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Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd

Discussion


Determining the amount of each carcass consumed by Yellowstone wolves was difficult because it was often unknown when the wolves abandoned a carcass. When wolves were away from a carcass, scavengers such as coyotes (Canis latrans), ravens (Corvus corax), eagles (Haliaeetus leucocephalus, Aquila chrysaetos), and grizzly bears (Ursus arctos) often fed on carcasses. Furthermore, even after we assessed a carcass, the wolves may have returned. Nevertheless, we were able to obtain information about the wolves' immediate use of carcasses because wolves usually consume a large amount of most kills within a few hours (Mech 1970).

The wolves in our study killed elk at rates similar to wolves hunting caribou (Rangifer tarandus; Mech et al. 1998:110) and tended to kill calves, old cows, and individuals with low marrow fat. This pattern is similar to that found in other areas where wolves prey on a variety of ungulate species (Mech 1970, Mech et al. 1998). Furthermore, the annual difference in winter severity yielded results similar to those of other studies. During 1997, at the end of 1 of the most severe winters on record, YNP wolves had a higher killing success rate, killed more prey, and consumed less of each carcass than in the mild winter of 1998. The nutritional condition of the prey killed in 1997 was poorer than of prey killed in 1998.

In addition, the wolves killed more calves in March 1998 than in March 1997. There are several possible reasons for this: (1) relative to adults, calves were disproportionately more vulnerable in 1998 than in 1997 because mild weather had less influence on 1998 adult condition; (2) the wolves killed more calves in fall 1996 than in fall 1997 and more calves died over winter, so there were fewer left in spring 1997 (D. W. Smith, unpublished data); and (3) calves were more abundant within wolf pack territories in spring 1998 than spring 1997 because more cows with calves migrate from wolf pack territories to lower elevations during severe winters (Shoesmith 1979, Coughenour and Singer 1996).

The cows killed during both years were older than the bulls, in keeping with their greater longevity (Houston 1982) and similar to the findings of Boyd et al. (1994). Proportionately, ages of the cows killed averaged about 70% of their maximum life span of about 21 years, whereas those of the bulls killed averaged only about 40% of their maximum age of about 15 years (Houston 1982). This age disparity is similar to that in other ungulate populations preyed on by wolves (Mech 1970, Mech et al. 1998).

The average age of all elk killed and of all adult elk killed in 1997 was higher than in 1998. This is opposite of what might be expected in that the severe winter of 1997 may have predisposed younger adult elk to predation as in Denali Park caribou (Mech et al. 1998). On the other hand, during neither winter did young adult elk comprise a very large proportion of the elk killed by wolves.

Comparison of the severe 1997 winter with the mild 1998 winter allowed us to distinguish the conditions that may have contributed to incomplete carcass consumption. Originally, we hypothesized that regardless of the severity of the winter, there would be enough elk in poor nutritional condition that the wolves would kill more than they could eat at the time. The fact that the wolves behaved as expected in 1997 but not in 1998 tends to refute that hypothesis. Although YNP wolves killed prey in poor condition during both mild and severe winters, they had a lower success rate during the mild 1998 winter, and a lower kill rate (despite an increased wolf density), and they tended to promptly consume all that they killed. Their 1998 kill rate was still high compared with published data (Schmidt and Mech 1997). Further, they were not aggressive or persistent in defending their kills from scavengers, indicating they had adequate provisions. Nevertheless, the degree of consumption of their kills showed that they did not kill more than they could immediately eat in 1998.

Snow conditions appeared to be the main factor influencing kill rate and the degree of carcass consumption in our study, and elsewhere (Pimlott et al. 1969, Mech and Frenzel 1971, Peterson and Allen 1974, Boyd et al. 1994, DelGiudice 1998, Mech et al. 1998). High elk density and lack of previous culling by wolves were insufficient to predispose elk to easy killing, even in late winter when elk are in their poorest condition of the year.

Our results also demonstrate that severe winters are not necessary for individual elk to become malnourished. Marrow fat content of adult wolf-killed elk during March 1998, after a mild winter, averaged 70, a direct indicator of low total body fat and marginal condition (Mech et al. 1998:136).

In summary, the relationships between reintroduced YNP wolves and previously wolf-free elk did not differ in any way that we could detect from wolf-prey relations in long-extant systems. This was true despite the high ratio of prey available to wolves and the large number of unculled prey. Furthermore, the degree of winter severity affected the new wolf-elk system in much the same way it affects long-extant systems. These findings suggest a dominating influence of winter severity on wolf predation patterns (Mech et al. 1998).


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