Plasma concentrations of immunoreactive relaxin activity and progesterone in the pregnant Asian elephant (Elephas maximus)

Abstract Immunoreactive relaxin activity and progesterone concentrations in weekly plasma samples were measured throughout six pregnancies in five Asian elephants ( Elephas maximus ). One animal aborted, one delivered a stillborn calf at term and the remaining pregnancies concluded normally with living young. Immunoreactive relaxin activity increased by week 20 of pregnancy to reach a mean peak concentrations of >6 ng/ml in the second trimester. This was followed by a slow decline beginning approximately at 30 weeks before term. A smaller, secondary relaxin rise was observed during the final eight weeks preceding parturition and circulating concentrations remained above pre-pregnancy baseline values for 72 h after birth. In contrast, progesterone concentrations increased immediately after mating and rose to a maximum of >2 ng/ml by mid-gestation. As with relaxin, progesterone concentrations began to decline gradually approximately 30 weeks before birth. More frequently collected samples before, during and after parturition revealed a decline to pre-pregnancy levels between 2–5 days before parturition, in contrast to elevated relaxin levels (1 ng/ml) during this time. The aborted pregnancy at week 35 was accompanied by a precipitous decline in both immunoreactive relaxin and progesterone concentrations one week before the visible termination of the pregnancy. The delivery of the term stillborn calf occurred five weeks after both immunoreactive relaxin and progesterone concentrations had declined to pre-pregnancy values. Results suggest that immunoreactive relaxin may be important, along with progesterone, in the maintenance of pregnancy in the Asian elephant as well as playing a role in preparation for birth and possibly to indicate foetal well-being.