Semen-induced luteal phase and identification of a LH surge in the koala (Phascolarctos cinereus)
Johnston, SD, O’Callaghan, P, Nilsson, K, Tzipori, G & Curlewis, JD 2004, Society for Reproduction and Fertility, DOI: 10.1530/rep.1.00300
The successful induction of a luteal phase in female koalas following insemination and no other stimulus suggests that koala semen possesses characteristics able to induce ovulation.
This study had two aims. The first was to describe the luteinising hormone (LH) surge in female koalas induced by the act of copulation. It was found that after a natural mating interaction, female koalas exhibited high concentrations of LH approximately 24–32 hours after copulation. The same LH surge did not occur in female koalas that were presented with a male but not allowed to mate. The second aim was to determine whether mechanical stimulation of the urogenital sinus of the female koala, and/or the deposition of semen, could induce a luteal phase. Out of nine koalas inseminated without any urogenital stimulation, four entered a subsequent luteal phase. Out of nine koalas that were inseminated and received urogenital stimulation with a glass rod designed to imitate the stimulation of the penis during copulation, seven entered a luteal phase. Statistical analysis confirmed that the deposition of semen had a significant positive effect on luteal phase induction and that urogenital stimulation with the glass rod did not. By extension, this may imply that the ejaculation of semen during mating is more important for inducing a luteal phase than the act of mating itself.
Although it has been established that copulation induces a luteal phase in female koalas, the precise factors responsible for the luteal phase induction are less clearly understood. While previous studies have demonstrated that koalas may have a ‘copuloceptive reflex’ by which ovulation is induced by the mechanical stimulation of the urogenital sinus by the penis during mating, an alternative explanation is that the biochemical composition of semen has ovulation-inducing qualities. This phenomenon occurs in a species of camel. The findings of this study are, however, challenged by prior contradictory findings that the longer the thrusting stage of mating, the more likely the female was to enter a subsequent luteal phase, highlighting the importance of urogenital stimulation in luteal phase induction in koalas. It is possible that the glass rod method of stimulation employed in this study was not an effective substitute for that provided by the penis during copulation. Alternatively, the timing and type of koala semen fractions that are ejaculated during natural copulation may be compositionally different from those collected in an artificial setting.
This study may be the first to document the LH surge in female koalas following mating and the importance of copulation in inducing this surge, as well as to demonstrate that koala semen may contain factors that can induce a luteal phase either in isolation or in combination with mechanical urogenital stimulation. Together, these understandings contribute to improvements in the timing and induction of ovulation for artificial insemination programs that will create new opportunities for koala conservation.
Summarised by Joanna Horsfall
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