Fertility of male and female emus (Dromaius novaehollandiae) as determined by spermatozoa trapped in eggs

Abstract

Changes in the fertility status of 10 pairs of emus were investigated using egg break-out and numbers of sperm in the perivitelline membrane of the germinal disc (GD) region. After the sexes were separated, sperm in consecutive eggs declined approximately logarithmically at a mean (±SEM, n = 10 females) rate of –0.148 ± 0.021 per log day. Sperm continued to be detected in eggs for 16.5 ± 1.7 days during which 5.6 ± 0.6 fertilized eggs were laid. Fertilized eggs that did not contain detectable sperm were laid by five females for a further 2.2 ± 0.9 days. Based on break-out fertility, the fertile period continued for up to 18.7 ± 2.1 days, for which the mean number of laid eggs was 6.3 ± 0.8. An egg with a 50 : 50 chance of being fertilized would contain 3.5 sperm mm^–2 of GD. Based on the sperm decline model, an egg containing that many sperm would be laid 21 days after the last copulation. In emus that were not separated and allowed to incubate their eggs (n = 3 pairs), the number of sperm in eggs laid before and during incubation declined in a manner similar to that after the last copulation and egg-laying stopped after the females had laid 3.3 ± 0.3 eggs. After incubation was terminated, females resumed laying within 8.3 ± 1.2 days and the number of sperm in eggs gradually increased but it did not return to pre-incubation levels. In non-incubating emus (three pairs), the number of sperm in eggs declined as laying progressed, although lit was higher during the period when the first seven eggs were laid than during the period when the rest of eggs were laid (214 ± 39 v. 100 ± 16 sperm mm^–2 of GD). Sperm numbers varied between successive eggs but a sharp increase followed by a decrease acted as an indicator of recent copulation. There were 8.7 ± 0.3 such increases per laying period (one per 2.8 ± 0.2 eggs), a frequency that suggests that emus copulate once weekly. In conclusion, as long as a female emu is supplied with sperm on a weekly basis, she will be fertile but, when copulations stop, she will stop laying soon after. Male fertility appears to fall towards the end of the laying season and it can be affected by egg incubation at any time of the season.