Friday, July 9, 2010

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.)