In nature, timing is everything. Most animals give birth in the spring just as new sources of food become abundant. But for many species, the mating season begins much sooner.
Male white-tailed deer, for example, are already shadowing groups of does. Occasional brief chases foretell the escapades to follow in October and November. But when the rut begins, does are only fertile for 24 hours every 28 days. Bad weather or too much human activity can disrupt deer behavior, so some does can miss a chance or two to breed and extend the rut into early February. The still spotted fawn I saw last week, for example, is the offspring of a doe that bred late, perhaps in December or even January.
Deer pregnancies average about 200 days so fall mating is essential to insure that fawns drop with sufficient time to mature by the following autumn.
Black bears breed in June and early July, when food is abundant. Though embryos form in early summer, they do not implant on the uterine wall until the sow enters hibernation. This enables sows to acquire vast amounts of fat to get through winter without devoting much energy to developing cubs.
And even after fetal development begins, it proceeds slowly with minimal stress on the mother. Black bear cubs weigh just six to 12 ounces at birth in January. Most bear cub growth occurs after birth. Cubs open their eyes at about four weeks, and leave the den with their mother when about three months old.
Fishers, river otters, and long-tailed and short-tailed weasels take this strategy of "delayed implantation" to extremes. These members of the weasel family mate in March or April, but do not give birth until about 10 months later. When the embryos finally implant in late winter, the actual pregnancy lasts only four to six weeks. This breeding strategy is particularly adaptive for predators because there are usually plenty of prey available for nursing mothers. And when the young leave the den, young prey are abundant so learning to hunt is less difficult.
Avoiding the costs of reproduction during the most energetically demanding times of the year is not restricted to mammals. Many insects and other invertebrates mate in late summer and overwinter as eggs, larvae, or pupae. Longer, warmer spring days trigger the development of these life stages and life cycles continue without the burden of reproductive behavior. Wooly bear caterpillars, for example, overwinter in its familiar larval caterpillar stage, and tomato hornworms drop to the ground and burrow beneath the surface where they overwinter underground as pupae.
Even some familiar fish spawn in the fall to avoid breeding in the spring.
The brook trout story begins with fall's shorter days and colder water temperatures. These environmental cues trigger hormonal changes in the brook trout that inhabit cold, clear waterways. Males get colorful and their lower jaws grow and turn upward. Females transform into egg-making machines for the fall spawn.
Along the shores of beaver ponds, small rivers and even the tiniest spring-fed mountain streams, females choose clean gravelly spawning sites. The water must be between 40 and 55 degrees F. And most importantly, there must be an upwelling of ground water directly beneath the nest or at least a current to carry away silt and sediments.
While the females build nests, the males establish a dominance hierarchy based on size and aggression. The largest male usually wins breeding rights.
When the female releases her eggs, the male fertilizes them. As the milky cloud settles into the nest, the spawn is complete.
The male leaves immediately, perhaps to search for another mate. The female completes the nest by using her fins to shovel a load of clean gravel atop the fertilized eggs. The eggs are now hidden, safe and ready for a winter of dormancy. The sediment-free nest keeps the eggs well oxygenated during the winter months. The eggs hatch in early March.
Brown trout also spawn in the fall when water temperature dip into the upper 40s.
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