Indian rhinoceroses: Reproductive tract tumors reduce female fertility in early stages

At the age of three years Indian rhinoceroses become sexually mature. They can reach an age of up to 40 years. Although females should reproduce until the very end of their life, on average the females in zoological gardens give birth to their last young at the age of 18 years. This is revealed by records in the international studbook for Indian rhinoceros which registers the life histories and reproductive success of all 189 Indian rhinos living in zoos worldwide. The researchers found another curiosity while investigating the studbook: If rhinos gave birth to their first young by the early age of five years, they produced up to six juveniles. Rhinos with an age at first reproduction older than that rarely gave birth to more than two calves.

It is known that female Indian rhinos often suffer from tumours in the reproductive tract. Robert Hermes, Frank Göritz und Thomas Hildebrandt, all veterinarian scientists at the IZW, and their colleague Monica Stoops from Cincinnati Zoo analysed ultrasound data of 23 female Indian rhinos. Over the last 20 years these animals were examined several times.

The finding: "Already at the age of 13 years all animals had developed tumours! The tumours grow rapidly, uncontrolled and lifelong until the vagina and cervix are completely covered in them." This process may be associated with necrosis and inflammations. "Besides the pain which the female must be experiencing because of the tumour, the mating act becomes almost impossible and even if successful, there is no free passage available to the sperm anymore," explains Robert Hermes.

Looking for an organism with comparable tumours the researchers found an unexpected model species -- the human being. "Eighty per cent of all women have such leiomyomas at the onset of their menopause. Leiomyomas can already start to develop in adolescence and they grow as a function of ovarian activity. In contrast to the rhinos the mostly benign tumours in women are located in the uterus and remain without symptoms," says Hermes. Because of the special hormonal condition during pregnancy or lactation, leiomyomas do not grow during these periods. Furthermore women, who gave birth to at least one child, have a 40 % reduced risk to develop leiomyomas.

This analogy provides an indication why those female Indian rhinos who have their first pregnancy at a younger age produce a higher number of offspring than females whose first pregnancy is delayed to an advanced age: Pregnancy inhibits tumour growth. This also suggests a prospective therapy for animals that suffer from inflammation and pain because of massive tumours. "By suppressing ovarian activity, we should be able to induce the menopause, thus putting a halt to further tumour growth," Hermes stated.

Indian rhinos are threatened with extinction. Only 2,900 animals remain in the wild. Robert Hermes cautions: "We should not rely on higher rates of reproduction in free-living Indian rhinos than those in captivity," says Robert Hermes. "Zoological gardens are responsible for a coordinated breeding programme which uses the genetic diversity of the entire population. It would be disastrous, if only because of early tumour diseases breeding is confined to only a few animals who may produce a high number of offspring and the majority bear fewer or no calves." The authors strongly recommend to start breeding at an early stage and to optimise breeding conditions to maximise the chance of females becoming pregnant.

Most zoos hesitate too long until the conditions for natural mating are created -- sometimes 12 or 13 years, deplores Hermes. "This is too late, since all female Indian rhinos do already have tumours in this stage of life. More than one calf is then unlikely." Because several Indian rhinos usually grow up in close proximity in zoological gardens, young females may not have a regular ovulation. In such cases, assistance is necessary. There is a wide range of methods: from a hormonal impulse to a more active support for searching a suitable partner.