Natural mortality in deer: the inescapable comparison to humans

(An adult female Columbian white-tailed deer marked with an ear tag and a plastic collar for identification in the field.)

For my Ph.D. research I studied a population of white-tailed deer located on a national wildlife refuge in southwestern Washington.  The refuge was situated on the north bank of the mighty Columbia River and this particular subspecies of deer is called Columbian white-tailed deer.  They were placed on the Endangered Species List in the early 1970s, which led to my research project on this rare form of North American deer.  The only other form of white-tailed deer that is considered endangered is the diminutive Key Deer of Florida.

As you all know, most populations of deer in North America are subjected to sport hunting every fall.  Historically, hunts allowed for male-only kills, but this has been greatly liberalized in recent decades to allow hunting of females to reduce the size of this now abundant (= too abundant) species.  One of the demographic observations about deer populations is the sex ratio of adults---it almost always favors females significantly.  Sex ratios among adult deer typically are 3-4 females for every male, and this is generally attributed to the fact that males are hunted and females are not.  That is, more males were removed from the herd every year due to legal hunting than were females, and this resulted in a skewed sex ratio favoring females.  Reproductively speaking, this is not a problem because most species of cervids (which include elk, moose, and caribou) have a promiscuous breeding system, where each male breeds with as many females as possible.  An adult male white-tailed deer can breed with 10 or more females during a single breeding season in the fall, so the skewed sex ratio does not inhibit reproduction at all.  Essentially, all females get pregnant every year regardless of the sex ratio.  Larger, older, and more experienced males probably obtain more copulations than younger, smaller, and less experienced males and, therefore, the larger bucks sire more offspring.

So, I had at my fingertips a non-hunted refuge population of deer to study, and I was free to choose the research questions that were of interest to me.  I decided that this was an opportunity to study natural mortality and demography in this population of about 200 deer found on a somewhat contained (i.e., surrounded by water or habitat not used by whitetails) piece of land of about 2,000 acres.  I lived on the refuge and worked on the population daily for two years.  At the end, an interesting demographic pattern emerged, which informed my view of what makes male mammals tick.

My primary method of studying this phenomenon was to systematically search the refuge with my assistant, Bill Half Moon, for dead deer.  When we found a carcass, I estimated the month in which it had died, its location on the refuge, and I collected the skull for later analysis.  This analysis involved removing a tooth, and staining and sectioning the tooth to reveal cementum annuli that can be counted to determine the age of the animal at time of death.  It is sort of like counting tree rings.  Of course, the sex of the deer was easily determined from the skull as well.  If the carcass was fresh enough, I took it to Oregon State University to be necropsied, and to determine the cause of death.  I also cracked open a femur to examine the bone marrow, which can be scored subjectively for fat content, which is a crude method of evaluating the nutritional health of the deer at time of death.

It turned out that in this population the sex ratio among adults was still 3-4 females for every male.  However, we knew that the sex ratio at birth was nearly 1:1; in fact, there were probably slightly more males born than females, a typical pattern in mammals.  That is, the sex ratio started out about equal, but by the time males and females were two years old or older, there were many more females than males in the population.  We knew that males were not leaving the refuge, or emigrating, so the only other explanation for the skewed sex ratio was mortality.  Between birth and adulthood, males died at a younger age than females.    Males, on average, were living about 3.5 years, while females were living an average of about 6.5 years.  The oldest male skull I recovered was 7.5 years old; the oldest female skull was 13 years old.  In other words, males were cycling through the population at a faster rate than were females.

To put it bluntly, males are just more reckless than females. They get hit my cars, they get caught in barbed wire fences, and they drown in ponds more often than females.  But the most common cause of death in males was their poor physical condition immediately after the rut, or breeding season.  In this population, the rut began in November and lasted about two months.  At the end of the rut, we are in the middle of winter when conditions are not conducive to recovering body condition, and males paid the price.  It is known that adult male deer spend so much time and energy locating and tending females in heat during the rut that they lose significant body weight.  They increase physical activity during this important process and they decrease the time they spend feeding.  The result is that males are worn out and emaciated come January, all because they want to make love to as many females as possible.  In fact, you could say that many males literally mate themselves to death.

The similarities to other mammals including humans is inescapable.  The mortality rate of male humans is higher than females, especially among those just entering age of reproduction.  Males take dangerous chances, largely in an attempt to increase their status in the eyes of females, whether they know it or not.  The winners can win big, with multiple mates during their life and the possibility of siring many offspring.  Of course, modern contraception has changed the outcome of this male behavior somewhat, but our behavioral tendencies produced over the past 4 million years of human evolution continue to play out regardless.

(See full citation and an Abstract of the monograph produced from this research.)

When a snake bites your student on his buttocks

(Would you check the white Swiss butt of this biologist for a snake bite?)

When you do field work in places where there are venomous snakes, you think about it. Because you see these snakes only rarely, you become somewhat habituated to the fact that they exist in your location, but it is always in the back of your mind. You think about where you put your hands and feet, where you sit down to have your lunch, where you go to the bathroom, and how you pick up a backpack that has been on the ground for several hours.

We know that humans actually do get bitten by venomous snakes. I have had two colleagues receive serious bites from snakes, and it is not pleasant. You spend days or weeks in the hospital receiving doses of anti-venom and other drugs, battle pain and nausea, and often undergo reconstructive surgery to repair the muscles that experienced necrosis and atrophy near the site of the wound.

It was a tense moment when one of my graduate students appeared unexpectedly at the door of our little house in southern Costa Rica one evening and announced to me: “Tom, I think I’ve been bitten by a snake.” I was studying birds, so my schedule was that of an ornithologist. I got up at 4:30am, went to the field at 5, came home about noon, and went to bed at 9pm. Martin, who is the focus of this story, was studying frogs and lizards. He went to the field about 2pm, but never returned home before midnight. We rarely saw each other until the weekend when we took some time off. But on this day, I heard his car pull up to the house in the dark about my bedtime, saw him trudge past the window in his yellow rain gear, and watched him make his startling appearance at the back door. He was slightly hunched over, his face was pale, and he stared me straight in the face as I digested the words “…….bitten by a snake.”

He explained that he and his assistants were sampling lizards after dark in a pasture next to the forest. This technique involves crouching low to the ground and, using a flashlight, searching every square meter of your assigned area, capturing all lizards you see by hand. The individuals were then taken to a processing “station”, where they were weighed, measured, and marked, before being returned to the area where they were captured. At one point, the student felt a sharp “prick” on his buttocks and at that very moment a small snake, striped red and black like some coral snakes, crawled between his legs. The temporal proximity of the prick and the presence of the snake led him to conclude that the snake had caused the prick. Not an unreasonable conclusion, in my opinion. The snake was definitely NOT a fer-de-lance, which we feared the most. But there are many other venomous snakes in Costa Rica. He waited a few minutes, felt nothing, and assumed that either the snake was not venomous, or it had not really bitten him, or, or, or. But the student was about an hour from any medical help, so his Costa Rican assistants demanded that he return home, just in case he needed to go to the hospital in town. He would be that much closer.

So Martin returned to our house and appeared at the door as described. The next question out of his mouth was almost more shocking than the statement that he might have been bitten. “Tom, would you check my buttocks?” I explained that this might be going further than the faculty-student contract, that this was not in my job description, that I needed to go to bed to get my sleep, but, geesh, this had to be done. He dropped his trou and I put on my examination face as if I had done this a hundred times before, and not at all sure what I would find. I looked it over, carefully, but I could see absolutely nothing—no wound, no mark, no swelling, no redness. I pronounced that he would probably live, although the scientist in me was quick to point out that I had no baseline data with which to compare. I could only assume that what I was seeing was a normal-looking, very white, pasty, Swiss butt (the student was, in fact, from Switzerland). We both laughed and the incident ended.

I got a lot of mileage out of this anecdote. I repeated the story when I introduced Martin to an audience before he gave a presentation on his research. I emailed everyone I knew and told the story. My son Matt replied to the email with a sobering thought: “Dad, it is a good thing he had not been bitten. You would have had to suck out the venom.” What could have been a really serious event turned out to be nothing but fodder for an amusing anecdote. But our fascination with snakes continues, and we think about them, and we watch for them, and the stories about them are remembered for a long, long time.