4) and LOW (TSH) prolactin elephants compared to NORMAL counterparts. Additionally, HIGH prolactin elephants exhibited lower glucose, G:I ratios, and low-density lipoprotein values, but higher cholesterol concentrations compared to normal cycling females. Overall, LOW prolactin elephants exhibited higher body condition scores and G:I ratios, and higher concentrations of insulin, high-density lipoproteins, cortisol and testosterone, but decreased triglyceride values compared to NORMAL elephants in the study. Data suggest that HIGH prolactin elephants may be experiencing hypothyroidism as a comorbidity, whereas LOW prolactin elephants may be exhibiting aspects of insulin resistance, albeit different from human clinical data. Specific studies are suggested that would help elucidate the underlying mechanisms at play and increase our understanding of prolactin’s role in elephant physiology.