Animals can't stand the heat either: millions of years of evolution completed in just a few decades

Climate change is accelerating, and animals are facing unprecedented challenges. Can they adapt to this pace?

By Andy Carstens

Compiled by Wang Chao

Figure 1. South African ground squirrel (Xerus inauris) | Source: Wikipedia

The adorable South African ground squirrel (Xerus inauris) lives in the arid savannahs of southern Africa and in tropical and subtropical scrub. To cope with the intense heat here, they have evolved a series of strategies, such as having extra-large hind paws to facilitate heat dissipation; lying flat on their backs to dissipate heat from their hairless bellies; and bending their furry tails to protect their heads like parasols. When the heat is too much, these burrowing mammals retreat into their holes to cool down. However, climate change is accelerating, with the highest daily temperatures in South African nature reserves increasing by 2.5°C in just 18 years. Miya Warrington, a conservation ecologist at the University of Manitoba in Canada, said that although South African ground squirrels have mastered so many cooling techniques, they may soon be unable to bear it in a rapidly changing climate.

Figure 2. South African ground squirrels protect themselves from the heat in various ways. One way is to lie flat on their backs and dissipate heat from their bellies.

Warrington observed that in less than two decades, the squirrels' already very large hind legs grew by about 11% relative to their body size, while the length of their spines shortened by about 6%. The huge environmental pressure brought about by rising temperatures may be the reason why their bodies have undergone such a short period of deformation.

The South African ground squirrel's metamorphosis is not an isolated case. Evidence is growing that many species have undergone subtle changes in body shape in a short period of time. But we don't know whether animals can adapt quickly enough to keep up with rising temperatures, or how close they are to the tipping point of population collapse.

Small body, long limbs

What is the relationship between temperature and body size? In the late 19th century, two biologists proposed two independent but related hypotheses. Bergmann's rule states that animals living near the tropics will be smaller, while Allen's rule predicts that animals in warmer areas will have longer limbs. The two hypotheses have a common meaning, that is, the body size of warm-blooded animals will change with latitude based on temperature differences, and these trends are temperature adaptations that animals produce to meet different heat dissipation needs.

“When you’re smaller, you have more surface area per unit volume, which helps dissipate heat better,” explains Casey Youngflesh, a quantitative ecologist at Michigan State University. Bergmann’s rule accounts for the effects of latitude, and Youngflesh was trying to figure out whether birds would get smaller as climate change heats up North America as a whole.

Youngflesh and his colleagues scoured bird data compiled by the Institute for Bird Populations. Looking across the entire range of 105 bird species, they found that 80 species in North America had significantly reduced their body weight over the past 30 years. After analyzing no fewer than 250,000 birds, they found that the average weight of all birds decreased by about 0.6%, with the tree swallow (Tachycineta bicolor) experiencing the largest drop, about 2.8%.

Figure 3: A scientist measures and prepares this Indigo Bunting (Passerina cyanea) for tagging as part of the ongoing monitoring effort at the Bird Population Research Institute.

While these numbers may seem small, most evolutionary changes occur on geological timescales, so the fact that these birds' weights changed in just three decades is astounding.

Ornithologist and evolutionary biologist Phred Benham agrees with Youngflesh: "The scale of their project is huge. The fact that so many species have changed in such a short period of time does suggest that some global factor has affected these birds - most likely climate change."

Youngflesh's research found that although the absolute length of bird wings did not change, the relative length of wings compared to the body became larger because the body shrank. Although Allen's law believes that limb lengthening is related to heat dissipation, Youngflesh believes that the phenomenon of bird wings becoming longer is not so much related to heat dissipation, but more due to the need for seasonal migration. The farther the bird population has to migrate, the longer their wings will become. Youngflesh believes that this finding can show the extent to which birds need to maintain the ability to fly long distances seasonally.

Benham is concerned about the changes in bird beaks. He believes that, unlike changes in wing length, changes in bird beaks may really be due to temperature. The larger the surface area of ​​the bird's beak, the better the passive heat dissipation effect. 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 Savannah Sparrow (Passerculus sandwichensis) and found that, as predicted by Allen's Law, the further south a population goes, the larger its beak. However, only the subspecies P. s. alaudinus, which lives in coastal Northern California, had its beak enlarged attributable to the hot weather brought on by climate change. These birds live in inland California's high-salinity tidal marshes, where fresh water is scarce, and their beak surface area has increased by about 7% in 150 years. This is estimated to reduce water loss by about 16% per day.

Benham suspects that P. s. alaudinus's beak grew so much because the cost of staying cool in a water-deprived environment is likely to be greater as the weather gets hotter, so he was curious to see whether birds in dry areas in Youngflesh's analysis would shrink more dramatically than birds in wet areas.

Pollination issues

Bergmann's rule and Allen's rule suggest that homeotherms have evolved different sizes over thousands of years to adapt to temperature gradients caused by latitude. Another rule, the temperature-size rule, describes the phenotypic plasticity that is prevalent in poikilotherms. "In almost every insect that scientists have studied, when the temperature of the environment in which the larvae develop increases, the adult size always decreases," said Michelle Tseng, an insect and aquatic ecologist at the University of British Columbia. "This is because warmer environments speed up biochemical reactions during development and shorten the maturation process of poikilotherms."

However, when studying the impact of these changes on the real world, scientists often do not clearly distinguish between statistical significance and biological significance. The latter is statistical significance that has a significant impact on the health or survival of organisms. Statistical significance is only evidence to support biological significance, and statistical significance does not mean that there is really a difference in the biological system. As for why the two types of significance are not distinguished, sometimes it is because researchers really do not know the impact of their findings on the real world, but sometimes it is because statistical significance is more important for paper publication. In order to study biological significance while paying attention to statistical significance, Tseng designed an experiment to study the impact of temperature-induced body size reduction on butterfly pollination behavior.

Pollinator-plant interactions are critical to biodiversity because plants rely on pollinators to mix genes. But how insect body size affects plant-insect interactions is not well understood. Tseng first studied how climate change is changing the body and wing size of the cabbage butterfly (Pieris rapae). Tseng grows kale in pots outside her home, and after the butterflies lay eggs on the leaves, she gently removes the kale and brings it to her lab.

After the eggs hatched, Tseng and his colleagues placed the larvae in incubators at 18 ° C, 24 ° C, and 30 ° C. Compared with the butterflies raised in the coldest environment, the ones raised in the warmest environment matured about twice as fast, had the lowest weight, the smallest wing area, and flew slower.

To determine whether these morphological changes are biologically significant, Tseng and her colleagues collected the same species of Pieris rapae from the wild and divided them into large and medium groups of similar size based on the size of the Pieris rapae raised in the laboratory, and then analyzed how much pollen the different groups accumulated after flying. The experimental team used gelatin to collect pollen from the faces and mouthparts of the Pieris rapae, and evaluated the amount of pollen carried by the Pieris rapae and the corresponding plant species under a microscope. They found that smaller Pieris rapae (equivalent in size to laboratory Pieris rapae raised in the warmest environments) carried fewer pollen plant species. Tseng believes this result is important and may have practical significance in terms of biodiversity.

Meredith Johnson, a graduate student in insect physiology at Arizona State University, has discovered another pollinator that may become smaller due to climate change. Looking at data collected in the field over the past five years, she found that male digger bees (Centris pallida) have a decreasing head width, an important feature that describes their body size. Digger bee males are dimorphic, that is, they appear in two different body sizes, which have different mating behaviors. Although all males have become smaller, the larger morph of the dimorph has the largest decrease in head width - about 8 percentage points. Johnson said that although it is unclear what the phenomenon will have, because larger males are more likely to mate successfully, as males continue to become smaller, the population may decrease accordingly.

Figure 3: A male digger bee of the trumpet variety copulates with a female bee on a mesquite (Prosopis) bush.

There are two reasons for the reduction in male digger bees: the rise in ambient temperature during their development and the mismatch between the phenology of bees and their host plants caused by climate. Phenology refers to the relationship between biological cycle phenomena (such as plant flowering or bee nectar collection) and seasonal climate. Although Johnson has not yet tested these two speculations, the most likely reasons are related to climate change. In her words, "I can't think of any other reason."

Johnson believes that climate change is a greater threat to oligophagous bees like digger bees than to polyphagous bees. Most of the 20,000 existing bee species are oligophagous, meaning they only eat nectar from a few specific plants. Digger bees, for example, rely on nectar from a tree called Parkinsonia. When flowering is abnormal or nectar production is reduced, digger bees have no other way to feed themselves.

It is not clear whether the changes in insect body size are due to body plasticity, a fast-forward version of evolution, or both. Although many scientists have done research for many years, no one has been able to provide evidence to answer this question. Currently, people are trying to analyze how plasticity and evolution affect insect body size changes through a large number of studies.

Suffocating in water

The temperature-size rule also applies to aquatic cold-blooded animals, but it is difficult to separate the effects of temperature from those of fishing: fishing always removes the largest fish from a given population, which itself is a selection pressure that favors smaller fish.

For this reason, many speculations about the future of marine life come from the fossil record and other paleontological samples. Renato Salvatecci, a paleoceanographer at the Center for Ocean and Society Research at the University of Kiel in Germany, analyzed sediment cores from the coast of central Peru about 120,000 years ago. They mainly observed the Eemian interglacial stage, the last interglacial period on Earth, during which the weather was warmer than it is now. The results show that when the temperature of the sea water was about 2°C higher than it is now, the fish in this area of ​​the South Pacific did not become smaller, but seemed to have migrated to more suitable areas for survival. However, they have seen a large-scale reduction in the size of fish in many places, but it is difficult to distinguish the reasons behind it. Perhaps different species have different reasons.

When studying how marine animals respond to climate change, changes in the oxygen content of seawater complicate matters. Most marine animals do not surface to breathe, so they must draw dissolved oxygen from seawater. As ocean temperatures rise, oxygen becomes less soluble, and there is less oxygen in the water for fish to breathe, just as at high altitudes on land, the thin air can make people feel suffocated.

The subtle thing is that rising temperature will increase the diffusion rate of oxygen and reduce the viscosity of water, which to some extent compensates for the effect of reduced oxygen solubility.

Using previous experimental data on the effects of temperature and oxygen on the body size of cold-blooded marine animals, paleontologist Jonathan Payne of the Stanford School of Sustainability built a model to understand the current distribution of species and predict how these species might respond under different climate change scenarios. This model takes into account the sensitivity of metabolism to temperature and the imbalance between oxygen supply and demand caused by ocean warming. In theory, the oxygen supply must exceed the oxygen required for animal survival, and for larger organisms, the oxygen supply and demand gap decreases faster.

Payne's model estimates that for species weighing about 1 gram, such as zooplankton, their biomass decreases by 10 percentage points for every 1°C warming of the seawater. Therefore, a 1°C warming will not bring about a super disaster, but we don't know what the exact ecological impact will be; and in the case of a 5°C warming, body size needs to be reduced by 25%. If this continues, at some point in the future, organisms 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. Just 1°C of warming would require them to shrink by 20%, while 5°C would require them to shrink by 80%. "That's a lot, right?" Payne says. And it could have serious consequences for many other species. "The big fish eat the small fish, and the small fish eat the shrimp," he says. "It's definitely going to spread through the food web."

Although we don't know yet whether Payne's model is accurate for the next 100 years or more, one clear conclusion from the model is that if large organisms don't move to cooler habitats or change their behavior, they will have to make changes in size in a short period of time, which they will most likely not be able to do, and thus may lead to extreme selective extinction.

Ripple Effect

Warming up temperatures affects more than just marine biomass. Because large animals transport nutrients over long distances, the impacts are felt across entire ecosystems.

Take salmon, for example: salmon eat in the ocean, absorb phosphorus, migrate upstream and reproduce in the river; then bears come to the river and eat the salmon, taking phosphorus from the river, and then move over land and excrete it somewhere on the hillside. Payne calls this phosphorus movement "anti-gravity," and this mode of nutrient transport is impossible in the non-living world.

“Think about the schools of fish in the ocean, think about the animals that are churning up sediment on the seafloor, think about the insects and other animals that are turning over the soil and transporting nutrients across the land, and suddenly you realize that the flow of nutrients in the modern world is clearly deeply biologically imprinted,” he said.

Tseng puts it more bluntly: “If it weren’t for those dung beetles working silently, it would be you and me shoveling cow dung every summer.”

Almost all relevant professionals believe that if species disappear because they cannot adapt quickly to climate change, it will inevitably bring similar ripple effects. Youngflesh pointed out that in the past 50 years, 30% of migratory birds have disappeared from the world, which is why we need to study this seemingly insignificant 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 understand more about how the various pieces of nature are put together and understand the response of ecosystems under climate change, "maybe we won't be so helpless."

This article is authorized to compile from
https://www.the-scientist.com/news-opinion/animals-are-shape-shifting-in-response-to-a-warming-world-70869, title is editorially created.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.