What is fitness?

When you think of fitness you probably think of someone lifting weights or running x number of miles every day and while the idea of a cat lifting weights is adorable, that's not the kind of fitness we are talking about. In the case of animals (or any living thing) fitness is an individual's ability to pass on their traits to the next generation. But fitness isn't just about reproduction, it's also about how long an organism can survive. The longer an organism can survive, the more offspring they are going to produce and the higher the chances of some of those offspring surviving to maturity and reproducing.  

How do you measure fitness? It seems like a simple question but the answer is actually complex. I know what your thinking, what do you mean it's complicated, you just said that fitness is just an organism's ability to survive long enough to produce offspring so all you need to measure is how many offspring a given organism produces and your done. Let's start with a nice and simple hypothetical situation. Say you've been asked to measure the fitness of two species of fish. For the sake of simplicity, let's say that these two fish species live in the same habitat, produce the same number of eggs, and have the same predators. Species 1 decides to invest a lot of energy into producing larger eggs that are too big for other fish in the area to eat. Species 2 doesn't invest as much energy into egg size, so its eggs are smaller and some get eaten by other fish. Now at this point, you're probably thinking that species 1 has a higher fitness but that's not necessarily the case. You see because species 1 invests so much energy into egg size the adult females are fairly small in size and are easy prey for larger fish while species 2 is able to invest more energy into growing larger making it too large for the predators to eat. With this new information, you could jump to the conclusion that species 2 has a higher fitness because the adults survive longer and can produce more eggs over time. However, that would depend on how long species 2 has to survive in order for enough eggs to hatch for more offspring of species 2 to be produced than species 1. You would also be assuming that no new predators are going to be introduced and that people aren’t fishing in the area and catching more of one species to take home for dinner.

Let’s use moose as a real example. In the Autumn, bulls will fight for the ability to mate with cows. The bulls clash antlers and frequently suffer from either minor or fatal injuries. As you would guess, the bull that is larger, stronger, and more experienced is the winner. In this case, it seems obvious that the larger bulls are going to have a higher reproductive fitness and in this particular case, you would probably be correct in assuming that larger bulls have higher fitness. However, you should also keep in mind that when people go hunting they aren’t likely to hunt the smaller bulls with damaged antlers.

At the end of the day how you measure fitness is going to be determined by the individual, you are studying. In some cases, it would be the most logical to measure size while in other cases age of maturity or number of offspring per breeding season may be more accurate.

 

Image credit: https://animals.desktopnexus.com/wallpaper/1135161/

Sources: https://www.nps.gov/articles/denali-moose-rut.htm#:~:text=Moose%20are%20very%20active%20during,as%20they%20prepare%20for%20winter