Effects of levonorgestrel on ovulation and oestrous behaviour in the female tammar wallaby

Abstract

Subcutaneous hormone implants are a useful method for managing overabundant marsupials in restricted enclosures in Australia. Levonorgestrel induces long-term infertility in the kangaroo, tammar wallaby and koala, although the contraceptive mechanism of levonorgestrel is unknown for any marsupial. In the present study, it was investigated if insertion of a single levonorgestrel or control implant at the time of reactivation of the diapausing blastocyst affected the subsequent post-partum oestrus or the preceding follicular development. Twenty levonorgestrel-treated and 16 control animals were autopsied the day before birth and the accompanying post-partum oestrus (Day 25), and 10 levonorgestrel-treated and five of the nine control animals were autopsied 3-4 days (Days 29-30) after the expected birth and oestrus. Peripartum behaviour was observed and birth and mating times were recorded. Levonorgestrel treatment did not prevent follicular growth because there was no significant difference between treatment and control animals in the size of the dominant follicle at Day 25. None of the levonorgestrel-treated females autopsied at Days 29-30 had ovulated (n = 10), in contrast to controls, where four of the five that were autopsied had ovulated. Mating occurred in eight of nine control animals but in only three of 10 levonorgestrel-treated females. Males showed a more sustained period of interest in the three that were mated than in the controls, and mating took place significantly later after birth (36 v. 10 h; P = 0.038). Follicular growth and development was not blocked in any female but only one-third of the animals mated and none ovulated after levonorgestrel treatment. These results suggest that levonorgestrel inhibits the preovulatory surge of luteinising hormone.

New insights into the reproductive physiology and management of the female koala (Phascolarctos cinereus): factors affecting the control of the oestrous cycle

“…Whilst here we have not determined if LNG treatment blocked ovulation in the koala, it certainly blocked oestrous behaviour in this induced ovulatory species. Interestingly, our results in 6-12 koalas and that of Crawford et al (2011) in the possum, appear to be in contrast to those previously reported by Hynes et al (2007) for the tammar wallaby, where by follicular activity continued unabated during progestogen treatment. This difference between species is not surprising given follicular growth in the tammar wallaby occurs during a period of elevated progesterone in the final third of pregnancy (Tyndale-Biscoe and Renfree, 1987).…”

“…These observations suggests that LNG suppressed oestradiol secretion from the existing dominant Graafian follicle (at oestrus) and also subsequently prevented development of the dominant follicle of the next cycle. These results are consistent with progestogen studies in eutherian species, but somewhat contrary to what has been observed in the tammar wallaby (Hynes et al, 2007).…”

“…This difference between species is not surprising given follicular growth in the tammar wallaby occurs during a period of elevated progesterone in the final third of pregnancy (Tyndale-Biscoe and Renfree, 1987). Consequently, it is likely that progestogen treatment prevents pregnancy in the tammar wallaby, through the suppression of the LH surge, thereby inhibiting ovulation (Hynes et al, 2007) but in the koala, LNG appears to inhibit mating success by the suppression of adequate secretion of oestradiol from the dominant follicle, preventing behavioural oestrus and consequently coitally or semen induced ovulation (Johnston et al, 2004).…”

“…Studies on the use of synthetic progestogens for contraception in the tammar wallaby (Hynes et al, 2007) have revealed that although the progestogen levonoregestrel (LNG) was capable of preventing a LH surge, it was unable to suppress follicular activity, and consequently this compound is unlikely to be suitable as an oestrous synchronization agent in the tammar.…”

“…Alternatively, ovulation in LNG treated koalas may be prevented, even if mating is achieved through inhibition of the LH surge, as is the case in many other spontaneous ovulating species including the tammar wallaby (Baldwin et al, 1994, Bettinger et al, 1997, Savage et al, 2002, Hynes et al, 2007. The ability of luteal or exogenous progestogen to suppress gonadotrophin secretion and subsequently inhibit follicular growth in the koala has not been specifically investigated, however, Johnston (1999) has shown that behavioural oestrous is at least suppressed during the luteal phase in both pregnant and non-pregnant koalas.…”