Monkey stories: The ons and offs of black and white leaf monkeys

Primates are close relatives of humans. The most familiar primate to everyone is probably the monkey. There are thousands of species of monkeys. Today I want to introduce to you the white-headed langur and the black-headed langur, a "love-hate relationship" that is "one black and one white" and that breaks up and gets together again and again.

Part 1

The Darwinian mystery of white-headed and black-headed langurs

White-headed langur and black-headed langur are endangered primates in China, with a population of less than 2,000 each. The white-headed langur is a primate endemic to China and is only found in Chongzuo, Guangxi. The distribution range of black-headed langur is slightly wider, and it is commonly found in Guizhou, Chongqing, and Guangxi in China, and in Vietnam and other places abroad.

The distribution range of white-headed langurs and black-headed langurs (provided by Zhao Xumao)

In Guangxi, the white-headed langur and the black-headed langur are separated by a river. They share the same origin. The only difference is the white color on the top of the white-headed langur, which looks like a white hat.

White-headed langur (Photo credit: Huang Chengming)

In addition to the color on the top of the head, the differences between the white-headed langur and the black-headed langur are also reflected in behavioral habits and habitat selection.

Studies have shown that the diet of black and white-headed leaf monkeys is mainly leaves, especially young leaves, while fruits and seeds only account for a small part of their diet. In the dry season, black leaf monkeys will increase their intake of mature leaves and seeds, while white-headed leaf monkeys like to eat a lot of young leaves every month throughout the year, but they will also increase the types of plants they eat when the dry season comes.

White-headed langur (provided by Li Ming)

White-headed langur and black langur share the same origin, but modern taxonomists define them as two different species. So what is the reason for the separation of the two?

The formation of species has always been called Darwin's mystery, attracting countless experts in the fields of evolution and conservation biology. Zhao Xumao's team from the School of Ecology at Lanzhou University and Li Ming's team from the Institute of Zoology at the Chinese Academy of Sciences combined molecular biology and species distribution model evidence to reconstruct the suitable distribution range and effective population changes of white-headed langurs and black-headed langurs since the last interglacial period, solving the mystery of their separation.

According to genomic evidence, white-headed langurs and black-headed langurs completed their differentiation 290,000 years ago. After the differentiation, they still have close connections and there is genetic exchange.

From the genetic exchange between the two, it can be seen that the white-headed langur and the black-headed langur were able to "intermarry" for a long time since they separated 290,000 years ago. The second genetic exchange occurred during the last interglacial period (140,000 years ago), and until 12,000 years ago (the last glacial period), they were still able to intermarry and did not completely separate.

Part 2

What influences the separation and reunion of black and white langurs?

The black and white leaf monkeys have been in a state of separation and reunion for tens of thousands of years. So what are the reasons that influence their separation and reunion?

Based on the assessment of the species distribution model, after the white-headed langur separated from the dark-headed langur 290,000 years ago, due to the warm and humid climate of the earth at that time, the suitable habitats of the dark-headed langur and the white-headed langur could be connected together, and their populations were large enough, so there were many opportunities for "communication".

But the world is full of unexpected events, and the monkeys are subject to misfortunes at any time. The good times did not last long. After the white-headed langur and the black langur separated, they encountered the Guxiang Ice Age (the second to last ice age). Under the influence of the ice age, the white-headed langur and the black langur lost the opportunity to exchange genes, and the two went their separate ways and began to evolve adaptively.

140,000 years ago, the Earth entered an interglacial period and the climate became warmer. The suitable habitats of the white-headed langur and the black-headed langur were connected again. At the same time, the population numbers of the two also showed an upward trend, so they were able to communicate again and rekindle their old friendship.

However, the arrival of the last ice age (21,000-12,000 years ago) broke up the relationship between the two species and interrupted their genetic exchange. During the last ice age, the suitable habitats of the black and white-headed leaf monkeys were separated.

In addition, the cold environment is a serious challenge to the survival of species, and species began to reduce their numbers. According to molecular clock calculations, the populations of white-headed langurs and black langurs both decreased significantly during the last ice age. The remaining populations lived in mountain shelters, forming isolated islands.

Moreover, as the storms of nature became more severe, the number of suitable habitats that became "isolated islands" gradually decreased, and the population also decreased. As a result, the white-headed langur and the black-headed langur became isolated and lost communication with each other again.

The effective population changes of white-headed langurs and black-headed langurs. The green line represents the white-headed langur, the blue line represents the black-headed langur, (a) and (b) represent two different algorithms. (Provided by Zhao Xumao)

After the last glacial period, the cold weather was gone, spring came, and the climate began to have a new impact on the distribution of species, giving white-headed langurs and black-headed langurs the opportunity to communicate again. However, more than 6,000 years ago, the increasing human activities changed the original appearance of the earth, and the "isolated islands" still did not connect to form a "continent", which made them lose the last chance to communicate.

In this way, the original opportunity to reunite was broken by human activities, resulting in the current distribution pattern of white-headed langurs and black-headed langurs. Black-headed langurs are isolated in "isolated islands". Although they are distributed over a large area, they are isolated from each other and cannot communicate. White-headed langurs are isolated by the Zuojiang River and Mingjiang River in Guangxi, and they are like the Cowherd and the Weaver Girl, who are separated by a river and cannot communicate with the black-headed langurs.

At this point, under the dual pressures of climate change and human activities, the black-headed langur and the white-headed langur have completely differentiated and become independent species.

So the next question is, since they have become independent species, can they hybridize if they are put together?

There is no problem with them interbreeding, since they have not been separated for that long, but this cannot happen in nature because of geographic barriers and small changes in behavior that cause reproductive isolation.

Part 3

Protecting the endangered black-and-white leaf monkey

"The fragmented habitats are like isolated islands, which not only lead to the reduction of suitable habitats, but also affect the migration, spread and colonization of species. Different subpopulations living on "isolated islands" have unique population genetic structures. Some of these small populations face a greater risk of extinction than the entire population. The population genetic structure they carry is more likely to be lost, which in turn causes the genetic diversity of species to continue to disperse and die out.

In addition, the rapid decline in the black and white leaf monkey population is also due to human activities, especially illegal poaching and habitat destruction in the last century. Poachers covet the huge profits brought by their fur, bones and other tissues, and sell them to zoos and other institutions for commercial profits.

Currently, the International Union for Conservation of Nature (IUCN) has classified the black leaf monkey and white-headed leaf monkey as endangered (EN) and critically endangered (CR) respectively (Mittermeier et al., 2013). In order to save these lovely natural spirits, we should take practical actions to protect them:

First, strict law enforcement must be implemented to eliminate illegal hunting and provide a legal umbrella of protection for the reproduction and survival of the black and white leaf monkey population.

Second, through publicity and education, people can be made aware of the importance of habitat protection in saving these rare and endangered species.

Third, habitat fragmentation forces primates to interact with humans, and infectious diseases from human groups pose severe challenges to the survival of primate populations. Therefore, it is necessary to conduct research and monitoring of rocky mountain leaf monkey epidemics and zoonoses to create a "buffer zone" between primates and infectious diseases.

The "emotional entanglement" between the black-headed and white-headed leaf monkeys has lasted for tens of thousands of years, resulting in more separations than reunions. For species protection, the separation of black-headed and white-headed leaf monkeys has undoubtedly increased the difficulty of protection. However, for species diversity, this is also a good thing, because their separation has enriched the species on the earth.

References:

[1]Li X, Ru D, Garber PA, Zhou Q, Li M, Zhao X. 2022. Climate change and human activities promoted speciation of two endangered langurs (François' langur and white-headed langur). Global Ecology and Conservation 38 (2022) e02185.

Produced by: Science Popularization China

Produced by: Wei Chang (School of Literature, Lanzhou University, animal translator)

Editor: Wang Tingting

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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