Dogs are getting more and more used to drinking milk as their owners do.

Produced by: Science Popularization China

Produced by: Liu Yanhu (Kunming Institute of Zoology, Chinese Academy of Sciences)

Producer: Computer Network Information Center, Chinese Academy of Sciences

As the saying goes, sweet in the south, salty in the north, spicy in the east, sour in the west, everyone has their own preferences. In this complex world, people in different regions have different eating habits. What about dogs who eat and live with humans? For example, do they drink milk?

Cattle were domesticated in the Middle East during the Neolithic Age between 11,000 and 10,000 years ago, and then domesticated cattle appeared. They spread to ancient Greece 8,400 years ago and to Central Europe 7,500 years ago.

Cows and milk (Photo source: Veer Gallery)

Domestic cattle can help humans with labor and improve labor efficiency. At the same time, they also provide humans with daily necessities and food such as meat, milk and cowhide.

However, when cattle were first domesticated or spread to Europe, local adults were not allowed to drink large amounts of milk directly because this could cause lactose intolerance.

What is lactose intolerance?

Lactose is the main sugar in milk and needs to be broken down by lactase before it can be absorbed. Mammalian infants and young children express a large amount of lactase in their intestines, so they can use breast milk as their main food.

However, most mammals lack lactase in their intestines after adulthood. Lactose is broken down by bacteria in the intestines, producing a large amount of gas, causing symptoms such as abdominal bloating, abdominal pain, and diarrhea. This is lactose intolerance.

How to avoid lactose intolerance?

As early as 7,000 years ago, our ancestors gave the answer. They invented cheese. In the process of making dry cheese, most of the lactose is filtered out or decomposed by microorganisms, and the cheese made is both delicious and safe.

The history of the spread of milk food culture to Europe (Source: The milk revolution)

The study found that the proportion of lactase tolerance is higher among Northern Europeans, while the proportion is slightly lower among people in the Middle East and Southern Europe, and the proportion is lowest among non-nomadic peoples in Asia and Africa. Why is this?

About four thousand years ago, the lactase gene of the European population mutated, resulting in large amounts of lactase in the intestines of European adults, allowing them to drink large amounts of milk like infants.

Milk provides people with rich nutrients and water, and people carrying this mutation are more likely to survive, causing the frequency of this mutation to continue to increase in the European population. This is the effect of positive selection.

However, the vast majority of adults in East Asian populations today do not have this mutation. At this point, many people wonder, “That’s not right. I drink milk every day, so why don’t I have symptoms of lactose intolerance?”

In fact, people with lactose intolerance are not completely unable to drink milk, but they cannot drink a lot of milk. The commonly used dose for lactose intolerance experiments is 50g of lactose in a glass of water, which is roughly equivalent to drinking 1 liter of milk at a time.

The proportion of lactase tolerant adults (Source: The milk revolution)

Following their owners, the dogs are getting more and more used to drinking milk

But did you know that not only do people in Europe and Asia have different lactose tolerance levels, but dogs in Europe and Asia also have different tastes, with European domestic dogs being more adaptable to a milk diet?

Image source: Made by the author

This is actually closely related to Europeans’ eating habits!

Before dog food was invented, dogs usually ate what their owners ate, so dogs living in Europe would drink more milk.

Recently, the team led by Academician Zhang Yaping from the Kunming Institute of Zoology, Chinese Academy of Sciences, the team led by Wang Guodong from the United Kingdom, and the team led by Liang Bin from Yunnan University discovered that the lactase gene is also subject to positive selection in European domestic dogs, and confirmed that the mutation in European dogs increased the expression of lactase, thereby making European dogs accustomed to drinking milk.

These results suggest that European dogs and humans have undergone the same or similar evolutionary process on a milk diet, which requires us to introduce the concept of "convergent evolution".

Dogs and people: Convergent evolution through mutual companionship

Convergent evolution refers to the phenomenon that different species evolve similar phenotypic or molecular-level characteristics in order to adapt to similar living environments.

In addition to the digestive aspects we talked about, dogs and humans actually show convergent evolution in many aspects such as energy metabolism and nerves. For example, the domestication process from wolves to dogs was accompanied by changes in eating habits.

Ten thousand years ago, human ancestors mainly lived by hunting animals and gathering wild fruits. With the origin of agriculture, the hunting-gathering lifestyle of most humans was gradually replaced by agricultural society. During this transition, both humans and dogs gradually adapted to starchy foods.

Image source: Veer Gallery

Studies have found that people who mainly eat a high-starch diet have more copies of the salivary amylase gene AMY1 and express more amylase to better digest starchy foods.

Dogs have a similar phenotype: Dogs from regions where agriculture developed earlier have more copies of the pancreatic amylase gene AMY2B and higher levels of gene expression.

Over the past tens of thousands of years, domestic dogs have gradually evolved from gray wolves in Eurasia to docile dogs that accompany humans. In response to the different preferences and needs of their owners, more than 400 modern dog breeds with various shapes have been bred in the past 200 years.

In fact, in addition to the convergent evolution of digestion between dogs and humans, many animals exhibit convergent evolution at the phenotypic, physiological and molecular levels, which is a common phenomenon in nature.

Two species with extremely similar phenotypes may be very distantly related. For example, whales living in the ocean have morphologies similar to fish, but whales are mammals.

Whales have a well-developed sonar system and rely mainly on echolocation when they are active. The same is true for echolocation bats, which are common in the evening. Echolocation bats emit ultrasonic waves with their mouths and receive the reflected sound waves with their ears to achieve echolocation. This shows that whales and echolocation bats have convergent evolution in echolocation. At the molecular level, mutation sites of convergent evolution have also been found in the genes Prestin, Cdh23 and Otof related to echolocation.

Finally, let’s take a look at what unique characteristics dogs who can “learn to drink milk from their owners” have compared to other animals?

A dog drinking milk (Photo source: Veer Gallery)

First of all, domestic dogs have a close relationship with humans and their living environment is most similar. Studying the genetic mechanism of domestic dogs' adaptation to the environment provides an important reference for analyzing the adaptive evolution of humans.

Secondly, domestic dogs are also important disease models. Of the 809 known diseases or traits in domestic dogs, 503 are potential animal models for studying human traits or diseases. Many of the tumors that dogs develop are very similar to human tumors in terms of histological appearance, tumor genetics, biological behavior, and response to traditional treatments. Coupled with the large number of dogs, it provides an excellent model for studying the occurrence and progression of tumors and the development of drugs.

At present, the research on the genetic mechanism of adaptive evolution and disease in domestic dogs mainly focuses on the stage of discovering related genes, and there is still a lack of research on how genes affect phenotypes. Research on domestic dogs still requires the joint efforts of multidisciplinary teams and the active participation of all sectors of society.

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[11] https://omia.org/home/