Study Guide

Animal Behavior Themes

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  • Evolution

    Sexual Selection

    All behavior has to pass the test of natural selection. However, sexual selection can override traits that would normally be selected for through natural selection. Sexual selection makes animals do some weird stuff.

    We already talked a little bit about peacocks, but because they are so funny looking we are going to revisit them. Peacock tails are extremely long, heavy, and energetically costly to grow and maintain. Not to mention that it must be a little hard to fly with something that long trailing behind you. They are not the best things to be carrying around while running from a predator. Males with shorter tails should be less likely to get caught by a predator, meaning those guys with short tails actually live long enough to reproduce. So natural selection should favor short tails. Right?

    Not so fast. Peahens (those are female peacocks) just think long tails are super attractive. The short-tailed males just don’t do it for them. Females only mate with the guys who have the best looking, longest tails. This means the genes for long tails get passed onto the next generation and short tails do not succeed, evolutionarily.

    It seems like females are setting their sons up for failure; if they inherit their dad’s long tail, they will draw more attention from predators and have a harder time flying. Over generations, natural selection should work against long tails. The thing is, it doesn’t. This is because sexual selection can beat out natural selection, at least for certain traits. Even though peacocks have these ridiculously long tails, they are able to overcome them and fly, find food, avoid predators, and eventually mate. Even though a trait may seem to give an individual a disadvantage in surviving, if it helps him mate the trait may be favored by evolution because of sexual selection. Sexual selection is all based on behavior—females like to mate with males with certain traits. Why? No one is really sure. One idea is that males who have a disadvantage, such as a heavy tail, must have really good genes if they are still able to fly and avoid predators. The tail signifies to the females, "Despite this bulky thing I’m carrying around, I’m very good at surviving." The girls go crazy, and the rest is history.


    Parasites can change animal’s behavior in sneaky ways to benefit themselves. Fungi of the genera Cordyceps and Ophiocordyceps parasitize insects and brainwash them. Ants infected with the fungus climb up stems of plants and grab on for dear life, just before the fungus kills them. Then, the fungus bursts out of the ant’s body and releases its spores. Making the ant climb up a stem gives the fungus a better vantage point from which it can spread its spores. You may have seen this on Planet Earth.

    Ophiocordyceps unilateralis growing out of dead ant bodies. Image from here.

    Fossil evidence suggests this fungus has been infecting ants for millions and millions of years. There are tons of different fungus species that do this to ants, butterflies, moths, and termites, and each is specific to its host ant (or other insect) species. The insects are not just taking this sitting down. They are under attack and they know it. If an ant comes down with the fungus, other ants do not let it infect them. The sentries guarding the queen kill the infected individual and remove its body from the nest before the fungus can spread to everyone. If the fungus were to infect everyone in the colony, that would be the end of the colony.

    The interesting thing about parasites is that they cannot be too effective, or they won’t have any hosts left. The fungus needs ants to infect. If it kills all the ants, it will have nothing left to grow on, and that will be the end of the fungus. In Cordyceps, multiple species have evolved by switching hosts, perhaps because they were running out of individuals to infect. At some point millions of years ago, a Cordyceps spore landed on an insect species it had never infected before. It managed to make that insect climb up to the top of the closest tree and spread to other insects in that group. Species of this fungus have invaded many different insects, making them modify their behavior to spread fungal spores. Could this fungus infect humans and turn us into zombies? One writer explores that topic here.

  • Regulation

    Physiology and endocrinology have big roles in regulating behavior. Hello, hormones, take me to prom? We’ll get to that in a minute. Hormones and physiology make animals do things. They make us hungry, thirsty, tired, alert, and ready to mate, just as part of their daily job.


    Animal physiology varies greatly and this affects behavior. Warm-blooded mammals can do things very differently than cold-blooded reptiles. Warm-blooded animals can afford to exert more energy on hunting than cold-blooded animals. Cold-blooded animals cannot expend energy chasing prey for hours. Reptiles spend a lot of time basking in the sun because they can’t regulate their own body temperatures as well as warm-blooded animals can.

    The time of day an animal is active depends on its physiology too. Nocturnal animals have special features that make them good at navigating the night—for example, cats can see well in the dark, allowing them to hunt at night. Cat physiology allows them to see in the dark—like many nocturnal animals, they have large eyes, large pupils, and protect their eyes from bright light during the day by having a slit pupil. Look at a cat’s eye. See how the pupil (the dark part) is a slit?

    Now go look at your eye in the mirror. Your pupils are round, or if they aren’t you better go see a doctor. Image from here.

    In the desert, daytime temperatures are oppressively hot, so animal behavior and physiology are tightly linked. Most desert animals are nocturnal, sleeping during the hottest part of the day and foraging at night. However, if everyone is awake at night, that means predation risk is high at night. In the Sahara desert, one the most heat tolerant animals on the planet can be active during the day. It is the Sahara desert ant and it can survive in temperatures up to 122º Fahrenheit. And we thought 80º was hot. Desert ants have a few physiological adaptations that allow them to deal with this heat. They have longer legs than other ants, which hold their bodies away from the hot ground. Their bodies produce heat shock proteins before leaving the nest to protect cellular function in extreme heat. And of course, they work in groups to drag food back to the nest.

    The thing is, even this little heat tolerant ant can only spend 3-5 minutes in the extreme heat before getting fried. So everyday when the sun is highest and all the other animals hide in cooler spots, Sahara desert ants leave the nest to forage. These ants monitor their location in relation to the sun’s position in the sky, always knowing the shortest route back to the nest. Check out the BBC’s video clip of these ants foraging.

  • Hormones

    Hormones are responsible for a lot of important things inside the body, like regulating digestion, sleep patterns, and mood. They also are at the root of a lot of behavior. As we said earlier, finding a mate is what drives a great deal of animal behavior. Hormones prepare an animal for sex and for nurturing offspring, and they also make you drool over Taylor Swift, Ryan Gosling, or whomever you might find attractive.

    Hormones dictate pretty much every part of a teenager’s life—as soon as puberty hits, those hormones say, "go find a mate, go find a mate!" Whether you realize it or not, those hormones are telling you to look your best, impress the other sex, show off your muscles/beauty, and drive recklessly.

    Hormones also affect other animals’ lives. Have you ever slept in the woods and heard frogs ribbiting all night? If not, listen to bull frogs here. They are not singing you a lullaby; they are calling for mates. Frog mating calls happen because of hormones called androgens, which are male sex hormones. Testosterone is the most well-known androgen, and is also responsible for a lot of male physiology. In humans, androgens are responsible for the development of male sex organs and secondary sex characteristics such as body hair, deep voice, and muscle development.

    Frogs are not the only ones who make mating calls because of testosterone—birds do, too. Male birds experience an increase in testosterone concentrations in the spring, when days get longer and food becomes more abundant. This is the perfect mating season, so male birds start to sing. As testosterone levels increase, males sing more and may have more complex songs, hoping to attract a female ready to mate.

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