It's finally spring here in Western Massachusetts! We've transitioned from they gloomy gray of an unpredictable winter to the bright colors of fresh leaves and blooms. This week, why not celebrate spring with equally bright and colorful baby monkeys?
Meet Trachypithecus cristatus! Also known as the silvery lutung, the silvered leaf monkey, the silvered langur, or the silvered monkey--you can take your pick!
The silvery lutung is an Old World Monkey (and member of the Colobinae subfamily) found in mangrove and coastal forests of Peninsular Malaysia, Sumatra, and Borneo.
Fun fact: they are considered to be the most silent of the colobines! Some researchers have called these guys as grave, slow, and expressionless. Nonverbal communication between individuals in the group is extremely important. Things like grooming and play are crucial for maintenance of group social bonds.
They are diurnal (active during the day) and mainly folivorous, with leaves occupying between 60% and 80% of their diet (although they have been known to eat fruit, seeds, and flowers). These monkeys have larger guts to allow for the fermentation and break down of leaves. Adults are a silvery gray color (who would've guessed?) with a pointed crest and cheek hairs on the face. Trachypithecus cristatus has a long, non-prehensile tail (they can't grasp with it) for balance while traversing tree branches.
These monkeys are primarily arboreal and quadrupedal, with some brachiation. Silvery lutungs maintain a polygynous mating system, with single male-multifemale groups--where one male mates with and defends multiple females--and smaller all-male groups that may try to take over a dominant males group.
In the wild, these monkeys can live up to 20 years of age. In captivity, they can live up to 30 years of age. Like the siamangs in our last post, these primates are unfortunately in decline with a IUCN status of 'near threatened' due to logging, hunting for bushmeat, and capturing for the pet trade.
But, it's springtime! So let's focus on something bright!
Silvery lutungs reach their adult size at 5 years of age, and reach sexual maturity between 4 and 5 years of age. They do not have a specific breeding season, but more births occur December through May--corresponding to an abundance of food. Females typically give birth to one infant per year (twins are extremely rare) and have a gestation period of 6 months.
Infants nurse for 18 months, while females only lactate for 12 months (both in the wild and in captivity). Due to communal care and nursing from other females in the group, this isn't an issue for growth and development.
Primates are the most colorful mammals. As such, silvery lutungs are born with bright orange fur and white skin, in contrast to the adults. They are born with their eyes open and a strong forearms to allow them to grip their mother's fur. Within days of being born, their skin turns black like the adults. At around 3 to 5 months of age the infant's fur--beginning with the head--slowly transitions to the silvery grey this species is known for.
Thomas Marent, the photographer who took these featured photos, managed to capture this color change in Kuala Selangor Nature Park, Malaysia.
But why are they orange in the first place?
There are three theories on the infant coloring of these langurs:
1. Babies are bright orange so that mothers can easily find their 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 alloparenting.
The first theory is pretty self explanatory. Bright orange is, well... bright. It'll stick out among the green leaves of the trees.
The second theory only seems to be contradictory. T. cristatus are preyed upon by snakes, tigers, leopards, and even jackals--there are no predatory birds preying on these monkeys. Most mammalian predators, especially felines, are red/green colorblind. This makes bright orange a great camouflage color for newborns.
For the third, Old World Monkeys are trichromatic (they can see red/green/blue like us) and thus would be able to easily 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, like the communal nursing mentioned earlier. This can also include babysitting, carrying, and feeding. Alloparenting can increase reproductive success by providing more protection from prey, assist in the infant's social development, and by helping the non-parents learn mothering skills before raising one of their own.
No one knows which theory is the "right" one--maybe they all are, maybe none are. We may never know why silvery lutungs are born bright orange. Scientists have not reached consensus on the origin of hair; similarly, current research on the loss of hair in humans in comparison to other primates brings in more questions than answers.
One of the lab's current research projects focuses on the evolution ecology of primate hair morphology. Stay tuned to learn more about it in a future post--but for now, check out the Research page for a quick overview of what we're doing!