Genetics or yams in the Land of Twins?
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Igbo-Ora, Nigeria - Igbo-Ora, a sleepy farming community in southwest Nigeria, welcomes visitors with a sign proclaiming The Land of Twins.
"There is hardly a family here without a set of twins," said community leader Olayide Akinyemi, a 71-year-old father of 12, as he settled a dispute between two neighbours.
"My father had 10 sets, while I had three sets. But only one set, a male and a female, survived," he said.
The town's high incidence of twins has baffled fertility experts - underscoring a more regional twin trend and an array of elaborate African rituals around them.
The rate of identical twins is pretty steady throughout the world at about 0,5 percent of all births, according to a 1995 study by Belgian researcher Fernand Leroy, who has worked extensively on twins.
But West Africa bucks that trend, particularly with a much higher incidence of fraternal, or non-identical twins than in Europe or Japan. That is especially true, experts say, amongst Nigeria's Yoruba community which is largely concentrated in the southwestern part of the country where Igbo-Ora is located.
Overall, almost five percent of all Yoruba births produce twins, the Belgian study said, compared with just around 1,2 percent for Western Europe and 0,8 percent for Japan - although fertility drugs in the developed world are changing those figures.
Yam consumption may be one explanation for Africa's largesse, some West Africans and Western experts believe. Yams contain a natural hormone phytoestrogen which may stimulate the ovaries to produce an egg from each side.
For their part, Igbo-Ora's residents appear nonplussed about their twin phenomenon.
Some like Akinyemi support the yam theory - and point specifically to the reputedly high oestrogen content of agida, the local name for yam tubers.
"We eat a lot of okro leaf or Ilasa soup. We also consume a lot of agida. This diet influences multiple births," he said.
Others are not so sure.
"The real cause of the phenomenon has not been medically found," said Akin Odukogbe, a senior consultant gynaecologist with the University Teaching Hospital (UCH) in Ibadan, the nearest big town.
"But people attribute the development to diet," he continued, adding that studies have shown that yam can make women produce more than one egg which can be fertilised.
Chief nursing officer at the hospital Muyibi Yomi, who records a monthly average of five twins for every 100 births, puts it all down to genetics.
"If a family has a history of multiple births, this will continue from generation to generation," she said.
That should be good news for Yorubaland, where twins are regarded as a special gift from God and bearers of good luck, Akinyemi said.
"Twins are treated with affection, love and respect. Their birth is a good omen," he said.
But while many African cultures see twins as blessed, they often believe twins also have divine powers and the ability to harm those who cause them displeasure.
In pre-colonial times some communities used to kill twins and occasionally their mothers, believing a double birth was an evil portent and that the mother must have been with two men to bear two children at once. A Scottish missionary is credited with ending this practice.
In Yorubaland and indeed in large swathes of sub-Saharan Africa, twins are also believed to possess one soul between them. This belief accounts for a whole series of distinctive, and in some cases macabre rituals that are often country specific.
If one twin dies in a Yoruba family, the parents order a wooden figure called an "ibeji" to be carved, to take the place of the dead twin. The half soul of the deceased twin is thought to live on in the ibeji figure - which is clothed, "fed" and carried by a mother exactly in the same way as the living twin.
When living twins reach maturity they take responsibility for the ibejis' care.
A twin who dies in Malawi is buried with a piece of clothing belonging to the surviving sibling.
But when a twin dies in South Africa, the surviving twin is made to lie face down on his sibling's coffin the night before the burial, to mourn his death and say goodbye properly.
Another variant has the surviving twin being made to lie face up in the freshly dug grave the day before his sibling is buried. If not, communities fear the surviving twin will pine so much for his dead sibling that he will also die.
Amongst the Yoruba - one of Nigeria's dominant ethnic groups who are also present in Benin, Ghana and Togo - a mother who loses both twins will take part periodically in ritual ceremonies where she dances with both ibeji figures, either one in each hand, or both tucked into her shirt.
Anthropologists say the elaborate rituals surrounding twins go back to the days when perinatal mortality was very high for twins - the increased chances of premature delivery compounding the problem of inadequate healthcare in traditional societies.
The rituals were destined to help communities come to terms with the loss of the babies.