For animals in the Northern Hemisphere, this is a pretty exciting time. The day length is steadily increasing, which is the primary signal to seasonal breeders that it is time to seasonally breed. Hibernating species such as chipmunks wake up with enlarged gonads and ready to go; songbirds start singing and their brains grow new song-specialized neurons in response to more daylight.
But not every species is driven by day length. What other cues help animals decide when it's time to commit to swapping their gametes?
For frogs and toads living in the Arizona desert, rain is the sign for a massive amphibian orgy. They need to lay their eggs in standing water, with enough time for the eggs to hatch into tadpoles and for the tadpoles to mature into land-dwelling adults. The desert's temporary pools may last only a few weeks, so at the first rain, the amphibians start hopping.
Horseshoe crabs' mating frenzy is triggered by extremely high tides. They deposit their eggs high on the beach, where they won't be washed away by subsequent tides. (This strategy is fortuitous for migrating shorebirds, which are passing through just in time to feast on the horseshoe crab eggs as the birds head north to their own breeding grounds.)
Rabbits are among the most efficient breeders. Females don't have a particular season or cycle of fertility; instead, they ovulate shortly after they mate. Female bats store sperm over the winter and use it to fertilize their eggs in the spring, possibly to encourage sperm competition. More gruesomely, male lions that take over a pride kill females' cubs to make the females become fertile again.
One of my favorite recent examples of extreme breeding behavior comes from the banded mongoose, a communally living African carnivore. In a given group, 64 percent of births happen on the very same night. Why such synchrony? If a pup is born too early, other adults kill it. If it's born too late, the other pups get all the food and attention, and it's likely to starve. Romantic, isn't it?
What are your favorite weird breeding cues?