What could possibly bring down an animal that stands seven feet and weighs 1500 pounds? Not much, right? Attempting to kill a moose comes with plenty of risks, but under the right circumstances, grey wolves (Canis lupus) can accomplish this feat. The wolves could have their skulls crushed by hooves or bodies pierced by antlers. So, for wolves to be successful at stalking and chasing moose, wolves must possess plenty of skills and a little bit of luck.
According to the Yellowstone Wolf Project Annual Report 2012, researchers detected 255 kills made by wolves during the year. Of those, only two were confirmed moose. Compare that number with 159 elk and you come to the realization that it must be easier to take down an elk. Wolves in Yellowstone don’t have anything to gain from hunting moose when there are plenty of elk. However, wolves that live in areas where there is no other prey must be careful. One strategy is to wait until the moose is already in trouble. For example, winter time may be an ideal time to find a moose in peril because of the lack of food and depth of snow. The energy demands are sometimes too great for the moose. Snowy conditions may not be the only factor that contributes to an energy-depleted moose. According to some research, the wolves may be getting help from an unexpected source.
Echinococcus granulosis, a type of tapeworm, hails from the phylum Platyhelminthes and class Cestoda and is a common larval parasite of moose. Wolves and other canines serve as definitive hosts. How could tapeworms no longer than 5mm help wolves hunt moose more efficiently? To answer this question, it is important to understand the life cycle of this tapeworm. E. granulosis live as adults in the intestines of wolves where they produce eggs. These tapeworm eggs contain an embryo (called an oncosphere or hexcanth). This embryo has six tiny hooklets. The outside portion of the egg protects the embryo during its journey. These eggs will eventually pass through the wolf in the feces.
Moose will accidentally ingest these eggs as they graze on vegetation. The eggs hatch inside the small intestine of the moose, release the embryos, then the embryos penetrate the intestinal wall (remember the hooklets?) and invade the circulatory system of the moose. From here, the embryos enter into organs, such as the liver and lungs, where they will develop into cysts. Each cyst will grow as daughter cysts form inside. To complete the life cycle of E. granulosis, wolves become infected again after consuming the moose organs.
The tapeworm does not seem to negatively impact the wolves. It’s the cysts that grow in the moose that may cause problems. Some researchers think that these cysts leave the moose more susceptible to predation by wolves because they form and grow in the lungs (Mech 1970, Peterson 1977). It would make sense that a lung infection in an individual moose would make it an easy target for a wolf, especially if that moose is already struggling due to difficult environmental conditions. Joly and Messier (2004) devised some very cool methods to test this idea. They stated that if E. granulosis did, in fact, increase moose vulnerability to wolf predation, then they should be able to tell by looking at the concentration of tapeworms across a moose population. Highly concentrated, or aggregated, tapeworms in an area simply means that few individual moose harbor most of the parasitic tapeworms. In a moose population that experiences high predation pressure from wolves, the distribution of these parasites should be less concentrated (and more evenly distributed) than in a moose population with low predation pressure. Their results did support this, but how does this make sense?
It can be explained pretty easily. In an area where there are lots of wolves that actively hunt moose, one would expect that those moose that are highly infected (high concentrations of tapeworms) would be at a disadvantage. They may be slower and weaker because of the large cysts in their lungs. Once the wolves kill those highly infected moose, only the moose that have a lesser concentration of the parasites, or none at all, would be left. However, in an area where there are low numbers of wolves hunting moose (if any at all), the parasitic tapeworms would be found in higher concentrations. Those moose with bad infections would not be “weeded” out of a population as fast because of the lack of active predators.
Lots of questions remained unanswered. Do parasites, like these tapeworms, have the ability to control relationships in ecosystems? Are moose and wolves just pawns in this parasitic game of natural selection? It seems like these tapeworms definitely rely on predator/prey interactions, but is it also possible that the presence of E. granulosis is necessary for a healthy wolf population in some areas?
Give a moose a tapeworm, and find out.
Joly, D. and Messier, F. (2004). The Distribution of Echinococcus granulosis in Moose: Evidence for Parasite-induced Vulnerability to Predation by Wolves? Population Ecology, 140: 586-590.
Mech, D. (1970). The Wolf: The Ecology and Behavior of an Endangered Species. Natural History Press, New York.
Peterson, R. (1977). Wolf Ecology and Prey Relationships on Isle Royale. National Park Service Monograph no. 11, Washington, D. C.