In humans, the frequency of multiple fetuses varies significantly among different ethnic groups. Twins occur in 1 out of every 100 pregnancies in white women compared to 1 out of every 79 pregnancies in black women. Certain areas of
The frequency of twins also depends on the type of twins. Identical twins (monozygotic) occur about once in every 250 births around the world. This type of twin formation appears to be uninfluenced by age, ethnicity, heredity, number of pregnancies or the intake of fertility drugs. The occurence of fraternal twins, on the other hand, is influenced by ethnicity, heredity , maternal age, the number of previous pregnancies and the use of fertility drugs.Heredity also plays a part in the occurrence of twins. A study found that the chance of a female twin giving birth to a set of twins herself was about 1 in 58 births. Triplets occur once in every 8000 deliveries and quadruplets, quintuplets and so on are extremely rare under natural conditions.
The occurrence of twins in Old World monkeys and apes is extremely rare, as it is with
In the past 17 years, for exemple, only 6 cases of twin orangutan births have been recorded. In 2005,
So what happens when twin babies are born to one of those species? Well, first of all, as in the human species, there is an increased risk associated with twin pregnancies, for the babies, and probably for the mother, and the occurence of twin fetuses is probably more common than we think. A number of them are most likely aborted while others probably result in still-born babies. When twin babies are born alive, still, it is a sad reality that many of them won't survive longer than a few days or weeks. Twins are often born smaller and weaker (often before the end of the normal gestation period) than single babies and they put an additional strain on the mother for feeding them. In some cases, the mother will receive help from other group members to carry her babies.
In Patas, for example,mothers cooperate and share child-raising activities, whereas in Capuchins, the whole group, including older brothers and sisters and mature males, will take turns carrying babies. But it is not the case in every species. Monkeyland's squirrel monkey mother had to carry both her twins on her own: quite a strain when you think a baby squirrel monkeys weighs 100 grams at birth, 1/8 of the monter's weight! One of the two was probably weaker, not fed quite enough and also had to cling on to a second choice position on the mother's back: it probably fell of at some stage, and we saddly lost it after one month.
Though howler monkeys sometimes have twins and are, apparently capable of cattering for both of them, a study in
Another study on banded leaf-monkey showed that while infant transfer is rare in this species, when one mother gave birth to twins, she often gave one of the twins to another to carry it. Allomothering, or alloparental care , may increase because it is difficult for a mother to travel and forage for food with having two infants to carry. As twins became older allomothering by adult females decreased while that of juveniles increased. Twins tend to stay with their mothers for a longer period of time as compared to single young, they became independent at a later age compared to single young.
In a rare case of twins among female Presbytis melalophos many patterns of behaviour were observed which are uncommon or absent when single infants are born. Here too, allomothering increased greatly, but it was often associated with maltreatment of twins by allomothers, even though this was never observed with single infants. The behaviour of the twins also differed from that of single infants, moving away from their mother much less than did single infants by their second month of life - except when in the care of allomothers.
In yet another study on Japanese macaques twins, it was noted that there were no clear differences in mother-infant interactions toward either twin or single infants, but the twins showed a clear nipple preference. The twins also spent much time in contact with and proximity to each other and as their interactions were peaceful no dominance relations between them were assumed. Each twin interacted with other infants and juveniles less frequently than did the single infants.
All the above cases concern monkeys species for which the norm is a single birth and where mutiple birth is, at least uncommon, if not extremely rare. There are a few exceptions, though, in the primate world. The first one is that of the "miniature monkeys", specifically marmosets and tamarins. In those species, indeed, single births are by no means the common rule. In cotton-top tamarins, for example, only 34% of births are single ones and 64% are twin births; the remaining 2% being triplets. In the same way, golden-handed tamarins give birth to twins 3/4 of the time. For common marmosets too, the "normality" is rather twins (62.5%) and single births only represent 12.5% (1 out of 8) of deliveries. In 21.4%, of cases marmosets give birth to triplets and even to quadruplets in 3.6% of cases.
Another exception to the single offspring rule is the prosimian (or "early" primates) group, in general. Indeed, prosimian reproductive anatomy differs from those of anthropoid primates. For example, they have a different shaped uterus and a different type of placenta. Many prosimians have at least four nipples and produce litters rather than a single offspring. Bushbabies, for instance, have between 1 and 3 babies at a time, whereas lemurs are much more likely to give birth to multiple offsprings than monkeys. A single baby is the rule for Aye-aye, for example, but for other species such as mongoose lemurs, black lemurs and ringtailed lemurs, twins, though less frequent than a single baby, are not uncommom. In other species, such as the black-and-white ruffed lemur, there can be even up to 6 babies in a litter -though 2 or 3 are more common-, as there are 6 nipples to feed them!
The number of nipples availbale for babies to suckle from is of course key to the success of raising triplets or more. For those species having normally, or frequently, multiple births, another key element might be the group involvement in caring for the infants. In the miniature monkeys, for instance, the father, and sometimes other group members, help by carrying the babies. These monkeys also increase the chance of success in bringing those babies to adulthood by having only one female in the group giving birth during a given mating season: even though there are 2 or 3 babies to look after, the whole group is devoted to those 2 or 3 only. But for those species too, more babies often means that each one of them is more at risk of not making it. A couple of years ago, one of our ringtailed lemur females gave birth to 3 babies. One of them was almost half the size of the other 2 and incapable of competing with them for the only 2 nipples available. Its chances of surviving were, from the beginning, minimal. Likewise, in the rare cases were a cotton-top tamarin gives birth to triplets, the chance of the 3 of them making it are very scarse (often one is still-born or dies in its first days of life).
For species who produce litters, like the black-and-white ruffed lemur, the risk involved in having multiple offspring births seems integrated in the breeding strategy: by having many babies, there is a better chance that some of them will make it. And infant mortality is, indeed, extremely high in that species with an estimated 64% of babies dying within the first 3 months in the wild. Maybe to increase the likelihood of infant survival, not all females in the black-and-white ruffed lemur group have babies every year and adults of both sexes show alloparental behaviour such as guarding infants and nonmaternal nursing