A study found that 52 of the most common species (物种) of birds in the USA have become smaller in size in the past 38 years. Scientists say species shrink (缩小) in size because of climate change.
This shrinking in size follows a rule called Bergmann's Rule. The rule says that animals in cooler areas are usually larger than those in warmer areas, which is because larger animals are able to keep warmer.
As global warming influences the planet, animals now have to make changes to face rising temperatures. Some of them shrink in size. Scientists studied over 70, 000 species of birds. They discovered that some birds in North America were shrinking in size. Studies on deer, insects and fish also show the same fact.
What does this mean for these species? First, global warming is happening faster than ever before. So, animals may not be able to change fast enough. Besides, when a species shrinks, other things will get influenced. When an animal's body shrinks, it may not be able to give birth to as many young as before-this may influence the size of the species population.
It is possible that scientists haven't yet completely understood how changes in size may influence the ability of animals to live. This shows the bigger problem with climate change-we aren't always able to predict what will happen when the Earth's temperature keeps rising. And that is the real danger.
