The Theme of Evolution in Animal Behavior
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.
Co-evolution
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.