How Do The Seasons Affect Animals?

Most young mammals are born in spring when it is getting warmer and food is plentiful. Image by John Delano of Hammond.

Baby deer are tiny and when they are first born, they might have trouble keeping warm. If you were a mother deer, it would probably make more sense to have a fawn in the warmth of spring or summer rather than the cold of winter. But planning this seems difficult.

Many parts of animal biology, especially those related to breeding, must take season into account. An animal that gives birth during the wrong time of the year, like when food is scarce, will have a very hard time raising young. How do you think animals know when to start raising families?

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Days get longer as winter thaws into spring. This slow increase in daylength brings about the need to breed in many animals that only raise young during a certain time of the year.

House sparrows know when it's time to breed based on changes in the length of the day. Image by tgreyfox.

For example, house sparrows, common birds which love to nest in the nooks and crannies of your house, prefer to raise their young in the spring and early summer.

The increase in daylength slowly pushes these birds into breeding condition. Because of this, the house sparrow breeding season typically begins in early March.

In fact, you can see the changes in breeding condition in male house sparrows. Breeding males have black beaks and nonbreeding males have yellow-ish beaks. Once summer begins and daylengths become shorter, the breeding parts of house sparrows quickly shut down until the next year.

Waiting for Warmth

Garter snakes come out of their dens in the spring when the temperature starts to increase. Click to enlarge.

Animals that hibernate underground through the winter and breed during the spring cannot track the length of daylight to figure out when to reproduce. Garter snakes have solved this problem.

They use the changes in temperature between seasons to time their hibernation and reproduction. When daytime temperatures drop to 0°C (32 °F), these snakes slither into their underground dens to wait out the cold. As air temperatures gradually warm to 20-28°C (68– 82°F), garter snakes awaken. They emerge from their dens to court and reproduce.

Resting Before the Rains

When it comes to finding the perfect time to breed, spadefoot toads have it rough. These toads live in the Sonoran Desert, where daytime temperatures in the summer can soar over 43°C (110°F). In July, however, monsoon season arrives.

Spadefoot toads know when it is time to breed when it rains hard enough. Click to enlarge.

Intense thunderstorms pour over the desert and temperatures cool. Plants sprout and insects swarm across the sky. When the first big rains hit, spadefoot toads, which have been patiently waiting underground, will emerge to furiously court and mate. By timing their breeding with the monsoon, their young can grow during this brief period of mild weather and abundant food.

Once the desert begins to dry again, all the toads, including the tiny new toadlets, use their hind legs to dig their way underground where they will wait for the next monsoon.

Additional images via Wikimedia Commons. Tide pools image by Brocken Inaglory.