Many animals have lived the same way for thousands of years. But when humans enter the picture — and with them, threats like hunting, pollution, and climate change — something has to give. Thanks to the principle of natural selection, the animals who have best adapted to survive in their environment are the ones who live the longest and reproduce the most. So the species as a whole evolves in order to adapt to new threats, like humans, in their environment.
Below, check out some of the most fascinating — and heartbreaking — ways that animals have adapted to the threats posed by humans.
1. Elephants have adapted to go without tusks
For an easy example of the way people force animals to adapt, Grist recommends looking to the population of elephants born without tusks. In Zambia, the proportion of tuskless female elephants went from 10% to 40% in just 20 years. In Sri Lanka, fewer than 5% of male elephants now have tusks. The elephants can thank poachers for that change. Poachers kill the animal to take their ivory. So elephants without tusks live longer and reproduce more. But the change isn’t without its consequences: Elephants use their tusks to dig for food and water. And according to The Independent, “conservationists say an elephant without tusks is a crippled elephant.”
Next: Thanks to hunting by humans, these animals have developed smaller horns.
2. Bighorn sheep have developed smaller horns
Similarly, bighorn sheep have evolved smaller horns in response to intense trophy hunting by humans. Phys.org reports that human selection leads to what scientists call artificial evolution. Hunting sheep specifically because they have big horns selects against that trait, since the animals who have that trait don’t live to reproduce as long as other animals in the population. So the animals’ horn size has declined over time.
Next: These animals changed color to keep pace with climate change.
3. Tawny owls have gotten more brown than gray
Humans don’t just affect animals — and their gene pools — by hunting them. Climate change brought about by people’s actions also has an effect. Scientific American reports that Finnish tawny owls have become more brown in the last half-century because milder winters have brought less snow in recent decades. As Scientific American explains, “Less snow means less chance of brown owls standing out against a white landscape as they stalk their prey—or falling prey to the dread Bubo bubo (the eagle owl).”
Next: These animals have changed their diet as their hunting grounds disappear.
4. Polar bears have changed their diets
Polar bears offer another well-known example of how animals have adapted to climate change. As National Geographic explains, the Arctic sea ice on which polar bears hunt progressively disappears during the summer. Sea ice forms later in the fall, and disappears earlier in the spring. So as the sea ice disappears, polar bears have to look for alternate food sources, such as on land. But scientists haven’t found evidence that’s a sustainable alternative that will enable polar bears to survive long-term.
Next: These small animals have begun breeding earlier in the season.
5. Red squirrels have started breeding earlier in the season
Another interesting example of how climate change has forced animals to adapt? Grist reports that many different species of animals now breed earlier in the year to account for global warming. “Red squirrels, for instance, are breeding earlier in the spring as the climate warns,” the publication explains. But red squirrels seem pretty fortunate in comparison to other species, like a bird called the pied flycatcher, that can’t shift their schedule. Those species may end up threatened as climate change progresses.
Next: These creatures migrate and breed earlier as the earth gets warmer.
6. Birds nest, breed, and migrate earlier in the season, too
National Geographic reports that squirrels aren’t the only animals that have been forced to change their behavior in response to the climate change brought on by their human neighbors. “Many birds are nesting, breeding, and migrating earlier as spring arrives sooner than before,” the publication explains. The Guardian explains that “birds are reaching their summer breeding grounds on average about one day earlier per degree of increasing global temperature.” It’s important that birds reach their breeding grounds at the right time, because if they miss it even by a few days, they’ll miss out on resources such as food and nesting places. That, in turn, affects when their offspring hatch and how likely those offspring are to survive.
Next: This create changed color during the Industrial Revolution.
7. Moths changed their color to deal with pollution
As The Conversation reports, the story of the peppered moth “is a classic example of natural selection, and of how animals can act as indicators of environmental change.” The peppered moth, found throughout Eurasia and North America, can be either white or black. These moths hide by blending in to their surroundings. The white moths used to blend in better with the white bark birch trees in England. But during the industrial revolution, the white moths became much rarer than the black moths. The reason why? The soot and pollution made it much easier for the black moths to hide from their predators, birds.
Next: These animals have begun to mature earlier in their lives.
8. Fish reach sexual maturity earlier in their lives
Grist also reports that animals who are frequently killed for food, like fish, also tend to adapt by reaching sexual maturity earlier in their lives. “If you’ve got a pretty good chance of being slaughtered as an adult,” Grist explains, “you’ll want to start passing on your genetic material as soon as you can.” Fish, in particular, have also adapting with earlier sexual maturity to get through deadly disease outbreaks.
Next: These creatures have adapted to high levels of pollution.
9. Some kinds of fish adapt to pollution, too
Especially for animals that live in water, pollution brought about by humans is a very real threat. Fortunately, Grist explains, some species of fish have learned to adapt to high levels of pollution. A species called the Atlantic tomcod lives in the Hudson river. Grist reports that this species “has become a true city fish.” In fact, it’s adapted so that it can swim in water polluted with toxic chemicals, and suffer no ill effects.
Next: This animals have adapted to pesticides used by humans.
10. Insects adapt to pesticides
We don’t typically poison fish on purpose. But people do develop and use pesticides to try to kill the insects that threaten their crops. However, as Grist explains, insects have learned to adapt. “By the time the insecticide DDT was outlawed, 165 or more insect species were immune to its effects,” the publication reports. The same thing applies to bigger pests, including rats and mice. Some are becoming resistant to warfarin, the pesticide used to kill them. And some have even become immune to warfarin’s replacement, superwarfarin.
Next: Even these gross bugs have grown resistant to pesticides.
11. Bedbugs have adapted to pesticides, too
A particularly skin-crawling example of an insect that’s adapted to the compounds that humans use to kill it? The bedbug. As The New York Times explains, bedbugs “practically vanished at the dawn of modern biology in the 1940s, thanks to the widespread use of DDT.” But the insects have “returned with a vengeance” in recent years because they’ve evolved resistance to pesticides. Sure, this example doesn’t sound quite as heartbreaking as some of the others. (After all, nobody wants bedbugs in their house or their hotel room!) But it’s still an impressive example of an insect evolving in response to humans.
Next: This fish has adapted to resist poisoning by humans.
12. A fish called the cave molly has developed resistance to poison
The BBC reports that in southern Mexico, a species of fish called the cave molly has slowly developed a way to survive a local religious tradition that involves poisoning them each year. “In a ceremony designed to ask their gods for rain, every year in spring the Zoque people of southern Mexico head to the local cave and toss leaves containing a paste made from ground barbasco root into the water.” The fish then float to the surface because barbasco contains rotenone, a powerful anesthetic. But some fish have developed resistance to the compound. And as if that weren’t impressive enough, this particular population of fish has also adapted to life inside a cave poisoned with a toxic gas called hydrogen sulphide.
Next: These animals have adapted when humans bring new, invasive species to their habitats.
13. Reptiles have adapted to the other species that ‘invade’ their habitats with human help
As Grist explains, people have long traveled the world for trade, exploration, and even leisure. But along the way, they tend to pick up “hitchhikers,” or animals that belong in one place but end up in another. The problem is that these hitchhikers can actually become invasive species in their new habitat. However, animals have learned to adapt even to this strange situation. When people accidentally brought cane toad to Australia, snakes began eating them. The problem? Snakes with bigger mouths were eating bigger toads, which contained more toxins. So snakes with smaller mouths lived longer — and entire populations of snakes adapted to have smaller mouths. And in parts of the United States invaded by fire ants, lizards have evolved longer legs so that their stomachs don’t drag on the ant mounds.
Next: These animals have adapted to human roads.
14. Coyotes learn to follow the rules of the road
As Yes Magazine explains, animals have also learned to adapt to the hazards of city living. One impressive, but heartbreaking, example? In Chicago, coyotes have needed to learn to follow the rules of the road. “Collisions with cars may be the biggest killer of Chicago’s estimated 2,000 coyotes, but many of them have learned a thing or two about navigating city streets safely,” the publication reports. Coyotes in the city will wait on the sides of roads, waiting for traffic to stop, before they try to cross. They also seem to understand divided highways, and have learned to cross them safely.
Next: These animals have learned how to avoid hunters.
15. Boars have changed their schedules during hunting season
Yes Magazine also reports that many animals living in or near urban environments have changed their schedules to cope with the dangers posed by humans. In Germany, for instance, boars change their activities during hunting season. “During hunting season, the animals are known to commute from rural forest areas to the suburbs of Berlin during the weekend to avoid hunters, and then head back to the country during the week when the forests are safer,” the publication explains.
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