News & Events

World’s Rarest Eggs Fail to Develop

The female Rafetus nested this year during the night of June 16 (63 eggs), again during the night of July 2 (63 eggs), and laid a third clutch on July 17. All eggs of the first two clutches were numbered and about half of those eggs were artificially incubated (29, 31 and 33 degrees Celsius, various substrates), the other half was left in the nests constructed by the female. The third nest was not disturbed at all and the eggs were left in place, without being handled.

Dr. Gerald Kuchling checked and candled the artificially incubated eggs on July 3, July 5, and August 9. Apart from 2-3 eggs of each clutch which showed yolk sedimentation and had a possible white blotch (no clear banding) all eggs (which were not cracked and rotten) appeared homogenous – which indicates that they were not fertilized.

Yolk sedimentation and banding is generally a sign of early development (and thus of a fertilized egg), but even in the eggs which showed some signs of fertilization, the sedimentation was weak and white blotches were not very clear. For this reason, it appears that in the first two clutches, only 5-10% of the eggs were fertilized, and none of those continued to develop. This compares to an estimate of 25% in 2009.

All three nests on the beach have been fenced and they are checked daily in case any hatchlings emerge from the nests dug by the female.

After hibernation this year, the male was observed swimming and basking on March 19 and the female was observed swimming on April 18. The turtles were put together from April 20-May 15, from June 18-28 and from July 3 to the present. Mating activity was observed from April 21-May 5 and the female was clearly receptive in late April and early May.

We are learning more about this rarest of species each year. The most likely spermatogenetic cycle of Rafetus is that spermatogenesis takes place in summer (June to August/early September), that sperm is transferred into the epididymis (a sperm storage organ in turtles) in early autumn (spermiation) and the male is ready to copulate from about October to spring. Androgen levels may be high in summer and decline during autumn. Due to this dissociated androgen cycle the (extremely old) male in at the Suzhou Zoo may actually be better primed for copulation in autumn than in spring. There is a very good possibility that Rafetus may mainly mate in late autumn prior to hibernation, with mating activity tapering out in early spring.

By pairing them only in spring and summer in previous years, we narrowed the opportunity for the male to inseminate the female to a short window. It also means that by that time the (extremely old) male had to store his sperm for more than half a year. It may be better for the sperm if the presumably much younger female stores it. It was the plan last year to let male and female stay together over the winter. Thus, the male stayed together with the female in the small breeding pond from early August to early October 2009. By late September, he again showed mating behavior which resulted in the female developing lesions and white scar tissue at her neck and forelimbs due to biting of the male when he tried to grab her for copulation attempts. This resulted in zoo personnel becoming concerned about her safety and separating the animals.

For this reason, Dr. Kuchling worked very hard in 2010 to get a glass barrier around the large breeding pond (completed in July 2010 with funds from the Chinese government) and we opened all gates of the Rafetus facility on 24 August 2010. This will allow both male and female to roam through an area more than three times as large as what they had for the last 2.5 years. This will make it more difficult for the male to corner and bite the female, but will give him access to the female for mating all through autumn and winter and spring. Our hope is that by giving the male the chance to inseminate the female with fresh sperm while still primed by a higher androgen level in October/November, we will have better fertilization results next year.

The nutrition and breeding pond setup for this pair has improved each year and hopefully allowing them to remain together throughout the autumn will improve the chances for successful breeding in 2010/2011.