Female elephants remain fertile in old age
Oldest mums Elephants are one of the longest-living land mammals and now it seems they could also claim the record for oldest new mothers.
A Zimbabwean study has found that female African elephants can potentially remain fertile until their death as their ovaries have an adequate supply of oocytes.
Researcher Dr Fiona Stansfield, of the Save Valley Conservancy in Zimbabwe, says as in humans, a female elephant's eggs are formed during gestation and then slowly reduce in numbers throughout their life.
However in humans, studies indicate that egg loss accelerates from the age of 37 with menopause signalling the end of fertility from around age 50.
Stansfield says her study indicates elephants do not experience what she termed "pachy-pause".
She says there is a significant reduction in elephant egg numbers after the first calving between the age of 15-25.
While egg numbers then decrease slowly, her study of female elephant ovaries shows the ovaries may still contain as many as 11,000 eggs at the age of 69.
"The results indicate that although the follicle reserve may finally become exhausted in wild African elephants it does not do so until very late in the total lifespan," she writes in the journal Reproduction, Fertility and Development.
Stansfield says the study suggests that advancing age is an important key to successful breeding for wild African elephants.
"Male elephants only have input into the gene pool after 35 to 40 years of age," she says.
This mating success is tied to the bull's position of dominance among his peers as a result of his body size.
"It seems that longevity and age are selected for in successful males and it could be the same for females as well, as other studies have shown that matriarchs of the herd are more successful at rearing calves," says Stansfield.
The ovaries examined in the study were removed from wild elephants that were culled or hunted under licence, Stansfield says, adding that Zimbabwe, unlike its neighbours to the north, has more wild elephants than the available land can currently support.
Thirty-one pairs of ovaries were obtained during management off-takes within private conservancies and from professional hunting safaris in Zimbabwe between 2005 and 2011.
The ovaries came from five age groups, ranging from 10 through to 70 years.
Seven sets of ovaries came from elephants aged 57-70 years and all showed low numbers of the small follicles, which contain the eggs that are released during ovulation.
Stansfield says despite this, the four oldest animals still exhibited up to 24,278 small follicles - with an average of 7067 - in their ovaries.
This compares with an average of only around 1000 oocytes - or unfertilised, immature eggs - that remain in the ovaries of women when they approach menopause at around 51 years of age.
The only other long-lived mammals that are capable of supplying oocytes right up to maximum life expectancy are the baleen whales, which may live and breed until 100 years of age, says Stansfield.
Stansfield says the findings could potentially help to artificially bring older elephants to an infertile state.
"If we can find a way to bring them [older female elephants] to an infertile state, they can remain in the herd as the matriarch and font of knowledge without adding to herd numbers," she says.
This could offer an alternative management tool to the culling of complete family groups which is practiced in Zimbabwe to reduce elephant numbers in areas where human-elephant conflict is an issue.
Stansfield says the next step in the research will be to look at the quality of the eggs remaining in the elephant ovaries.
The study also supports views that a late start to breeding may reduce elephant fertility.
Stansfield says in the wild female elephants conceive for the first time around the age of 12 to 14 years. However in US zoos about 75 per cent of the captive population is aged 20 to 35 years.
"Once they are nulliparous (haven't been pregnant) at an age 25-30 years many captive African elephants have a real problem conceiving because the reproductive cycle doesn't kick in."