Have you ever wondered why adult animals are so much bigger than juveniles? Many decades ago, evolutionary biologists proposed an explanation: there must be an advantage to growing large. But what advantage? Relative to what? If an animal’s genes are going to be passed on to the next generation, they need to ensure somehow that they can reproduce better than their competitors.
In the particular case of fish, reproductive success is achieved by producing as many good-quality eggs as possible. The great majority of fish eggs die within the first couple of days after being released by their mothers. They are either eaten by a predator or simply do not survive harsh environmental conditions.
Just as we see with chickens, fish eggs have a yolk – a stash of food that keeps them going for a few days until they mature and can feed themselves. Female fish reproductive strategies range between two extremes: they can make a few large eggs with lots of yolk, or spawn many tiny eggs.
When we look at fish mothers of different sizes we see that larger females produce more eggs. This would seem to explain why young fish grow to become large adults: so that they can produce more eggs and increase their chance of spreading their genes to future generations.
Scientists were not satisfied by this simple observation and explanation. They wanted to quantify exactly...
