We spend great effort looking for sites where moose have died. During winter we discover moose carcasses from aerial surveys. During summer discovering a moose carcass requires, on average, about ten miles of difficult off-trail hiking. They are discovered just by hiking around, being observant and fortunate. Finding the bones is only part of the effort. Some of the bones have to be carried back to base camp for further analysis. The bones we collect weigh about eight pounds and include the skull, mandible (jaw), metatarsus (foot bone), and any bone showing signs of arthritis. 
From the site where a moose has died, we judge the cause of death and date of death. For example, if wolves killed the moose one can often find blood on nearby trees and many broken branches of nearby shrubs. Also wolves typically chew on and scatter the bones. The carcasses of moose that die of starvation are commonly fully intact and the bones not scattered and or chewed. To help us find and collect moose bones, we rely importantly on the help of Earthwatch volunteers. Finding and collecting these bones is a great deal of work, but still only a fraction of the effort. Once collected, the bones are cleaned, catalogued, measured, analyzed, and stored for future analysis.
Copyright © John A. Vucetich — All rights reserved
Teeth
Collecting bones
As moose age they tend to get arthritis and osteoporosis – just as humans do. In humans, severe arthritis pain is associated with bone deformities that require a trained eye to detect. By this standard, we regularly detect extraordinarily severe arthritis. A quarter of prime-aged moose killed by wolves have at least mild arthritis, and half of older moose have arthritis. Males are more vulnerable than females.
From a moose’s tooth we can determine its age. Every year a new layer of cementum develops in a tooth. When the tooth is cut in cross section, the layers appear as lines that can be counted. The number of lines is equal to the age. These cementum lines are so close together that they must be counted with the aid of a lower power microscope.
The metatarsus
Arthritis and Osteoporosis
By knowing the year of death and age at time of death for many of Isle Royale’s moose, we are able to estimate how the number of moose in various age classes changes over time. Prime-aged moose (3-8 years old) are tough for wolves to kill, where as calves and senescent moose are much easier. Population age structure is also dynamic. Whereas the number of moose has varied five-fold over the years, the number of vulnerable moose has varied ten-fold.
Moose do not walk on their feet. They walk on the tips of two of their toes. Consequently, a moose’s foot looks like the lower portion of their leg. Most specifically, a moose’s metatarsus is homologous (or evolutionarily equivalent) to the two largest bones in a human foot. On a moose these two bones are fused together and look like the lower, rear leg.
We also assess the fat content of the bone marrow from moose carcasses that we discover during winter. The fat content gives us an idea about the nutritional status of moose at the time they die.
A starving mammal mines its body of all available fat reserves. In the last stages of starvation, fat is taken from the bone marrow. Moose with less than ~70% marrow fat in mid-winter are at greater risk of starving before the arrival of spring, are more likely to be weakened by malnutrition and thus more vulnerable to predation. Depending on the severity of the winter and other factors, between 30 and 60% of the adult moose on Isle Royale can have marrow fat contents below 70%. In this way wolves have little impact on moose dynamics –they merely quicken the death of those whose death is eminent. They are scavengers that don’t quite wait for their food to die.
Jaw & Palate Necrosis
Fat content of bone marrow
As moose grow old, their teeth loosen up just a bit. Moose eat very coarse material, including medium-sized twigs. Sometimes a twig gets lodged between their teeth. Because moose don’t floss, the twig just remains and rots. This can eventually lead to a severe infection of the jaw bone or palate. Eventually, the flesh and bone just rot away. The process is called necrosis. A moose may go from being relatively healthy to having severe necrosis in a matter of just a few months or half a year.
Earthwatch volunteers with bone-laden packs
Bone storage facility on the mainland.
A summer’s worth of mandibles.
diagram of an incisor
photograph of a moose incisor
Cementum lines of a tooth under microscopic magnification. Red box in center panel show from where rings are counted.
The graph to the left shows fluctuations over time for calves (X’s), old moose (filled circles), and total moose abundance (o’s). Notice that lots of moose does not always translate into lots of old moose or lots of calves. For example, in the early 1980s there were not all that many moose, but there were quite a few old moose. To understand wolf-prey dynamics one has to understand not only how many moose there are, but also the relative abundance of moose that are easier for wolves to kill - the young and the old.
The metatarsus is one of the bones we always collect. This bone stops growing when a moose is about 12 months old. Thus, the length of this bone is an important indicator of a moose’s overall size and nutritional health during its fetal development and first year of life.
Prime-aged bulls are difficult to kill compared to cows, calves, and old bulls. When wolves kill a prime-aged bull they tend to select those with smaller than average metatarsus length. In this way, the impact of wolves on moose is fundamental. It represents natural selection itself – evolution occurring one kill at a time.
It also seems that moose with smaller metatarsal bones have a greater likelihood of developing arthritis later in life. That is, moose with poor early nutritional health are more vulnerable to arthritis later in life.
Remember moose are eating machines. The teeth are their eating instrument. Moose with necrosis in the jaw or palate are vulnerable to malnutrition and consequently wolf predation.
Wolves from East Pack killed this moose in the winter of 2007. The arrow points to a large hole in the moose’s palate (roof of the mouth). The nasal cavity of this moose had become impacted with a wad of rotting vegetation that was the size of a softball.
lower jaw is healthy, upper jaw has severe jaw necrosis
a closer view of the necrotic area
an arrow points to the moose’s metatarsus
moose metatarsus
human foot, showing location our our five metatarsal bones
Lower hip socket is normal. Upper hip socket, from a different moose, has severe arthritis.
Arrows point to two of several lesions in this dorsal (top) view of a moose skull. The lesions represent osteoporosis - regions of bone with low bone density.
