Sitka Black-tailed deer biology
Sitka black-tailed deer are a small, forest-dwelling subspecies of mule deer, and are key species economically, ecologically, and culturally in the temperate rainforests of North America.
Reproduction is a key component driving deer population growth across deer and other ungulate species- most variation in year-to-year population growth is driven by differences in fawn survival among years, not differences in female survival or pregnancy rates.
Sitka black-tailed deer typically have 1-2 fawns, conceived during their 2nd fall ad born when they are just turning 2 years old. Pregnancy rates are typically high, even in populations with less high-quality forage available, but twinning rates are lowered when food is scarce. Twinning rates are also lower for first-time mothers, who typically have a "singleton" instead.
Conception typically occurs mid-November, but if does do not conceive during their first estrus cycle (if they are first-time breeders, or if severe weather disrupts the rut), fawns may not be conceived until December or even, in rare cases, January.
As a result, while the "peak" of fawn season occurs from late May through mid-June, fawns can be born later in the summer as well, all the way into August. These late-born fawns can do just fine in mild winters, but are much more likely to die in sever winters with deep snow and/or long cold spells. This is because they are lighter-weight and smaller, and cannot move through deep snow as well as larger, stronger fawns. We found that in a harsh snow winter, 85% of fawns did not survive until 1 year of age.
Similarly, fawns that are born lightweight are more vulnerable to bear predation, although bears will also eat normal-weight fawns. Black bears on Prince of Wales Island eat ~40% of fawns over the course of the summer.
Female deer with fawns face some tough trade-offs- they have some very high nutritional demands through late pregnancy and while they are lactating (in fact, lactation requires just as many calories per day as the end of pregnancy), but they also need to keep their fawns safe from black bears, and keep themselves safe from wolves and bears as well.
As a result, we see them change their behavior through the spring, summer, and fall. As they become increasingly pregnant and birth nears, they go from not caring much about where black bears are on the landscape, to actively avoiding areas of high black bear occurrence.
Once fawns are born, they continue to avoid black bears, but start to relax a bit as their fawns get older and allow more overlap with black bears in space. If their fawns die, they quickly return to no longer caring about black bears, instead focusing intensively on finding areas with lots of forage. Bear diets on Prince of Wales consist of 10-20% deer in springtime, which we think is mostly due to predation of fawns.
Once deer reach adulthood, they face new challenges. They are no longer highly vulnerable to bear predation, and can handle deep-snow winters more effectively than can fawns, but they do face predation by wolves, harvest by humans, and death from malnutrition during harsh winters (particularly in areas of degraded or poor-quality habitat).
Summer is a key time for deer, as they must put on body weight and fat if they are to survive the winter. And this means finding high-quality forage!
High-forage habitats include alpine areas, medium- and low-volume old growth timber, and young second-growth timber. While bears use some of these areas as well, deer appear to be able to avoid bears enough of the time, given their strong population growth rates in years without deep-snow winters.
Some deer migrate to higher elevations, where alpine vegetation provides a rich food resource. Migration may also allow females to avoid predation risk to their fawns to some degree. The costs and benefits of migration for deer, and how important migratory behavior is to the total deer population, remains unknown. Likewise, the effects of habitat change, such as timber harvest in migratory corridors, also remains unknown.
fall & winter
As summer turns to fall, deer continue to forage hard to put on fat for the winter. For the does, putting on fat during this time period is critical to be able to gestate fawns, preferably fawns with high birthweights that are more likely to survive. For bucks, early fall is when they pack on the thick layer of fat that will get them through the breeding season (rut), when they expend huge amounts of energy moving across the landscape to check on does’ reproductive status, marking their territory, and fighting rival males for dominance and access to breeding opportunities.
Male deer behavior during the rut is not well documented for Sitka black-tails, but some data does exist from camera traps, and we can also infer a lot from Columbia black-tailed deer and other mule deer.
Some of the prime territorial marking rituals that Sitka black-tails use are antler-rubbing on trees and shrubs (often called a “buck rub”), usually accompanied by scent marking (rubbing glands near their eyes, and elsewhere, on trees and shrubs in the vicinity), and urination (which also includes urinating on their own tarsal glands, along the inside of their hind legs, to enhance their scent).
If they end up in a face-off with a rival male, there are a number of things that will likely happen before a fight gets started. Each buck will seek to make himself appear as large and formidable as possible to his rival. This includes making his fur stand up to appear bigger, presenting his side rather than his front to show off his body size, and also making some pretty incredible sounds, which we call the “grunt wheeze” or “snort wheeze.” Here’s a great youtube video of two columbian black-tailed bucks making those sounds:
As the rut ends, deer switch to conserving energy through the winter by avoiding deep snow and finding sufficient food to sustain them through spring. They also rely on reserves of fat that they built during summer and fall. High-quality winter habitat, allowing deer to avoid deep snow, access shrubs and evergreen forbs to eat, and move through the landscape to other high-quality patches, is key for deer survival and healthy deer herds.