Approaches to Field Investigations of Cause-Specific Mortality in Mule Deer (Odocoileus hemionus)

1. What they did

Mule deer numbers have been falling across their range since the mid to late 1900s, and predation is one of several causes. To understand why deer die, researchers and managers need to tell predation from scavenging, disease, starvation and accidents, and to know which predator was involved. This Colorado Parks and Wildlife (CPW) technical report gives general guidelines for investigating mule deer mortality sites in the field.

It is based mainly on several years of investigating the deaths of radio-collared mule deer in the Piceance Basin of northwest Colorado, as part of a long-term study of mule deer and natural gas development, together with a review of the literature. The authors give guidelines rather than step-by-step instructions, because every site is different. The report covers:

• Safety and equipment: gloves, masks for suspected zoonotic disease, a necropsy kit and thorough photographs.
• The site: approaching carefully, searching a wide area for remains and predator sign, signs of a chase or struggle, and the position of the carcass.
• The carcass: external and internal examination (necropsy), canine punctures, claw marks, and how to tell bruising and hemorrhaging from blood pooling and decomposition.
• Health and other causes of death: body condition, dehydration, age, season, disease, capture myopathy, vehicle collisions, fences, ice, poaching and old sites.
• Newborn fawns: how to tell whether a fawn was born alive.
• Identifying the predator: killing and feeding behaviour and tracks and signs of cougars, bobcats, coyotes, domestic dogs, black bears, golden eagles and scavenging birds.

It ends with two summary tables and a mortality investigation field form.

2. What they found

Predation or not

• An undisturbed carcass rules out predation. Deer that die of disease or starvation usually lie upright or on their side with legs folded under them. Deer killed or scavenged by predators usually lie on their side with legs extended, often in or near thick cover.
• Predator tracks along or behind deer tracks suggest a chase; blood confirms predation. Broken or trampled plants, clumps of deer hair and drag trails of blood are signs of a struggle.
• Wounds made while the deer was alive show bruising and hemorrhaging. Damage without hemorrhaging, such as canine punctures with no bleeding around them, comes from scavenging after death.
• Blood pooling after death (lividity) can look like hemorrhaging and can appear within 20 minutes of death. Cutting into the hide helps: bruising into the hide or muscle suggests hemorrhaging, while “white” tissue suggests lividity.
• The best proof of predation is canine punctures with hemorrhaging. Where little is left, blood trails or blood on plants with fresh predator sign, or even a bloody collar, show an attack.
• When the cause cannot be found, the authors recommend recording it as unknown rather than guessing.

Health and other causes

• Femur marrow is ranked from 1 (white, hard and waxy, a healthy deer) to 4 (deep red and gelatinous, poor condition). Fat is used up first under the skin, then around the organs, and last in the marrow. Young fawns naturally have little fat and pink-red marrow.
• Vehicle collisions cause “road rash”, broken legs, vertebrae, ribs or pelvis and widespread bruising. Deer also die in fences, by falling through ice, and rarely by tangling in shrubs.
• Capture myopathy (a muscle disease after extreme stress) can kill deer soon after capture, and the authors suggest a 7-day censor period after helicopter capture.
• Newborn fawns that breathed have spongy, light-pink lungs; those that walked have lost the soft pads on their hooves (within 24 hours); those that fed have milk in the stomach.

The predators (Tables 1 and 2)

• Cougars and bobcats: short chases (longer and messier for bobcats), usually a single bite to the throat, top of the neck or skull, and fine, razor-like claw marks. They usually drag the carcass to cover; cougar drag trails can be more than 300 m. They pluck hair and enter behind the ribs, eat organs first, set aside the rumen and intestines, leave clean-cut edges, and cache the remains. Bobcats reach about 0.3 m to cover a cache, cougars about 1 m. Cougars can break large leg bones; bobcats separate legs at the joints instead.
• Coyotes: often hunt deer in pairs or small packs, sometimes chase for long distances, and bite repeatedly, leaving many punctures and severe hemorrhaging, especially on the throat and hindquarters. They eat muscle first, scatter hide, hair, rumen contents and remains widely, chew ribs into ragged splinters, and sometimes bury the head. They also carry off and bury radio collars, at times more than 1 mile (1.6 km) from the site.
• Domestic dogs: attack like coyotes but often do not eat much, leaving mutilated carcasses with tearing wounds.
• Black bears: mostly kill newborn fawns, and rarely adults except when the deer is weakened or giving birth. They bite along the spine and leave massive bruising, flattened vegetation, beds and scat. They peel the hide inside out (“banana peel”) and crush the skull and muzzle, without scattering the remains widely.
• Golden eagles: the authors confirmed eagle predation on 3 fawns less than a week old in Colorado, with deep talon punctures, one into the brain.
• Scavenging birds: leave white droppings, pellets, tracks and wing marks in snow, and intact skeletons with flesh removed, and may carry fawn remains to cliff ledges.

The report gives track sizes, scat diameters, canine spacing and canine puncture diameters for cougars, bobcats, coyotes and black bears.

3. What they concluded

• Finding the cause of death depends on how quickly the investigator reaches the site and what evidence is left. Weather, decomposition and scavengers quickly destroy evidence, and collar mortality sensors take several hours (usually 4 to 8) to switch on.
• Investigators should search the whole site before touching the carcass, skin the whole carcass rather than only common attack zones, look at health as well as wounds, and take many photographs.
• Each predator has a typical way of chasing, killing and feeding, so the identity of the predator can usually be found from the signs at and around the kill site. All predator and scavenger signs should be weighed together, because most predators also scavenge.
• A conclusion of “undetermined cause of death” is acceptable and better than assigning a cause on weak evidence.

4. How tracking was or was not used

Tracks and signs identification is at the heart of this report. Investigators are asked to search for and photograph tracks, scat, urine, predator and deer hair, blood trails, drag trails, matted or broken plants, caches, scrapes, tree scratches, beds, bird droppings and wing marks. They read signs on the carcass itself: canine punctures, claw marks, plucked hair, feeding entry points, chewed or cleanly cut bones and the scattering of remains. Tracks are to be photographed with a scale, and length and width measured, with photos of several tracks to judge stride length and track patterns. Predator tracks along or behind deer tracks are read as a chase. The authors keep the tracks and scat descriptions brief and refer readers to track and sign books (Murie 1974, Halfpenny and Biesiot 1986, Elbroch 2003).

6. Other technologies, skills or innovations

• Very high frequency (VHF) radio collars with mortality sensors, and Global Positioning System (GPS) collars. Where a coyote has carried off a collar, the kill site may be found from clusters of GPS locations and periods of inactivity.
• Field necropsy, femur marrow scoring, organ fat scoring, tissue sampling for diagnostic laboratories, lung float test for newborns, and chronic wasting disease testing.
• Hair samples can be sent to a wildlife forensics laboratory; paper coin envelopes rather than plastic bags keep DNA from being destroyed by moisture.
• Appendix I: a two-page mortality investigation field form with a key to the cause of death.

7. Implications for conservation (from the paper)

• Accurate mortality investigations are needed to measure the causes of mule deer deaths, to address the concerns of wildlife managers, hunters, landowners and others, and to guide management.
• Investigating each mortality signal promptly, and checking collars at times when carnivores are usually inactive, improves the chance of finding the cause.
• Wrongly assigning a cause of death on weak evidence can mislead; “unknown” is the better record.