Back To CourseBiology 101: Intro to Biology
24 chapters | 220 video lessons
You may remember that a predator is an animal that hunts and kills other animals for food. Conversely, prey is a term used to describe animals that are hunted and killed by predators. Now, for the purposes of this lesson, when I talk about predators and prey, we can also think of herbivores as predators of plants and plants as prey of herbivores. The interactions between a predator and its prey, as well as the interactions between a herbivore and the plants it eats, are some of the most powerful driving forces behind evolution.
Predation is a strong, selective pressure that drives prey organisms to find ways to avoid being eaten. Prey organisms that are difficult to find, catch or consume are the ones that will survive and reproduce. The result is that over evolutionary time, prey organisms have developed a stunning array of strategies to avoid being eaten. Some defensive strategies are pretty obvious, and top among the list of obvious strategies is running away. Gazelle, deer, small mammals and lizards often rely on their speed and quickness to escape predators, and many birds rely on flight as their primary defensive strategy. Some organisms, like armadillos, tortoises, porcupines and thorny plants, use armor, quills and thorns to defend themselves against predators.
Some organisms have resorted to chemical warfare to defend themselves against predators. The very distinctive-smelling spray of the skunk comes to mind, and the bombardier beetle sprays would-be predators with a toxic chemical in a similar fashion. Some organisms are themselves poisonous, like this poison dart frog. Animals that use chemicals as a defense rarely use camouflage; quite the opposite, in fact. These animals are usually brightly and distinctively colored to stand out. These markings are called warning coloration and are bright or distinctive markings that serve as a warning to would-be predators.
A very common and universally known warning coloration is the black and yellow stripes of many species of wasps, hornets and bees. Some harmless species of flies, and even some beetles, have developed similar black and yellow coloration to mimic the potentially dangerous wasps and bees. This type of mimicry, where a harmless animal mimics a dangerous or unpalatable animal, is called Batesian mimicry. Many types of butterflies also practice Batesian mimicry. However, there are cases where two or more dangerous or unpalatable species all resemble one another. This type of mimicry is called Mullerian mimicry. The monarch butterfly shown on the left and the viceroy butterfly shown on the right will make animals sick or taste very bad if they are eaten. When two unpalatable species resemble each other, they reinforce avoidance by predators by increasing the frequency of unfavorable encounters. The predators learn faster, and fewer butterflies of both species are eaten during the learning process.
So far, we've focused mainly on defensive strategies used by prey, but predators can also use some of the strategies already mentioned to help them capture or kill their prey. Cheetahs use speed to outrun their prey, and eagles, hawks, owls and falcons use flight to their advantage while hunting. Venomous snakes, spiders and wasps use poisons to either kill or immobilize their prey. Some predators even use mimicry to capture prey. The alligator snapping turtle uses its pink worm-like tongue as a lure to draw fish right into its mouth. The turtle will sit submerged underwater with its mouth wide open and flail its tongue around like a writhing worm. When an unsuspecting fish goes after the tongue, the turtle snaps its mouth shut and eats the fish instead.
When it all comes down to it, predators and prey are in a constant battle to gain an advantage that will help them survive. Whenever a predator develops an advantage that helps them acquire prey, there is selective pressure on the prey to adapt and find a way to avoid this new method of predation. The organisms that most effectively adapt to and avoid predation will survive and reproduce. It works in the opposite direction too. When prey organisms develop an effective defense against predation, predators must adapt to the change and find a way around the defense, or find a new organism to prey on. Predators that don't adapt and can't capture prey will starve to death. The end result is that predators and prey evolve in response to interactions with each other. These tight evolutionary relationships can result in coevolution, which is when two species evolve in a coordinated fashion by adapting to changes in each other.
A good example of coevolution occurring between a herbivore and a plant is the coevolution of the Heliconius butterfly and passionflower vines. Passionflower vines contain toxic compounds in their leaves that make them inedible to most herbivores. However, Heliconius larvae have evolved enzymes that break down the toxic compounds and allow them to eat the leaves of the vines. Heliconius butterflies lay their bright yellow eggs on the passionflower leaves because they are a good food source for the larvae. They won't lay eggs on leaves that already have bright yellow eggs on them because too many larvae in one place will result in not enough food for all of the larvae, so Heliconius females search for leaves that don't already have eggs on them. Amazingly, some species of passionflower vines have bright yellow structures on their leaves that look like Heliconius eggs. These structures deter the butterflies from laying their eggs there, and more than that, these structures are actually nectaries that provide food for ants and other predatory insects that eat Heliconius eggs and larvae. So the yellow structure defense is two-fold: It deters the butterflies from laying eggs, and it attracts predators of Heliconius larvae. These back-and-forth counteradaptations are what cause coevolution between predators and prey.
Okay, it's review time. A predator is an animal that hunts and kills other animals for food. Conversely, prey is a term used to describe animals that are hunted and killed by predators. The interactions between a predator and its prey, as well as the interactions between a herbivore and the plants it eats, are some of the most powerful driving forces behind evolution. Both predators and prey can use a variety of strategies in order to survive, including speed, flight, physical protection, camouflage, chemical compounds, mimicry and countless other strategies that we don't have time to talk about here. While some animals rely on camouflage, others, especially those with chemical deterrents, have bright or distinctive markings that serve as a warning to would-be predators called warning coloration. Some harmless animals take advantage of warning coloration and mimic other brightly colored species. This type of mimicry, where a harmless animal mimics a dangerous or unpalatable animal, is called Batesian mimicry. There are also cases where two or more dangerous or unpalatable species all resemble one another. This type of mimicry is called Mullerian mimicry. The end result of all of these interactions is that predators and prey evolve in response to interactions with each other. These tight evolutionary relationships can result in coevolution, when two species evolve in a coordinated fashion by adapting to changes in each other.
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Back To CourseBiology 101: Intro to Biology
24 chapters | 220 video lessons