Prolactin (PRL) is a peptide hormone that is involved in a number of diverse physiologic roles, particularly with respect to reproduction, including: influencing sexual and parental behaviors, onset of puberty, regulation of seasonal reproduction, follicular maturation, ovulation, luteinization and corpus luteum (CL) function, steroidogenesis, mammary gland development and lactation, testicular and spermatozoal function, and immunomodulation of ovarian processes. Little is known about PRL's role in elephant reproduction. The present research was conducted to determine seasonal changes in PRL secretion in non-pregnant female African elephants. A corollary objective was to examine the potential functional interrelationships between secretions of PRL, cortisol and progesterone. Weekly blood samples for 18 months were taken from four female African elephants and the sera were analyzed by radioimmunoassay for progesterone, cortisol, and PRL concentrations. Estrous cycles averaged 14 weeks in length, and estrous cycle synchronicity was evident between pairs of elephants. The luteal phase was defined by serum concentrations of progesterone consistently above 200 pg/ml, and averaged 9 weeks in length (range: 5-12 weeks) with a mean (± SE) concentration of 750.3 ± 171.9 pg/ml. The follicular phase was defined by serum concentrations of progesterone consistently below 200 pg/ml, and averaged 5 weeks in length (range: 4-8 weeks) with a mean concentration of 103.1 ± 17.5 pg/ml. Mean (± SE) serum concentration of cortisol was 5.7 ± 1.3 ng/ml (range: 1.4-19.3 ng/ml), and concentrations of this adrenal steroid were negatively correlated with progesterone concentrations (r=-0.15; p<0.01). Serum concentrations of PRL averaged 3.91 ± 0.69 ng/ml (range: 0.84-15.8 ng/ml), were significantly lower during the luteal phase (p<0.0001; t-test), and were positively correlated with serum concentrations of cortisol (r=0.14; p<0.05). There was no significant effect of season on PRL concentrations. One of the elephants appeared to be hyperthyroid, but since removing her values from the data set did little to affect overall means, they were included in all of the calculations. These data suggest that stress may affect secretion of PRL in elephants, and cortisol and PRL may affect reproductive potential in elephants by altering luteal function. While this study did not demonstrate a seasonal effect on PRL secretion, it cannot be concluded that there is none because the reproductive effects of photoperiod are not always easy to detect. The higher serum concentrations of PRL detected during the follicular phase suggest that this hormone may play a role in modulating ovarian function in elephants during this stage of the estrous cycle
