ENDOCRINOLOGY OF THE OVARIAN CYCLE AND PREGNANCY IN THE ASIAN (ELEPHAS-MAXIMUS) AND AFRICAN (LOXODONTA-AFRICANA) ELEPHANT

In reviewing the literature, this paper assesses the current level of understanding of the hormonal control of the ovarian cycle and pregnan cy in the elephant. Data are compiled from two genera, Elephas maximus and Loxodonta africana, recognizing differences where known to exist. Measurements of various steroid and peptide hormones, but most notabl y immunoreactive progesterone (iP), indicate an overall cycle length o f 15-16 weeks, comprising an 8-11-week luteal phase and a shorter inte r-luteal (follicular) period of 4-6 weeks. Oestrogen related phenomena suggest the occurrence of two (possibly more) waves of follicular dev elopment with a 3-week periodicity, although oestrogen levels are low and fluctuate without clear pattern. The inter-luteal period is charac terized by two discrete LH peaks also 3 weeks apart, but only the seco nd is associated with an iP rise. Ovulation rate is variable and addit ional luteal structures lacking ovulation points are probably formed e ach cycle. The post-ovulatory period is associated with elevated proge stogen levels, mainly Sa-reduced compounds, while progesterone itself is a minor secretory product. A revised model for the elephant ovarian cycle is presented. Detailed information on the endocrinology of preg nancy is lacking. Elevated progestins beyond the normal luteal phase i ndicate that conception extends the functional lifespan of the CL, wit h maximum steroidogenic activity between 3-15 months. The nature of th e luteotrophic support is not known and placental gonadotrophins have not been conclusively demonstrated. Progestins fluctuate at or slightl y above luteal phase values throughout pregnancy. There is a marked in crease in prolactin levels after 16-24 weeks and a relationship with o estrogen secretion may exist since circulating and urinary levels show a progressive increase from 6-8 months. A placental contribution to p rogestin secretion is likely, although direct evidence is lacking. Con siderable enlargement of foetal gonads during the second half of pregn ancy in African elephants suggests a possible steroidogenic function, but none has been defined. The endocrine events leading to parturition are unknown. In the Asian elephant, a fall in iP precedes parturition ; oestrogen levels decline and cortisol increases at the time of birth . The paper concludes with a brief selection of priorities for future research. (C) 1998 Elsevier Science B.V. All rights reserved.