When I was very young, maybe 4 or 5, I had a collection of dead bees. I found my specimens dead on the sidewalk, floating in a swimming pool, or lying in the corner of a sunny window ledge. At my behest, my mom would wrap the lifeless insect in a paper napkin, stash it in her purse, and bring the tiny carcass home for my collection. I stopped accumulating bees after a few years but remained fascinated by dead things. Over the years, I’ve found a desiccated frog on a granite slab, a Vaux’s swift cradled in an icy sun cup on a glacier, and the broken body of a red-tailed hawk lying between jagged rock outcroppings. These encounters and others like them stick with me. I still find myself wondering: Where did these animals travel? How did they end up there? Why did they die?
It should come as no surprise then that in studying carnivore-ungulate interactions, it’s often the deaths of deer and elk that I find most interesting. While many of us glimpse animals through binoculars or the camera lens, it’s a rare privilege to document the bookends of their lives. We collar neonates shortly after birth, and for some of those, we later investigate their deaths. When they die, it’s our job to piece together what happened.
First, we look for tracks and sign around the scene. Paw prints and scats indicate which species have visited the site, and drag trails from a cached carcass can reveal blood-soaked earth or lead to a kill site. Bears often leave scratches on tree trunks, and carnivore hairs get caught in low branches. Using a microscope, we can identify which species left those hairs. We map out the comings and going of the scavengers, and look for signs of prey struggle.
The condition of the remains adds more to the story. Coyotes, wolves, and dogs scatter a carcass. Bobcats and cougars cache them. Cats eat the internal organs first, but leave the stomach intact, even placed delicately to the side. Bears peel back the hide and chomp on limbs – bones and all - like they’re hot dogs. Such clues indicate the scavengers that have visited, but they don’t reveal why the ungulate died.
For this, we get under the skin, performing a necropsy, or autopsy of an animal. Methodically, we dissect the carcass searching for clues: starvation saps the marrow of fat, leaving it red and gelatinous; disease can lesion the affected organs; vehicle collisions shatter bones and organs. We can identify predation by finding hemorrhaging at the site of lethal bite wounds. These factors don’t always occur in isolation – a sick deer may be a starving deer, and a sick or starving deer is easier for a predator to catch.
In cases of predation, we swab predator saliva from the exterior of hemorrhaged bites and later analyze the DNA in that saliva to confirm the predator. Where the substrate is poor for tracking, such as pine duff, and consumption patterns are unrevealing, this technique can be the lynchpin in cracking the case. It can also help us identify a cause of death when many scavengers have fed at a carcass.
Each tool on its own has its limits. Considering the clues together, we can often piece together the story of the deer or elk’s demise. Collectively, mortality investigations allow us to examine how different factors influence prey populations over time and across the landscape.
- Taylor Ganz