Producer: Popular Science China
Producer: Yang Xifu (Assistant Researcher, Institute of Zoology, Chinese Academy of Sciences)
Producer: Computer Network Information Center, Chinese Academy of Sciences
When I lived in the countryside as a child, what I hated most at night was the sound of "clicking" and "squeaking" from upstairs.
When the sound is too loud to bear, we will knock on the wall to make some movement, and they will be quiet for a while after being frightened, but it will not be long before they make the same sound.
The culprits of these irritations are some residential rats, such as brown rats and small house mice.
They belong to rodents, because of the damage to furniture, the destruction of food, the spread of diseases, etc., people have used a variety of methods to hunt them, but with little effect, let us have to lament their strong viability.
Wild mice, is not also very cute| Picture source: the author took it
Not only that, in early 2021, a study reported that many large mammals are on the verge of extinction due to increasing human activities, increasing climate change, and many large mammals, but small and medium-sized mammals such as rodents have become the biggest beneficiaries of major environmental changes, and predict that rodent diversity will continue to increase over the next 50 years.
<h1 class="pgc-h-center-line" data-track="10" why > are large mammals endangered or even extinct? </h1>
Before answering the question of why rodents are the biggest beneficiaries of environmental change, let's take a look at the main endangered mammals. Why can't they survive?
Perhaps we will have a more comprehensive understanding of the expansion of small mammals.
Since 1500, 322 species of terrestrial vertebrates have gone extinct, and the number of remaining species has shrunk by an average of 25%.
At present, the endangered mammals mainly include Malayan tiger, Indochinese tiger, Sola antelope, Mengjiala tiger, Tabanuri orangutan, giant panda, golden snub-nosed monkey, South China tiger, Tibetan antelope, elk and so on.
From the perspective of these endangered animals, they have several common characteristics: large individuals, low fertility, slow growth and development, and low adaptability.
A study in the journal Science compared weight data from all known mammals that have gone extinct since the Pleistocene (< 50,000 years) and found that larger animals are more susceptible to natural selection and more vulnerable.
Rodents are not threatened due to their small size| Image source: References[3]
Today, the decline in large and medium-sized mammal populations has received a lot of attention, and scientists are also trying to find the cause of the decline.
For example, large mammals such as mammoths and giant tree otters that lived on Earth more than 20,000 years ago disappeared more than 20,000 years later, and some researchers have blamed their extinction on human hunting.
Later, some researchers analyzed animal radiocarbon and paleoclimate data in fossils, arguing that sudden changes in climate were responsible for the spiraling decline in the numbers of these species.
However, this conclusion has also been questioned, as the authors argue that when fossils disappear in one place, it means species extinction, which is a bit of an over-interpretation of the integrity of the fossil record.
Skeptics argue that humans are the decisive factor in the extinction of large mammals, based on the fact that the extinction of many animals occurred after the advent of humans.
Extinction of large mammals and paleoclimate records| Image source: References[1]
Coincidentally, using the same fossil data, researchers found that climate change caused the concentration of carbon dioxide in the atmosphere to decrease, which in turn promoted the expansion of large mammals into grasslands, rather than the influence of ancient humans, and ultimately led to the extinction of species.
Recently, Chinese researchers presided over a study published in the Proceedings of the National Academy of Sciences, through quantitative analysis of the data in China's historical documents, found that in the past two thousand years, the population of large mammals (such as elephants, rhinos, giant pandas) declined not only earlier than medium-sized mammals, but also reduced rapidly.
In addition, they found that the local extinction rate of mammals was associated with the intensity of disturbances from human activities and extreme temperatures (cooling or warming).
Population decline and survival rate changes of 11 large and medium-sized mammals in China from 905 BC to 2006 AD|Pic source: References[8]
Due to the different objects of study, the time span, and the methods used by scientists, different results have been obtained.
However, the reason is that scientists believe that it is mainly due to the inability of individual organisms to adapt to environmental changes under the influence of external factors such as human interference and climate change.
Whether human interference and climate change work together or both need more research to verify.
<h1 class="pgc-h-center-line" data-track="10" > rodents are super adaptable and are the biggest beneficiaries of environmental change</h1>
Unlike the fate of large mammals, small and medium-sized mammal taxa such as rodents and rabbits have a lower risk of extinction.
Further research found that small sizes, smaller home areas, and flexible dietary combinations made rodents more likely to thrive in human-influenced ecosystems.
Using the fossil record and extant data, the researchers found that large mammals had disappeared from environments with frequent human activity and moved to colder alpine climates compared to the pre-Industrial Revolution period, while rodents had expanded their range of climate adaptation due to land use in the United States.
Recently, a report in the journal Molecular Biology and Evolution reported that based on data analysis of short segments of the genome and whole genome, researchers found that rodent species derived from Southeast Asia and habitat generalization have seen significant population growth in recent times, and speculated that their potential distribution areas may expand further in the future.
For example, gray-bellied rats distributed at high altitudes show an expansion trend in population size and may continue to expand to the plateau surface.
Further analysis revealed that this was due to significant changes at the genome level of gray-bellied rats to adapt to the environment of the Tibetan Plateau.
Differentiation time and phylogenetic history of the genus White-bellied rat| Image source: References[5]
The reasons for the evolutionary success of rodents may be diverse.
The first is the small size of the rodent individual
Small individuals, the daily amount of food is relatively small, you can open up, adapt to the environment that is not suitable for individual animals, so as to establish a large population.
The second is that rodents have a miscellaneous diet and good teeth
They eat a variety of seeds, grasses, leaves, insects, worms and all kinds of small vertebrates.
In other words, they are not picky eaters. And because there are two pairs of lifelong growth front teeth, sharp after polishing, there is almost nothing that cannot be eaten.
In addition, rodents have strong fertility and rapid growth and development
A female brown house mouse can reproduce up to 15 litters a year, each litter can give birth to 3-7 cubs; and their growth and development ability is very fast, and small brown house mice only need 3-4 months from birth to maturity.
Because of the rapid replacement, more genotypes can be preserved and mutated, so that when the environment suddenly changes, there are always some lucky people who can survive. Combined with other features, they were soon able to create a new rodent population.
Because of these superpowers, they have also become the most successful evolutionary taxa of living mammals, with almost the number of genera and species combined with all other orders.
There are 192 species in 9 families and 33 genera in China (2277 species in the world), including squirrels, Japanese sleeping mice, beavers, jerboas, prickly rodents, mole-shaped rodents, hamsters, rodents, and porcupines.
<h1 class= "pgc-h-center-line" data-track="10" > rodent populations do not fall but increase, is it a good thing or a bad thing for humans? </h1>
The rodent behavior mentioned at the beginning of the article makes us grit our teeth in hatred, in addition to affecting damage to homes, crops, and affecting our rest, they are also the wild hosts and vectors for a large number of animal pathogens (such as hantavirus, plague).
However, are rodents useless to humans? The answer is definitely no.
Rodents are small, easy to raise and maintain, and adapt well to new environments; fast reproduction, short lifespan, average 2-3 years, so generations can be observed in a relatively short period of time; and genetic composition and biological behavior characteristics are very similar to humans.
As a result, they are beloved by biomedical experimental researchers, such as the commonly used sized white rats.
Rodents also play an irreplaceable role in ecosystems, such as the main food source of large and medium-sized carnivores, the transmission of plant seeds and fungal spores, so they also play an important role in maintaining the ecological balance.
Not only that, rodents have a long history of evolution, and are highly differentiated in terms of morphology and habitat adaptation.
Therefore, it is also an ideal material for studying important scientific issues such as the impact of human interference and historical changes in the global environment on species differentiation, morphological evolution, and the formation of geographical distribution patterns.
Needle-haired rats processing plant seeds| Image source: Taken by the author
In short, we live in a global wave of biodiversity loss (such as species and population extinctions) caused by human activities, and the impact of humans on animal diversity is a global pattern of environmental change that is not yet fully understood.
This reduction in animal diversity will affect the functioning of ecosystems and the well-being of humans.
Rodents are not without negative impacts in the face of drastic changes in their environment.
Therefore, strengthening ecological governance, restoring green mountains and waters, and making human beings and nature live in harmony can slow down the trend of mammal decline, maintain ecological balance, and protect the earth on which people and wild animals and plants depend.
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