On the Scandinavian peninsula (Sweden and Norway), intensive persecution of wolf (Wabakken et al. 2001) and brown bear (Swenson et al. 1994) during the nineteenth and twentieth century exterminated these predators from most of the peninsula. Both wolves and bears were gone from central Scandinavia by the mid-to-late 1800s, and whereas wolves were functionally extinct from the whole peninsula by late 1960s (Haglund 1968) brown bears survived in a small remnant population in the northern part of the peninsula (Swenson et al. 1994). Recovery and expansion into the south-central parts of Scandinavia started in the 1950s for brown bears (Swenson et al. 1994) and as late as the 1980s for wolves (Wabakken et al. 2001). Contrary to wolves, the Scandinavian moose population has grown tremendously throughout the twentieth century (Markgren 1969; Lavsund & Sandegren 1989), and has been exposed to an intensive management that replaced most natural mortality with human harvest (Cederlund & Sand 1991; Saether et al. 1996; Stubsjoen et al. 2000; Solberg et al. 2003).

We studied wolf predation and HS on moose in the re-colonizing wolf population in central Scandinavia. We tested three predictions from the hypothesis that the history of predation will shape the behaviour of prey. First, Scandinavian moose unfamiliar with wolves will initially lack appropriate behaviour to escape wolf predation after wolves have re-colonized an area, and so are relatively easier to kill, i.e. naive compared to moose in populations that have been continuously exposed to wolf predation (Berger 1999). Second, the success rate of wolves hunting moose should decrease with time since occupancy of an area, as a result of moose re-adapting (i.e. learning) their behaviour to the presence of wolf predation (Berger et al. 2001; Laundré et al. 2001). Third, Scandinavian moose should express a higher level of avoidance and a less aggressive behaviour towards humans as compared to North American moose, which are mainly regulated by natural predators.

We also analysed possible confounding effects of other factors than geography/history on the variation of HS, i.e. wolf pack size, wolf body size, moose body size, percentage of adult females in kill, percentage old (>10yr) moose in kill, moose population density, moose population age structure, percentage of wolf-killed calves with depleted (<10%) marrow fat, snow depth, and month of data collection (table 2). We predicted that increased wolf HS would be positively associated with all these factors except moose size, which should be negatively related to HS. Two of these factors could only be qualitatively evaluated in relation to HS, i.e. month of data collection, which was neutral to variation in HS, and population age structure, which had opposite effect on HS than the one predicted. Population age structure was assumed to be skewed towards a higher proportion of older moose in the North American moose populations with none (Isle Royale) or low rate of human harvest (Alaska) as compared to the heavily harvested Scandinavian moose population. Among the remaining factors, four (pack size, snow depth, moose population density, percentage of wolf killed calves with depleted marrow fat) were inversely related to HS, and thus were not likely contributors to the higher HS found in Scandinavia. Four variables (moose size, percentage of calves in kill, percentage of adult females in kill, percentage of old moose in kill) were positively but weakly (d.f.=5, p>0.34) related to HS. One variable (wolf size) gave some support (r_s=0.62, d.f.=5, p=0.17) for the prediction that this factor could contribute to the variation found in HS among studies. Adult wolves in Scandinavia and Alaska were equal in body size but on average 25% heavier than Isle Royale wolves. In contrast, adult female moose in Scandinavia and on Isle Royale were of approximately the same size but on average 7–13% smaller than moose in Alaska. Thus, a moose size:wolf size ratio among populations/studies was negatively related to HS but not significantly so (r_s=−0.47, d.f. =5, p=0.29).

Although a number of other population specific parameters of wolves and moose were considered for any confounding effects on the results, only two (wolf and moose size) gave some support to the pattern found. However, although these parameters (and their ratio) correlated as would be predicted with variation in HS, the relationship was not significant among populations/studies. Therefore, we argue that this factor is not likely to be the ultimate cause to the pattern found in HS among populations.

Instead, we interpret our results as support for our first prediction that moose in Scandinavia are currently naive to re-colonizing wolves compared to populations where wolves have occurred continuously (Denali, Alaska) or been absent for a relatively short period of time, i.e. 40–50 years, (Isle Royale, Kenai Alaska).

In contrast to our first prediction, the second prediction failed, i.e. that HS will decrease with time since wolf occupancy of an area. Thus the hypothesis that moose swiftly will adjust behaviour in response to the new presence of wolves was not supported. Our results thus seem to contrast with some other studies where behavioural adjustments in ungulates to reduce predation from wolves and grizzly bears have been documented to occur within a single prey generation (Hunter & Skinner 1998; Berger 1999; Berger et al. 2001; Laundré et al. 2001). Our results were unexpected since a high predation pressure on calves by wolves (70% of all moose killed), as generally found in Scandinavia (Sand et al. in press), would favour a vertically transmitted learning process, which would be the most effective way to achieve a widespread behavioural change in the population, and ultimately reduce the HS of wolves. Surprisingly, our data also seem to contrast with a study of moose response to an expanding brown bear population in Scandinavia (Berger et al. 2001). Bears at the expansion front had a higher HS than bears at the core of the population, suggesting an adaptive change in moose behaviour. One explanation for the difference between moose response to bears and wolves may be that the bear population in Scandinavia has a longer history of sympatric distribution with moose in bear core areas (30–50 years) compared with the relatively recent establishment of the wolf population (10–25 years; Wabakken et al. 2001). This indicates that adaptive change in anti-predator behaviour in Scandinavian moose may take longer than the period covered by our data (up to 21 years). Alternatively, more subtle changes in the behaviour of Scandinavian moose, not reflected in the HS of wolves, may still have occurred in some wolf territories in this study.

Our third prediction however was supported by the fact that Scandinavian moose seem to be much more timid towards humans than North American moose. Aggressive behaviour towards humans has frequently been reported for North American calf-rearing female moose (Geist 1963; Mech 1966, 1970; Peterson 1977; Franzman 1998; Mech et al. 1998), which is why helicopters are normally required to separate calf-rearing females from the capture crew during capture of moose calves (Ballard et al. 1979; Gasaway et al. 1992; Franzman 1998). In Scandinavia, aggressive behaviour by calf-rearing females towards humans is extremely rare (Ekman et al. 1992) as was also corroborated by the behaviour of female moose towards human personnel during capture of newborn moose calves within the Grimsö Wildlife Research Area.

We argue that there are two important factors that either alone, or in combination, constitute the ultimate cause to the behavioural differences found in moose between continents and why Scandinavian, but not North American, moose so far have failed to re-adapt to wolf predation. First, moose in south-central Scandinavia have not experienced wolf (and brown bear) predation for 120–150 years whereas most moose populations in Alaska and Canada have a history of continuous exposure to wolf and bear predation during past centuries (Orians et al. 1997; Mech et al. 1998; Franzman & Schwartz 1998). The main mortality factor in Alaska is predation by large predators such as wolves, black bears (Ursus americanus), and grizzly bears (Orians et al. 1997; Ballard & Van Ballenberge 1998) with an annual harvest rate of <5% of the winter population (Van Ballenberge & Ballard 1998). In contrast, moose on Isle Royale arrived on the island in the early twentieth century and have been protected from human harvest ever since (Peterson 1977). In 1949, wolves colonized the island and are currently the sole moose predator there. Also, moose on the Kenai Peninsula in Alaska escaped wolf predation (but not black bear predation) due to extermination of wolves for a period (1915–1960) during the twentieth century (Peterson et al. 1984). Thus, the period moose were released from wolf predation was significantly shorter on Kenai and Isle Royale (40–50 years) than in Scandinavia (120–150 years).

Second, in most parts of Scandinavia, human harvest completely replaced natural predation on moose by wolves and brown bears during the twentieth century. During the last 40–50 years the annual moose harvest rate has been 25–40% of the total winter population (Markgren 1969; Lavsund & Sandegren 1989; Solberg et al. 2003), and constituted approximately 95% of the total mortality (Cederlund & Sand 1991; Saether et al. 1996; Stubsjoen et al. 2000). This harvest pressure has continued over most of the general wolf range after the return of wolves, while mortality due to wolf predation inside this range today is still generally low compared with human harvest (<5%). Even within present wolf territories moose mortality due to wolf predation is still only 25–50% of human harvest (Solberg et al. 2003).

Also the mode of moose harvest differs between continents. Throughout North America use of dogs for hunting moose is prohibited. In Scandinavia, the use of baying dogs to hunt moose is widespread and has a long (>100 years) tradition (Ekman et al. 1992). Certain dog breeds have been artificially selected for this type of hunting behaviour, which show close similarities to the hunting behaviour of wolves. Contrary to a wolf attack on moose, however, the hunting dogs never press the attack in order to kill the moose, but just try to keep the moose at bay, thereby distracting it and disclosing its location to the hunter, who can then stalk the moose. Approximately, 20–30% of all moose harvested annually within the current core of the wolf range are shot using this method (Sand et al. unpublished data). This means that the most successful anti-predator behaviour moose can use against wolves, i.e. being aggressive and taking a stand against the wolves, as demonstrated e.g. on Isle Royale (Mech 1966; 1970; Peterson 1977), is exactly the behaviour that has been selected against for more than 100 years in Scandinavia. This behavioural difference may also be reflected in the relatively low proportion of moose that stood their ground when attacked by wolves in Scandinavia as compared to moose in North America.

Because maintaining anti-predator behaviour (for example aggressiveness) in the absence of predators is assumed to be costly (Magurran 1999) we expect that it would be eliminated, or at least relaxed, by selection if it had no other advantage. If in addition a new predator appeared, against which the previous anti-predatory behaviour was even more costly, like moose aggressive behaviour towards human hunters, we would expect a strong direct selection against it. Human harvest has replaced natural predators for several centuries in large parts of the world and is currently the major cause of mortality for many large mammalian prey species. Thus, human harvest is likely to have been an important evolutionary factor shaping life-history strategies and behaviour of these species (Festa-Bianchet 2003), but our data support the notion by Berger (2005) that it is not likely to have been functionally equivalent to that by large mammalian carnivores.