Timing is everything for the seasons
December usually brings the first harsh winter weather of the season, so it would seem a bad time for wildlife to mate. For many animals, however, winter marks the beginning or even the middle of the reproductive cycle. Mammals use a variety of life history strategies to get a head start on spring by breeding in the summer, fall, or even mid-winter. White-tailed deer, for example, breed in November. Does carry developing fawns until late May or June. The prolonged seven-month gestation requires that mating occur in the fall so fawns can be independent by the following autumn.
Most species of bats breed even earlier - in late summer. But females have the ability to retain the sperm in the uterus until spring, when ovulation occurs. Females then release the sperm, and fertilization takes place. Biologists call this breeding strategy "delayed fertilization."
Bears and some members of the weasel family such as long-tailed weasels and river otters prolong gestation by a process called "delayed implantation." Mating and fertilization occur in summer or fall, but embryonic development stops after just a few cell divisions. The embryo does not implant on the uterine wall until mid-winter or early spring. Then development resumes, and birth occurs just a few weeks later.
In these species pregnancies that appear to be six to eight months long are functionally much shorter. Black bears, for example, breed in June or July, but the embryo doesn't implant on the uterine wall until the female enters hibernation. Then sows give birth to tiny cubs in January, while in the midst of their winter sleep. Fishers carry this reproductive strategy to the extreme. After giving birth in early March, females mate later that same month. That yields a gestation period of about 350 days, though active pregnancy after implantation lasts only 30 to 60 days.
The reason so many species breed before the onset of winter is simply a matter of energetics and timing. Adults of many species are in prime condition in the summer and fall. Why defend territories, battle competitors, and woo females in late winter or early spring when animals are weak and poorly nourished? By mating in the summer or fall when they're strong and well fed, parents need only worry about finding food in the spring.
But some species actually breed in mid-winter. Great-horned owl courtship begins in the fall and most pairs are on the nest by late January or early February. After an incubation period that can last 35 days, the eggs conveniently hatch just when prey such as rabbits and rodents are beginning to breed. That means food for nestling owlets.
Mating activity of fox and gray squirrels peaks in January. That's what all those wild chases are all about. After a 40 to 45-day pregnancy and seven to 10 weeks in the nest, young squirrels emerge just as spring foods become abundant. Again, perfect timing.
Foxes, coyotes, and bobcats usually mate in February. After a pregnancy that last 50 to 60 days and a two to three month nursing period, fox pups and bobcats kittens are ready for real food just as prey populations peak in late spring.
The common denominator among winter breeders is that their young emerge just as a new year's food supply becomes available - tender, emerging vegetation for herbivores; mice, rabbits, and baby birds for predators.
Animals that delay mating until spring do so for one simple reason. The time required for these species to produce and raise a brood or litter of young from conception to independence is short, often just a matter of weeks. Many invertebrates, frogs, toads, rabbits, rodents, and most birds fall into this category.
Two ecological lessons emerge from these patterns of behavior. One, there's no single best reproductive strategy; nature embraces any tactic that works. And two, timing is everything.