In the central United States, broad-tailed hummingbirds arrive at the end of winter. They've migrated all the way from Central America to breed. They've come for the local flower blooms, as the nectar gives the birds energy to mate and raise young. In return, the birds pollinate the flowers.

As the earth warms, the flowers bloom earlier, but the hummingbirds arrive on the same date. The time between their arrival and the bloom is growing. If this continues, the birds may eventually miss the entire bloom. Then the hummingbirds and the flowers will not be able to reproduce.

On a tropical island northwest of Australia, the wet season cues little red Christmas Island crabs. They exit their burrows and scuttle to the Indian Ocean shore. There, they mate before returning to their burrows. This mass migration depends on rain. The crabs won't migrate until 0.87 inches (22 millimeters) of rain falls. No migration means no mating.

In areas where seasons are based on rainfall, climate change can cause droughts. Drought can endanger animals like the Christmas Island crab, which need the rain to mate.

A small colony of emperor penguins has vanished from an island off the Antarctic Peninsula. Emperor penguins use the same areas year after year to mate and raise young. However, fewer penguins have been returning to those areas since the 1970s. During this time, the Antarctic Peninsula has been heating up. Changes in sea ice have affected fish and squid populations in the area, which means penguins have less food.

The American pika, a tiny cousin of the rabbit, has been climbing the mountains in the central United States. American pikas usually live in cold alpine areas near or on the top of mountains. They need the cold, as pikas can die if exposed to air temperatures above 78 °F (25.5 °C) for six hours or longer.

Many animal species need males and females to reproduce. For the alligator, warm temperatures may cause a shortage of fathers. The sex of a baby alligator is determined by the temperature of the egg. If the egg reaches temperatures greater than 86 °F (30 °C), it will most likely hatch as a female; below that, it will most likely be a male.

Mother gators can choose where to build nests, giving them some say over egg temperature. As it warms, mother gators may not be able to find places cool enough for any male babies. Without many male alligators, alligator populations will shrink.