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This article comes from the official account of Wechat: ID:fanpu2019, author: Andy Carstens

Climate change is getting faster and faster, and animals are facing unprecedented challenges. Can they adapt to this rhythm?

Write article | Andy Carstens

Compilation | Wang Chao

Figure 1. South African ground squirrel (Xerus inauris) | Source: wikipedia lovely South African ground squirrel (Xerus inauris) lives in the arid grasslands of southern Africa and in tropical and subtropical bushes. In order to cope with the extreme heat here, they have evolved a series of strategies, such as the large soles of the hind feet to dissipate heat, such as lying flat to dissipate heat from the belly with less hair, or bending the fluffy tail like an umbrella over the head to enjoy the cool. And when the heat is unbearable, these burrowing mammals fall back into the cave to cool down. However, climate change is getting faster and faster, and the maximum daily temperature in South African nature reserves has increased by 2.5 °C in just 18 years. Miya Warrington, a conservation ecologist from the University of Manitoba in Canada (University of Manitoba), said that although South African ground squirrels have mastered so many cooling techniques, they may not be able to stand it in a rapidly changing climate.

Figure 2. South African ground squirrels protect themselves from the heat in a variety of ways. One way is to lie flat and dissipate heat from your stomach. Warrington observed that in less than 20 years, squirrels' already large hind feet have grown by about 11 per cent relative to their size, while the length of their spines has shrunk by about 6 per cent. The huge environmental pressure caused by rising temperatures may be the reason why their bodies are deformed in a short period of time.

The deformation of the South African ground squirrel is not alone. There is growing evidence that the bodies of many species have undergone subtle changes in a short period of time. But we don't know whether the adaptation rate of animals can keep up with the rising temperature, and we don't know how far they are from the tipping point of population collapse.

What is the relationship between the temperature of a small body and a long body and the shape of the body? In the late 19th century, two biologists put forward two independent and related hypotheses. Bergmann's rule believes that animals living near the tropics will be smaller, while Allen's rule predicts that animals in warm areas will have longer limbs. The meaning of these two hypotheses is common, that is, based on the difference in temperature, the body shape of thermostat animals will change with latitude, and these trends are the temperature adaptation of animals to adapt to different heat dissipation needs.

Casey Youngflesh, a quantitative ecologist at Michigan State University (Michigan State University), explained: "when you are smaller, you have a larger surface area per unit volume, which is more conducive to heat dissipation." Bergman's law takes into account the effects of latitude, while Youngflesh tries to figure out whether birds get smaller and smaller as the whole of North America gets hotter because of climate change.

Youngflesh and his colleagues did a carpet search of bird data compiled by the Bird population Research Institute (The Institute for Bird Populations). Throughout the distribution of 105 species of birds, they found that 80 species of birds in North America had significantly lost weight in the past 30 years. After analyzing no less than 250000 birds, they found that all birds lost an average of about 0.6 per cent, with the tree swallow (Tachycineta bicolor) falling the most, at about 2.8 per cent.

Photo 3: a scientist prepares this indigo bunting (Passerina cyanea) for posture measurement and marking. This work is part of the ongoing monitoring work of the Bird population Research Institute. Although these numbers may seem small, it is important to know that most of the evolutionary changes are made on a geological time scale, and it is shocking that the weight of these birds has changed in just 30 years.

Phred Benham, an ornithologist and evolutionary biologist, agrees with Youngflesh. "their project is huge. The fact that so many species have changed in such a short period of time does suggest that some kind of globalization is affecting these birds-a factor that is most likely climate change."

Youngflesh's study found that although the absolute length of birds' wings did not change, the relative length of wings relative to the body became larger because the body shrunk. Although Allen's method believes that longer limbs have something to do with heat dissipation, Youngflesh believes that the phenomenon of longer wings in birds has less to do with heat dissipation and more to do with the need for seasonal migration. The farther a bird population migrates, the longer its wings become. Youngflesh said the findings could show the extent to which birds need to maintain the ability to fly seasonally over long distances.

What Benham focuses on is the change in the beak. He believes that, unlike the change in the length of the wing, the change in the beak may really be due to temperature. The larger the surface area of the beak, the better the effect of passive heat dissipation. This heat dissipation process does not require additional metabolism and does not rely on evaporative cooling, which is more conducive to water conservation.

The researchers evaluated four subspecies of the sparse grass bunting (English name: Savannah Sparrow, scientific name Passerculus sandwichensis) and found that, as predicted by Allen's law, the population's beak was usually larger as it went south. But only P. s. Alaudinus, a subspecies living off the coast of northern California, can be attributed to the hot and dry weather brought about by climate change-these birds live in high-salinity tidal swamps in the interior of California, lack of fresh water, and their beak surface area has increased by about 7% in 150 years. It is estimated that this will reduce water loss by about 16% per day.

Benham speculates that the increase in P. S. alaudinus's beak is so large because the hotter the weather, the greater the cost of staying cool in a water-scarce environment. So he wanted to see if birds in arid areas shrunk more significantly than those in humid areas in Youngflesh's analysis.

Pollination Bergman's law and Allen's law believe that thermostatic animals have evolved in different sizes over thousands of years to adapt to temperature gradients caused by latitudes. Another rule, the temperature-body shape rule (temperature-size rule), describes the phenotypic plasticity that is common in temperature-changing animals. " In almost every insect that scientists have studied, when the ambient temperature of larval development increases, the adult becomes smaller, "said Michelle Tseng, an insect and aquatic ecologist at the University of British Columbia." this is because a warm environment accelerates the biochemical response in development and shortens the maturation process of temperature-changing animals. "

However, when studying the impact of these changes on the real world, scientists usually do not clearly distinguish between statistical significance (statical significance) and biological significance (biological significance). The latter is a statistically significant effect on biological health or survival. The statistical significance is only the evidence to support the biological significance, and the statistical significance does not mean that there is a real difference in the biological system. As for why they do not distinguish between the two kinds of saliency, sometimes it is because researchers really do not know the impact of their findings on the real world, but sometimes it is also because statistical saliency is more important to the publication of papers. In order to focus on statistical significance as well as biological significance, Tseng designed an experiment to study the effect of temperature-induced size reduction on butterfly pollination behavior.

The interaction between pollinators and plants is critical to biodiversity because plants rely on pollinators to mix genes. However, people do not fully understand how insect size affects the interaction between plants and insects. Tseng first studied how climate change changes the body size and wing size of Pieris rapae. Tseng planted some kale in the flowerpot outside her house. After the butterfly laid eggs on the leaves, she would gently take them out and take them to her laboratory.

After the eggs hatched, Tseng and his colleagues placed the larvae in constant temperature incubators at 18 °C, 24 °C and 30 °C, respectively. Compared with the cabbage butterflies raised in the coldest environment, the cabbage butterflies raised in the warmest environment mature about twice as fast as the former, with the lowest body weight, the smallest wing area and slower flight speed.

To determine that these morphological changes were significant at the biological level, Tseng and her colleagues collected the same species of Pieris rapae from the wild, divided them into small, medium and large groups of similar size, and then analyzed how much pollen they accumulated after flying. The team collected pollen from the face and mouthparts of Pieris rapae with gelatin and evaluated the amount of pollen carried by the butterfly and the corresponding plant species under a microscope. They found that smaller rapeseed butterflies (the equivalent of laboratory butterflies raised in the warmest environment) carried fewer species of pollen plants. Tseng believes that the results are important and may have practical implications in terms of biodiversity.

Meredith Johnson, a graduate student in insect physiology at Arizona State University in the United States, has found another type of pollinator that may become smaller due to climate change. Looking at the data collected in the wild over the past five years, she found that the head width of male digging bees (digger bees, scientific name Centris pallida) is declining, which is an important feature of their size. The males of digging bees are dimorphic, that is, they appear in two different bodies, with different body sizes and different mating behaviors. Although all males became smaller, the larger variants of dimorphism had the largest decrease in head width-about 8%. Johnson said that although it is not clear what impact this phenomenon will have, because larger drones are more likely to mate successfully, as males continue to get smaller, the population may become smaller.

Photo 3: a small male digging bee is mating with a female on a leguminous shrub (Prosopis). There are two reasons why male digging bees become smaller, one is the increase of ambient temperature during development, and the other is the phenological mismatch between bees and host plants caused by climate. The so-called phenology refers to the relationship between the periodic phenomena of organisms (such as plant flowering or honey collected by bees) and seasonal climate. Although Johnson has not yet tested these two theories, the most likely causes are related to climate change. In her words: "other than that, I can't think of any other reason."

Johnson believes that climate change is a greater threat to oligophagous honeybees such as digger bees than polyphagous honeybees. Most of the 20, 000 species of honeybees are oligophagous, which means they only eat nectar from certain plants. For example, digging bees rely on the nectar of a kind of tree called Parkinsonia. When there is an abnormal flowering or a decrease in nectar production, digging bees have no other way to feed themselves.

It is not clear whether the change in insect size is due to plasticity, a fast-forward evolution, or a combination of both. Although many scientists have done research for many years, no one can produce evidence to answer this question. At present, people are trying to analyze how plasticity and evolution affect the change of insect shape through a large number of studies.

The temperature-body shape rule of asphyxiation in water also applies to aquatic animals with variable temperatures. But it is difficult to distinguish between the effects of temperature and the effects of fishing: the fishing industry always catches the largest fish from a particular population, which in itself is a selective pressure that benefits smaller fish.

For this reason, many speculations about the future of marine life come from fossil records and other paleontological samples. Renato Salvatecci, a paleooceanographer at the Center for Oceanographic and Social Studies at the University of Kiel (University of Kiel) in Germany, analyzed sedimentary cores off the central coast of Peru about 120,000 years ago. What they mainly observed was the Aimu Interglacial Age (Eemian interglacial stage), the last interglacial age on Earth, during which the weather was warmer than it is now. The results show that when the temperature of the sea is about 2 °C higher than the current temperature, the fish in this part of the South Pacific are not getting smaller, but seem to have migrated to a more livable area. However, they have seen a reduction in the size of fish in many places, but it is difficult to tell the reasons behind it, perhaps for different species.

Changes in oxygen content in sea water complicate the problem when studying how marine animals respond to climate change. Most marine animals do not surface to breathe, so they must draw dissolved oxygen from the sea. As ocean temperatures rise and the solubility of oxygen decreases, so does the amount of oxygen in the water for fish to breathe, just as at high altitudes on land, thin air can make people feel breathless.

Subtly, the increase in temperature increases the diffusion rate of oxygen and reduces the viscosity of water, which to some extent makes up for the impact of the decrease in oxygen solubility.

Using previous experimental data on the effects of temperature and oxygen on the body size of marine warm animals, Jonathan Payne, a paleontologist at Stanford University's School of Sustainable Development, built a model to understand the current distribution of species and predict how these species might respond to different climate change scenarios. This model takes into account the sensitivity of metabolism to temperature as well as the imbalance between oxygen supply and demand caused by ocean warming. In theory, the oxygen supply must exceed the oxygen needed for animals to survive, and for larger organisms, the gap between oxygen supply and demand decreases even faster.

According to Payne's model, for species weighing about 1 gram, such as zooplankton, for every 1 ℃ of seawater temperature, the biomass decreases by 10%. Therefore, a warming of 1 °C will not cause a super disaster, but we do not know what the exact ecological impact will be; in the case of 5 °C, the body size needs to be reduced by 25%. If this goes on, at some point in the future, the organism will not be able to cope with this situation either anatomically or physiologically.

For larger organisms, such as cephalopods weighing 100 grams, the model's predictions are even worse. A rise of only 1 °C in seawater requires a 20% reduction in body size, while a rise of 5 °C requires an 80% reduction in size. "that's a big impact, isn't it?" Payne said. And it could seriously affect many other species. "Big fish eat small fish, small fish eat shrimp," he said. "it will definitely spread to the food web."

Although it is not clear whether Payne's model is accurate in predicting the next 100 years or more, it is clear that large organisms will have to make body changes in a short period of time if they do not migrate to cooler habitats or change their behaviour. There is a high probability that this cannot be done for them, so it may lead to extremely selective extinction.

Ripple effect rising temperatures affect more than just marine biomass. As large animals transport nutrients over long distances, the entire ecosystem will eventually be affected.

Take the salmon as an example: the salmon feeds in the sea, absorbs phosphorus, then migrates against the current and breeds upstream; then the bear comes to the river to eat the salmon, takes the phosphorus away from the river, and then moves on land to release the phosphorus somewhere on the hillside. Payne calls the movement of phosphorus "antigravity", a mode of transport of nutrients that is impossible in an abiotic world.

"if you think about the schools of fish in the ocean, the animals that are turning over the sediment on the ocean floor, and the insects and other animals that are turning over the soil and transporting nutrients on land, you suddenly realize that in the modern world, the flow of nutrients is clearly with a deep biological imprint," he said. "

Tseng put it more bluntly: "without those dung beetles who work silently, it would be you and me who shovel cow dung there every summer."

Almost all relevant professionals believe that if species disappear because they cannot adapt to climate change quickly, it will bring similar ripple benefits. Youngflesh points out that 30% of migratory birds have disappeared from the world in the past 50 years, which is why we need to study this seemingly humble change in size. By studying the changes in animal size, we can figure out which species may be in the most dangerous situation. When humans know more about how the fragments of nature fit together and how ecosystems respond to climate change, "maybe we won't be so helpless."

This article is authorized to be compiled from https://www.the-scientist.com/ news-opinion / animals-are-shape-shifting-in-response-to-a-warming-world-70869 under the title of Editor.

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