Dusky Langurs. Why the orange babies?

A Dusky Langur mother with her orange baby

The Lutongs (lutungs), langurs or leaf monkeys are a group of Old World monkeys in the genus Trachypithecus. The name 'Lutung' from the Sundanese language means 'blackness'; and is the preferred name for members of this genus.

DNA genetic analysis indicates that the ancestors of the modern species of lutung first differentiated from one another a little over 3 million years ago, during the late Pliocene, The various species alive today then diverged during the Pleistocene, presumably driven by habitat changes during ice ages. There are 20 species in the genus Trachypithecus today, listed below.

The current lutung species, although living in different habitats have a number of similar habits and traits. One of these is that they mostly have orange coloured babies. This suggests that the genes encoding orange fur in infants is an ancestral gene(s) that has been maintained because it gives each species a survival advantage in their current habitat. It is very unlikely the 'orange baby' gene expression has evolved through parallel evolution in each species. This answers the question as to where the orange fur comes from. Clearly it is in the genome of the lutungs and is expressed appropriately within the life cycle of each species. Adult Gee's Golden langurs have orange fur and Javan Langurs have several adult fur colours with orange being one of them Capped langur adults have some patches of orange fur.

Phayre's Langur and baby

Cat Ba langur with an orange infant.

Francois Langurs; Mother and infant

Silvery Lutung with twins. It is rare for both to survive. Help from other troop members is vital.

Adult Gee's Golden Langur

Adult Javan Langurs; coat colours have several variations

Capped Langur; has patches of orange fur in adults

There have been several theories put forward as to what advantage the orange fur provides for the babies;

1. Babies are bright orange so that mothers can easily find the young in the forest when they wander off.

2. Babies are bright orange as a form of camouflage

3. Babies are bright orange to ease identification within the group to encourage, or more correctly, induce alloparenting.

The first theory is self explanatory. Bright orange is, well.....bright. It'll stick out among the leaves of the trees. The newborn is bright orange for only a few weeks as the colour transition to black/grey occurs soon after birth. During this time the infant is essentially glued to the mother's chest, thus very unlikely to stray.

In respect to the second theory, there are no predatory bird species preying on these monkeys. Most mammalian predators, especially felines are red/green colour-blind, This makes bright orange a great camouflage colour for newborns, but grey black is effective too,

For the third proposition, Old World Monkeys are trichromatic (they can see red/green/blue like us) and thus would be able to identify the infant against the dark adult coats. Alloparenting is a practice in some species where individuals other than the parents will provide care for the infant. This can include babysitting, carrying, and feeding. Alloparenting can increase reproductive success by providing more protection from predators, assist in the infant's social development and by helping non-parents learn mothering skills before raising their own.

I believe the appearance of a small orange-coloured infant is a visual signal that triggers a hormonal response in the mature females of the troop with the result that the infant gets the best support during its most vulnerable time, the first two weeks of life. The arboreal feeding does not cease and the new mother must keep up with the others in moving through the forest.

The social structure of the troop is pertinent to this discussion.

Lutungs live in groups of 5-20 animals, mostly in harems ie. A single male with a number of females. The young are the progeny of the alpha male. Young males leave their birth group when fully mature (4-5 years old), often forming bachelor groups. The troop, via the alpha male, coordinates a number of activities and moves through the forest together. Thus it is in the interest of the troop to support the most vulnerable members.

My evidence comes from observing and photographing one troop, with around 20 members and their behaviour surrounding the arrival of two newborns to the group during 2020, one in May and one in July. They live in a dense rainforest and cannot be seen at all times, so the evidence is incomplete. Some of the interactions seen depend on the personality traits of the mother and these were somewhat different.

I will describe the early journeys of Baby A and Baby B.

When Baby A was introduced to the troop it was reminiscent of da Vinci's painting of 'The Adoration of the Magi' Other females in the troop gazed in awe and admiration and wanted to hold the infant. The mother seemed very independent and would not give her baby to others but they would assist with grooming and toilet duty. This trait continued when the infant was a little older...where she snatched the baby back if others tried to hold it. The mother was daring and took on the highest jumps with the infant clinging tight. She was an attentive mother and introduced the young one to eating leaves, much to the infant's mirth.

Adoration of the new arrival (Baby A)

Adult on the right wants to cuddle the infant. The mother would not allow it

Weeks later the adult on the left wants to hold the infant but the mother and the infant resist.

The mother would make daring leaps when the infant was only days old. Survival depends on the infant's grip.

The mother would climb tall trees and jump from them into the canopy below

The life lessons are later done by the mother only.

Baby B was met with a similar adoring group when introduced to the troop. An aunty followed the mother for the first week. Several aunties took turns to hold the baby while the mother ate. An aunty also assisted with grooming while the mother nursed the infant. On another occasion the troop had to traverse a group of palm trees. It required precision jumping to grasp the thin stems. A group of three langurs arrived at the start of the obstacle course. One langur, possibly the mother, was carrying the infant. The trio appeared to have a conference and the baby was handed to another langur who completed the course with the young passenger. who was then handed back to presumably the Mum. It appeared they had selected the individual best able to complete the course with the impost. All these incidents (shown below) are clearly examples  of alloparenting. 

The arrival of Baby B also created an admiring audience.

In the first few weeks the mother had assistance from one or two aunties.

The mother is in the foreground eating while the infant is held by an aunty,

Another aunty arrives to look after the infant

The Aunties continue their alloparenting

While the mother nurses the baby an aunty grooms the infant

The palm trees where the most accomplished jumper took the infant.

The assistance the mother gets does not last more than a month and seems proportional to the intensity of the orange colour of the infant which starts giving way to the adult grey/black within weeks of birth. The adoration cluster seems to last only a week or less.

It was one of my main aims to record the colour transition on one of the infants I obtained a complete set with Baby B.  The numbers represent the number of days from the baby's birth. To see how long it took for the orange to fully disappear.

 

In the image below although mainly orange (around 50 days old) the infant is very mobile, very cheeky and sitting on its father's head.

At around 50 days old and very mobile the infant sits on the father's head.

As the infant becomes more adept at getting around trees the mother does not need any direct assistance.....although members of the troop will play with junior and prevent it straying too far.

So what triggers the response from the other females?

It is most likely the arrival of the small orange-coloured baby triggers a visually-induced hormonal response in the 'aunties'. Primate hormones are the same as those in humans and although it is sure to be a cocktail or hormones involved, it likely involves oxytocin.

Oxytocin is known as the 'cuddle' or 'love' hormone. It is a powerful peptide hormone that acts as a neurotransmitter in the brain. It plays an important role in reproduction, initiating contractions before birth as well as milk release. It is thought to be involved in broader social cognition and behaviour, potentially ranging from mother-infant bonding and romantic connection to group-related attitudes and prejudice. The hormone is produced in the hypothalamus and released into the bloodstream by the pituitary gland.

As a facilitator of bonding among those that share similar characteristics, the hormone may help set in motion preferential treatment of in-group members relative to those outside of one's group.

Listed below are the conclusions from this observational study;

1. The gene(s) conferring orange fur are ancestral in trachypithicus species and are expressed during various development phases but notably in newborn infants.

2. Any physical trait in a species is controlled by the expression of a specific set of genes and will be retained over time if it provides a survival advantage (in this case the retention has been for at least several million years).

3, The social group or troop is a team with a common interest for survival and inter-relationships.

4. Orange-coloured newborns trigger a visual response in mature females that leads to hormone release and alloparenting traits.

5. The assistance given is short-lived but assists the infant through the most vulnerable stage of its life.

6. The visual trigger fades almost straight away and is gone in four months. Every effective triggering system requires an off-switch as well.

All animal behaviour comes down to biochemistry

The final question to answer is; what do mothers do when their infant is being minded by aunties?

The answer;

References

This article is mainly based on observation and correlation. I have used few references but the following were sources of material: Done in modern internet referencing rather than traditional science requirements.

Best done this way due to blog constraints

Reference 1.  Here

Reference 2. Here

Reference 3. Here

Reference 4. Here

Graeme is a retired Professor of Biochemistry with a passion for wildlife photography. He has written over 100 scientific papers.

Website here