Infanticide refers to the behavior of adult animals killing cubs of the same kind, which is what we often call cannibalism. From the perspective of the species population, this is an unfavorable behavior and belongs to internal friction.
However, according to long-term research on 260 species of mammals by many scientists from Britain and France, it was found that about 46% of species (114 species) engage in infanticide. The protagonists of these infanticide behaviors are both male and female. Among them, males are only observed to kill the offspring of other males of the same kind, but females will not only kill the offspring of other males of the same kind, but also kill their own offspring .
Obviously, this behavior exists in so many mammals, and this behavior has not eliminated the species. This means that infanticide has benefits, the benefits either offset the disadvantages, or outweigh the disadvantages .
Let’s take two examples to understand. For example, female cats and dogs have the behavior of killing or even eating their own cubs, and the cubs they eat are usually individuals that are obviously weaker than other cubs or individuals with congenital deficiencies. This elimination will not affect the number of their own population, because these cubs will be eliminated sooner or later.
For another example, male lions killing other male lion cubs happened after the replacement of lion king by . After the replacement of lion king, the lion king changed owners. If there are still lion cubs under 1 year old in the lion group at this time, they will be killed by the new lion king. The purpose is In order to let the lioness enter the estrus again, so as to leave her own offspring as soon as possible , this is actually a link of survival of the fittest, because the original male lion is no longer capable of protecting his "wife and children", and it will be a matter of time before he is eliminated. The new lion king is stronger and the offspring he produces are better than the old lion king.
It can be seen from this that infanticide also has certain benefits. For these animals, the benefits are either balanced against the disadvantages, or the benefits outweigh the disadvantages, so they have not been eliminated by natural selection.
Although infanticide is relatively common among mammals, and polar bears , as the largest carnivores on land, have also been observed to engage in infanticide, this behavior is not normal for polar bears, and polar bears even rarely committed cannibalism in the past. Why do you say this? Let’s find out together.
Cannibalism of polar bears
Polar bears are the largest extant Ursidae animals, and are also the bears that live in the north. According to Bergmann's law, similar species live at higher latitudes, the larger their size, so polar bears comply with Bergmann's law among bears.
Because polar bears live in the ice and snow of the Arctic, they do not hunt on the ground like other terrestrial carnivores. Polar bears prefer and are good at hunting on ice floes.
More than 90% of the prey of polar bears is composed of seals and fish. They hunt on ice floes for a long time, and polar bears have also developed the ability to swim. Polar bears can swim nearly 100 kilometers a day at a speed of 10 kilometers per hour, and can dive into 3-6 meters of water and hold their breath for 3 minutes and 10 seconds. This is already very powerful for a behemoth with an average size of about half a ton.
Because seals and fish resources are relatively abundant in the ocean, for a long time, scientists only observed male bears fighting for mates during the estrus period, and such duels were often not fatal.
But by 2004, everything changed. When scientists were conducting scientific research in northern Alaska and western Canada, they discovered three incidents of polar bears cannibalizing each other. For the first time, scientists have observed polar bear cannibalism and infanticide. Two of the cases involved female polar bears being killed and eaten by male polar bears, and one involved a female polar bear who had just given birth being bitten to death in a cave, and her cubs were also eaten.
In 2020, polar biologist Polar Biology and his research team observed 14 incidents of polar bear cannibalism in Frans Josef Land, Novaya Zemlya, and Viz Island in the Barents Sea and Kara Sea, including intraspecific killing, cannibalism, and fatal attacks.
In other words, since 2004, polar bear cannibalism has increased year by year. Prior to this, Steven Amstrup, a scientist at the USGS Alaska Science Center, and his research team conducted 24 and 34 years of research on polar bears in the southern Beaufort Sea in northern Alaska and northwest Canada. During this period, they did not find polar bears killing their own kind or cubs.
It can be seen that the polar bear's temperament suddenly changed, and this temperament change became increasingly obvious. So, what makes the polar bear "crazy"?
The reason for polar bear’s “big change in temperament”
After scientists first observed cannibalism among polar bears in 2004, the reason behind it has been a mystery. Later, scientists believed that male cannibalism The reason behind the infanticide behavior of polar bears should be to increase the chance of reproduction. This theory is inferred based on one of the 14 polar bear cannibalism incidents in 2020, when a strong male polar bear killed and ate the one-year-old cub of a female polar bear.
After all, female polar bears have to take care of their cubs alone for 2-3 years. During this period, female polar bears do not have an estrus period. This also leads to the fact that female polar bears can only give birth to about 5 babies on average in their lifetime. However, if the cub dies, the female polar bear will enter the normal estrus period again, so that the male polar bear has the opportunity to leave his own offspring.
However, this only explains occasional incidents, because in most cases, males not only kill their cubs, but also spare adult female polar bears. Obviously this is not for breeding, and adult females have also been observed eating their own cubs and the cubs of other females. However, as polar bears behave more and more, scientists have found the real reason: food shortage.
Scientists compiled data on polar bear cannibalism (including infanticide) in the past 20 years and found a problem. Most of the polar bears that commit crimes have a common characteristic - they are very thin . As a mammal living in the polar regions, thinness means malnutrition. With such a body, it is difficult for them to face the severe cold. In the end, they will only die. The thinness is obviously due to hunger.
Secondly, cannibalism of polar bears mainly occurs in the summer when there is less floating ice. The location is usually on land. Under normal circumstances, even in the short summer, there is still a lot of floating ice in the Arctic sea. However, since the 21st century, the amount of summer ice in the Arctic sea is no longer enough to support all polar bear hunting. This is the reason for their thinness and the trigger for cannibalism.
The root cause of all this is actually related to global warming . As early as April 2021, an academic paper jointly published by scientists from many countries published in "Nature Science" revealed to us how fast global warming is after entering the 21st century?
The research team has conducted the most accurate and comprehensive assessment of 17175 glaciers around the world. The data shows that from 2000 to 2019, the annual melting volume of global glaciers was 67 billion tons. , this data is an average, and the rate of global glacier melting is gradually increasing. Among them, about 227 billion tons of glaciers melted every year between 2000 and 2004, and by 2015-2019, it increased to about 298 billion tons per year.
The accelerated melting of glaciers is the best evidence of increasing global warming. Global warming not only causes glaciers to melt, but also causes large areas of ice floes to disappear in cold seas. Polar bears hunt more than 90% of their prey on ice floes. The less ice floes, the lower the polar bear’s hunting success rate. After all, although it can swim, it is still much slower in the water than fish and marine mammals.
And Andrew, a scientist at the University of Alberta in Canada, discovered that ice floes are not only the main battlefield for polar bears to hunt, but also one of the food sources for their prey, because there are many seaweeds on the ice floes. These seaweeds play an important role in the polar marine ecosystem. They are the food source for marine zooplankton 70%, and zooplankton is the main food source of fish, and fish is the main food source of marine mammals. This is a complete food chain.
However, the large-scale reduction of ice floes has caused many seaweeds to lose their breeding grounds, ultimately affecting the entire food chain. This is also one of the reasons why polar bears attack similar species.
Summary
Although there are many animals among mammals that engage in infanticide, polar bears were not originally among these animals. Although they live in the ice and snow, they have abundant food and each have their own territories without disturbing each other. However, as global warming intensifies, the shortage of food has made polar bears’ cannibalism and infanticide more and more serious.
This is worthy of our deep thought, because global warming is a disaster for all animals on the earth, and humans cannot be immune to it.
