To survive better, of course! Particularly in climates that have a wide range of temperatures (such as Alaska, where the record low is -80 F and the record high is 100 F), animals need to be able to change their bodies to suit the surrounding climate conditions. Most mammals grow more hair during the fall to trap more heat for the winter, then shed it during the spring to allow them to cool off better in the summer. (This is why cats and dogs cover the sofa with fur every spring.)
Many animals have seasonal breeding cycles, so that young are only born during warm times of year, when they are more likely to survive. Some insects have a life cycle of metamorphosis that is aligned with the seasons.
Some animals, such as the arctic fox, even have skin or coats that change color from brownish in the summer to white in the winter, to better camouflage in their most likely surroundings.
In all these cases, these changes are evolutionary adaptations to help the animals survive and reproduce better in their environment.
I think it's worth talking about how this comes about, and how it shows the vital distinction between genotype and phenotype. No animal changes genotype on a seasonal basis; that's simply not possible. The genes of an arctic fox remain exactly the same whether it is summer or winter. So how does the change in phenotype come about?
There had to have been a mutation that changed gene expression---turning some genes on and others off---based on some signal that's correlated with the seasons. Perhaps the gene responds to sunlight, or temperature. Then individuals with that new mutation survived a little better than their counterparts without it, and so more of their genes were found in the next generation, and then even more in the next, and so on. Some of these changes are so complex that they must have involved many different genes, probably evolving for a totally different reason and then being co-opted thousands or even millions of years later to perform the seasonal change.
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