Monkeys rarely give birth during the day. The darkness of night gives them better protection from any predators that are lurking. As a result, monkey births in the wild have scarcely been observed.
It was previously believed that they usually give birth alone.
Now for the first time in several decades of observation, researchers have seen a wild golden snub-nosed monkey (Rhinopithecus roxellana) give birth during the day.
That was not all. The monkey had a "midwife", another female who was there to assist her during labour.
When the pregnant female first showed signs of agitation, her helper quickly came to her and groomed her. Soon after, her contractions were clearly visible. The midwife stayed close.
As the infant's crown became visible, the midwife twice attempted to pull it out of the birth canal. When the infant's head was fully exposed, the mother reached down and pulled the infant most of the way, with her midwife helping until it was fully out.
The midwife attended to the mother afterwards, and was allowed to hold and lick the newborn after the mother had first fed and licked it.
The helper and one other female were allowed to hold the baby, but only 77 minutes after it was born. Another female juvenile attempted to take the infant, but was rejected by the mother.
As soon as the mother had given birth, she cut the umbilical cord and spent five minutes eating the placenta. This provided her with valuable nutrition.
The birth itself only lasted four minutes and ten seconds. The researchers had not expected to witness it, but caught the subsequent behaviour in a series of photos, as pictured.
The team describe their observation of the birth in the journal Primates. The event was witnessed on the Qinling Mountains of China, the only country where these endangered snub-nosed monkeys live.
Social assistance during childbirth has often been proposed as a uniquely human behaviour, says co-author Bao-Guo Li of Northwest University in Xi'an, China. "Direct assistance from other individuals during birth is not common in wild primates."
However, we do know of similar instances in other monkey species. In 2014 BBC Earth reported on a langur monkey, also endemic to China, that acted as a midwife. In 2013, a black-and-white snub-nosed monkey also received help from another female.
As the infant's crown became visible, the midwife twice attempted to pull it out of the birth canal
Monkey "midwives" may therefore be more common than once believed. The fact that it is rare in itself to witness a monkey giving birth means it is hard to find out how frequent it really is.
We also do not understand why some monkeys receive help during birth, when so many others manage alone.
We now also know that bonobos, apes closely related to chimpanzees, do not give birth alone. In 2014 a team observed a wild bonobo birth for the first time, and noted that two other females were present.
Receiving help from a midwife could provide "several adaptive advantages in terms of physical effort, and social and emotional support," says Li.
The midwife attended to the mother afterwards, and was allowed to hold and lick the newborn
In this latest instance, the mother evidently benefited from additional grooming and attention both during and after the birth.
Sarah Turner of McGill University in Canada, who was not involved with the latest observation, agrees that there is a "much higher diversity of birth-related behaviours than was once thought", including more physical contact.
"Some of these contacts do seem to directly help facilitate the birth process," she says. "Most nonhuman primates are very social animals and a female may choose to give birth close to others in the group to help reduce her stress and labour pain."
The team is now planning to record the monkeys giving birth at night, to see if midwives really are common. They might discover even more monkey midwives "working" through the night.
The fact that our evolutionary neighbours sometimes use midwives might help us understand the origin of birth assistance in human society, Li says.
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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