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.)

Cats: our feline friends, or are they?

(Cat eating a bird it has just killed.)

About a decade ago I had a student in my Conservation Biology class named Scott Boomer. We were discussing the problem of non-native organisms that week, and Scott told me he had kept some interesting records on the behavior of three cats that he and his wife had at that time. The cats (one male, two females, and all neutered) had access to food and water in Scott’s apartment around the clock. The three natural predators had the habit of capturing prey outside and bringing it back to Scott, often dropping it at his feet or putting it in their bathtub. Scott is a biologist and he was able to identify all the prey items returned to his apartment over a 2-year period.

The list included:

Mammals: 5 deer mice, 2 woodland jumping mice, 5 Eastern chipmunks, 4 meadow voles, 1 gray squirrel, 6 star-nosed moles, 4 short-tailed shrews, 1 cinereus shrew, 2 little brown bats, 1 Eastern cottontail

Amphibians: 2 green frogs

Reptiles: 1 Eastern painted turtle, 3 Eastern garter snakes

Birds: 3 common yellowthroats, 2 black-capped chickadees, 1 house wren

Total: 43 animals

Now, there are about 90 million cats in the U.S., according to the 2005-2006 National Pet Owners Survey. A certain percentage of those cats never go outside. But anyway you run the numbers, the collective mortality on native wildlife by U.S. cats must total millions of individuals of dozens of species. In some places in the world, feral cats, those that have gone completely wild, are responsible for the demise of rare species of birds. The Stephens Island wren (a flightless species) in New Zealand went extinct in the late 1800’s due to the island’s cats, or so that story goes. The wedge-tailed shearwater in Hawaii is also impacted by cats. Conservation biologists actively control cats (as well as non-native rats, mongoose, etc.) in such places today, especially on oceanic islands.

We hear a lot these days about our “ecological footprint”, or the impact that a human has on the earth’s natural resources and ecosystems. I doubt that our pet ownership is included in these calculations. Remember that I tend to think in terms of quantity and quality of habitat for biodiversity. I usually think of our “habitat footprint” as defined by the boundaries of our house and the lawn surrounding it. But the effect of that living space penetrates further depending on the chemicals we use on the property, how far away we or our children trounce on the environment, and the influence of our pets, of which cats are probably the worst offenders. There are zones of concentric circles beginning with the epicenter of the house itself, which include areas of decreasing influence on the fauna and flora that is there now, as one moves respectively outward. Cats probably have an effect in each of those zones, but they may represent the only threat in the outermost circle, which is perhaps several hundred meters from the edge of the house.

In fact, last year someone built a new house about 100 meters from the edge of my woodlot. For the past few months, I have had two cats roaming my property that I am sure live at that house. I have not had cats on my property in 20 years. And so it goes. We increase our collective ecological footprint, we chip away at the quantity and quality of wildlife habitat and, in my opinion, the quality of life is diminished just a little bit more---again.

In these few paragraphs I wanted to increase your awareness of an idea that perhaps you have not thought much about--- how that lovable pet cat of yours is possibly reducing the biological diversity in your neighborhood. I do not intend to explore a detailed solution to this problem, although attaching a simple bell to your outdoor cat would probably reduce its kill rate. You might be thinking that cats kill organisms that people do not like very much anyway, so what the heck. But I can assure you that every one of those species killed by Scott’s cats represents a unique and interesting biological story. Remember that not so long ago, nearly everyone thought it was fine to shoot, trap, or poison wolves, mountain lions, and eagles.

The taming of the shrew

(A nest of Northern short-tailed shrews, only a few days old.)

Yesterday I found the first nest of northern short-tailed shrews (Blarina brevicauda) I have ever found.  I was transferring some straw from one pile to my compost pile, you know, the one that never reaches 170 degrees.  Beneath the straw were two nests.  One contained 4 or 5 babies, with gray fur and eyes still closed.  I did not measure them, but I estimate that they were about 40mm in length, minus the tail.  The other nest was empty and was about a foot from the babies' nest, but I am sure it was the nest for the mother.  I had no idea that she kept a separate nest from her babies, but she was close enough to detect what was going on with them.  I have captured this species in Sherman live traps many times, and I have watched the adults moving in the woods from time to time, but this was a novel event for me.


When I uncovered the nest, the babies began to scatter immediately.  I quickly gathered them up and put them back in the nest.  Shrews have a high metabolic rate, and I am sure these babies would die outside of the nest in short order, and it was a cool day.  I returned about an hour later, and they were all gone.  When I left them, they had been sleeping in the nest, all cuddled together (see photo).  I am sure the female returned, realized that the site had been compromised, and moved them.  She probably did this by picking each one up individually in her mouth, and moved them to a new location.  I was unable to locate this new site.

Whitaker and Hamilton's "Mammals of the Eastern United States" give many details about the life history of this interesting mammal.  This species feeds on numerous invertebrates, especially earthworms, slugs, and snails.  They have been known to kill mice and even small birds.  This species is one of only two shrews (and the only one in North America) with venomous saliva, and they are the only mammals in the world to have this feature, which they use to subdue their prey.  The idea is that they are able to paralyze an earthworm and then place it in a food cache for later use; the food item does not die and decompose and yet is unable to crawl away.  Young are born from early spring to late September, and a litter usually numbers 4-6.  Copulation between male and female may last 25 minutes, with the pair locked together, and with the male seemingly inactive and dragged around by the female all the while.  (No wise-crack comments, please.  We are talking real biology here.)  In addition, short-tailed shrews use echolocation (clicks in the range of 30-55 kHz) to navigate their environment, given their extremely small eyes and probable poor eyesight.

I have always maintained that there is still a great deal to learn about shrews and moles, given their relative secrecy and the difficulty observing them.  I also tell students that no matter how much you have seen in nature, I can guarantee there is much more to be seen.  I have been poking around fields and forests for 50 years, and the discovery of this nest taught me yet again that there is much I have to learn.  So get out there and make a new discovery for yourself.  And if you have children, take them with you.   If you have a spouse, you can bring them also, as long as they leave their iPod, and cell phone, and any other electronic thingamabob at home.  Cameras and binoculars are permitted, however.