Their situation is emblematic of understanding the effects of climate change: the last few decades have been quite difficult for polar bears in western Hudson’s Bay, Canada.
Declining Arctic sea ice, which is crucial for bears to hunt, has contributed to a sharp decline in the species’ population in the region.
Stephanie Penk, a biologist at the NGO Polar Bear International, told BBC News that the number of bears has dropped from 1,200 individuals to 800 since the 1980s.
But Penk and other researchers have also noted that the bear population isn’t the only thing declining: the bears themselves are also getting smaller.
The females got thinner and smaller, losing an average of 65 kg in weight and 5 centimeters in length between the 1980s and 2010. And that’s probably reducing their ability to raise young.
“We’ve seen females with fewer cubs than before, and those puppies are also smaller because the mother doesn’t have as much energy to give them (in the form of milk),” explains Penk.
“Reproduction rates are already low in healthy polar bear populations, so even a small drop in rates is important.”
But polar bears aren’t the only animals that are shrinking.
Scientists believe that rising temperatures on the planet are causing animals around the world to shrink in size.
The phenomenon has been observed in recent decades in hundreds of species, from fish to reptiles, amphibians, mammals and even insects.
And the list of affected species has grown.
Experts fear that these changes could have a profound impact on ecosystems and also affect humanity.
why size matters
In short, body size is a key factor in fertility, life expectancy and ability to survive events such as food shortages or drought.
Animals have always adapted to changes in their environment, increasing or decreasing in size, something that can be easily observed in fossils, for example.
But those changes are now happening very quickly, researchers say. Published in 2020, a study of a salamander species that lives in California found a 20% drop in body condition — a weight-to-length ratio — in just eight years.
This rate of change can affect the animals’ ability to feed or reproduce and can create a ripple effect throughout the broader food chain, especially if some creatures are more affected than others.
Which animals are shrinking?
Scientific studies on the impact of climate change on wildlife have observed that it can affect aspects ranging from where certain species live to changes at important moments in the lives of animals, such as migration and birth.
In the last decade, extensive research has been carried out on the size of the animals’ bodies and its possible link with the increase in the planet’s temperature.
A review of several academic papers on the subject, published in 2020, analyzed more than 50 studies on the subject.
One of the best-known studies is the analysis of more than 70,000 birds killed in collisions with glass windows in Chicago, which are part of the collection of the Field Museum of the American city.
The researchers found that between 1978 and 2016, the total size of 52 bird species declined—the bird’s leg bone length, which is a common measure of body size, shrank by 2.4%.
This shift coincided with a period of rising temperatures not just in America but on Earth as a whole. The United States Oceanic and Atmospheric Administration (NOAA) claims that our planet’s temperature has increased by 0.08°C per decade since 1880 and by 0.18°C per decade since 1981.
“We found that almost every species was declining,” said Brian Weeks, an assistant professor at the University of Michigan’s School of Environment and Sustainability and lead author of the bird study.
“The species were quite diverse, but they are responding (to warming) in a similar way. Finding out from these data was shocking,” he told BBC News.
A wide range of species
More extensive evidence of “reduction” in the size of animals was provided in 2012 by British scientists.
A team of researchers from the University of Liverpool and the Queen Mary University of London has published an analysis of experiments with 169 species of animals that live on land or in water.
The survey found that 90% of them reached maturity at smaller body sizes when temperatures were warmer.
“It’s a widespread phenomenon,” explains David Atkinson, professor of integrative ecology at the University of Liverpool and one of the study’s co-authors.
Another analysis of eight fish species in the North Sea showed that animals in six of them experienced a reduction in size over the space of 39 years as the water temperature has risen since the 1970s.
Fish and other aquatic organisms are an important food source for billions of people around the world, according to the Food and Agriculture Organization of the United Nations (FAO).
But could these changes in body size amid rising global temperatures be a simple coincidence?
Professor Atkinson explains that declining body size is what scientists call the “third universal response” to global warming, alongside changing species life events and geographic spread.
“Certainly more research needs to be done to analyze the effects of temperature on nature, but what we have seen are worrying signs”, said the professor.
“Changes in body size can affect individual survival and reproductive success, which can result in an impact on the structure and functioning of ecosystems.”
As not all animals are getting smaller at the same rate, the change could lead to a scenario where predators need to eat more prey to satisfy hunger, a situation that would be exacerbated if the fertility rate of animals that are decreasing in size also fell.
Animal temperature and sizes
Differences in the size of animals of the same species have long been observed in nature.
In the 19th century, German biologist Carl Bergmann found that species of warm-blooded animals — mainly birds and mammals, which are capable of producing heat within the body — tend to be larger in size in colder regions than in warmer places.
This pattern, although not universal, is known in zoology as the Bergmann rule.
In simple terms, larger individuals are better at retaining heat than smaller ones.
A similar effect, known as the temperature and size rule, has been observed in cold-blooded animals such as fish, amphibians and reptiles.
There are other factors to consider, of course — the body size of some animals seems to be more sensitive to vegetation and food quality than temperature.
Of course, there are exceptions to the general rule: a 2011 analysis of dozens of species, done by scientists at the University of Singapore and published in the scientific journal Nature Climate Change, found that some animals did grow in warmer environments, but four of them. five species analyzed recorded a decrease in size.
“Many studies are supporting this general trend. As more research comes out saying the same thing, we need to understand why this trend is happening and what it will mean for society,” wrote study authors, Professors David Bickford and Janet Sheridan
prehistoric ‘shrinkage’
There is also good evidence that temperature has influenced animal size for millions of years.
About 56 million years ago, Earth’s temperatures rose by as much as 8°C in the space of 10,000 years, according to paleoclimatological data.
Fossils from animals such as Sifrhippus, an ancient horse that lived in what is now the Midwestern United States, show that the species first experienced a 30% reduction in body size and then recovered, growing by 76%. Scientists have observed the same phenomenon in other creatures such as crustaceans.
Similar results were found in studies of land animals that lived in the UK over the past 750,000 years, carried out by the UK Museum of National History. Scientists have detected fluctuations in the size of the red deer’s bones.
This “shrinkage” was reversible in some cases. Researchers at the University of New Mexico, in the United States, studied fossilized fecal pellets from a species of rodent, the thick-tailed rat, and found an “accordion effect”.
The rats’ body size increased and decreased over 25,000 years, in line with the rises and falls in temperature.
“Body size decreased during periods of warm weather, as predicted by the Bergmann rule and physiological responses to heat stress,” the researchers concluded.
Many species have proven to be remarkably resilient to climate change in the past, but there are fears that, in contrast to earlier eras, man-made climate change is warming the Earth at a rate that would mean that any loss of body weight will not be easily regained.
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