Millions of years ago (before there was any life here), earth was hit by a huge meteor and knocked off its axis — at least that's what scientists think happened, but hey, nobody was here to ask! Anyway, because of this astro-collision, as the earth orbits the sun, sometimes it is tilted toward
it and sometimes its tilted away
from it. When the northern hemisphere tilts toward the sun, it receives more of the sun's energy and light (summer), but when the northern hemisphere tilts away from the sun, the energy and light is less intense (causing winter).
The tilt in our axis also affects the length of our daylight hours, which is more extreme the further from the equator you go. This is why summer days in Seattle, for example, have about 18 hours of daylight compared to about 7 hours of daylight on winter days.
So then, how does all this affect animal life?
Well, for starters, think about what would happen if we didn't have changing seasons. Birds and whales wouldn't migrate, bears wouldn't hibernate, and insect populations wouldn't cycle. Deciduous trees (those that shed their leaves before a cold season) and perennial plants would die, having been denied their required dormant season. The cycles of flowering plants would be disrupted. Even places where the climate seems to be relatively steady, such as the high tundra in Siberia or the sweltering deserts of Africa, undergo more subtle, but very necessary, seasonal changes. All plant life on earth would be affected, and because the feeding and reproductive stages of nearly all animal-life on earth follows the seasonal changes in the plant world, the results would clearly be monumental.
The always changing amount of solar energy our planet receives is also critical to our varying weather. Sunlight does a lot more than give you a pretty tan; it heats our atmosphere, grows our food, evaporates our water and causes rainfall, and even changes our moods. In some parts of the world, sunlight is what heats our homes and powers our appliances. The amount of sunlight reaching our planet affects every part of life as we know it.
Pretty awesome, isn't it?