I have read quite many wolf-related publications and while reading them, I have developed a few ‘hunches’ that are not wild imaginings but that have resulted from mental processing of wolf dietary data in different habitats and different regions and different wolf populations, globally.
Diet is influenced by many factors and selectivity for prey species is determined by the availability of this species as well as the availability of alternative prey.
Also, wolves have undergone local adaptations and they might have adjusted their body size and hunting strategies to the specific prey in the region although most grey wolves can learn to hunt new prey (see, e.g., Smith, D.W. et al., 2000) even if the prey is difficult to subdue and altogether formidable.
I have also read about wolves that recolonize ranges where the dominating (most available) prey species is different from that on their former range.
I think the study I am referring to was one conducted in Poland where the wolves switched from red deer to roe deer or from roe deer to red deer as their dominant dietary resource as the wolves settled in an area where the other species was more abundant (or the only available) compared to their area of origin.
Unfortunately, I cannot recall the title of the publication and if I come across it again, I will reference it here.
Species composition and selectivity would also be related to the landscape (forest vs. grassland prevalence) and the corresponding hunting success, or species migration patterns (e.g., in Denali National Park, Alaska wolves subsided on reindeer during reindeer migration periods but when reindeer were not found on wolves’ range, the wolves did not follow (unlike in some other wolf populations that track the ungulates to their winter yards) and hunted, e.g., Dall’s sheep instead: Murie, A., 1944).
Similarly, wolves are not known to preferably hunt in a rugged terrain in high altitudes therefore alpine ungulate species (sheep, goats etc.) might have lower predation rates unless they descend during certain seasons to feed in the valleys or on lower mountain ranges.
These effects are difficult to disentangle, still, I have developed this notion that wolves dislike prey species that mostly forage on browse (trees, specifically).
All ungulates forage on browse but some more than others (e.g., red deer compared to roe deer and elk compared to white-tailed deer or moose).
Certainly, I am not about to claim that wolves never prey on browsing ungulates because that is evidently not true.
For example, on Isle Royale, moose is the only prey species for wolves (apart from beavers that are more of a summer resource).
Also, wolves mostly feed on elk, e.g., in the Northern Rockies but in the Midwest black-tailed or white-tailed deer are taken because those are the available prey species.
However, I am not really prepared to give up my hypothesis because it appears to me that when grazers or intermediate feeders are available, wolves would select for them rather than browsing species unless:
- the grazers are a very dangerous and elusive prey (e.g., bison);
- the grazers/intermediate feeders are too scarce to be worth selecting for;
- the habitat determines that intermediate feeders have more browse in their diet becoming similar in their feeding habits to the more ‘traditional browsers’ – e.g., in predominantly forested landscapes);
- the landscape structure makes it difficult to access intermediate feeders or grazers compared to browsers, for example, because intermediate feeders stay higher above sea level while the lower mountainous ranges are forested and therefore browsed by ungulates.
There are several considerations that complicate the analysis:
- It would require for analyzing wolf diet data in particular regions respective to the prey species diet in particular regions because generalizing can lead to dismissing essential local-specific differences (e.g., red deer feeding on more browse in mountainous or forested regions or roe deer feeding on more forbs and grasses in regions where forests have been lost and browse is not as available);
- It would be necessary to compare not only ungulate availability but also their distribution and accessibility on the landscape throughout seasons respective also to wolf hunting behaviour patterns (types of habitat in the area where hunting and killing the ungulate can occur);
- In intermediate feeders and even some browsers the diet composition can be determined by habitat use and within an area different wolf packs might have access to the same species that still exhibit varied dietary content;
- The effects of the browse on ungulate physiology might be mediated by their size or other biological factors etc.
There are reasons why I have developed this perhaps strange theory.
While red deer are a more profitable prey than roe deer which serves as an explanation why they are taken by wolves more often in many parts of Europe, roe deer can be the dominating prey item but mainly in mountainous, forested regions (where elk might feed higher above sea level or have diets more similar in browse content to roe deer).
Of course, the explanation might also lie with species abundance, habitat use etc.
There is little difference in the taking of fawns by different ungulate species although their weight (profitability) can also vary among species.
While this could be explained with the mere vulnerability of the fawns (little cost to the predator), it is also possible that the impact of browse on ungulate physiology (if there is, indeed, some aspect of it that wolves dislike) is mediated through the nursing process.
In Italy where often roe deer and wild boar are available, wolves would hunt wild boar rather extensively despite wild boar being a dangerous prey compared to roe deer so that in many regions in Europe wild boar are only taken as piglets or carrion (the diet strategy by wolves in Italy could be explained also by roe deer comparatively low densities in many areas as the species has been recovering).
Unfortunately, I do not really know enough about white-tailed deer and black-tailed deer yet to give examples regarding the North American situation where elk appears to be the preferred species where many ungulates co-occur (but also the most abundant one).
If I have the time to study wolf diet and ungulate diet in the publications already accessible, I will add more commentaries to this post.
Currently, I am sharing my ‘suspicion’.
There could be some chemical reasons for this avoidance but I have not found any studies that compare the qualities in biomass between grazing / intermediate feeding / browsing ungulate species.
If wolves avoided browsers that mostly rely on woody plants, this could be related to, e.g., some effects on the ungulate’s physiology caused by tannins or lignin or woody plant chemical defenses (secondary metabolites).
Some secondary metabolites or other chemical elements can affect hormonal level in the species consuming them but thus far, I have only encountered studies on invertebrates (e.g., Wink, M., 2018).
Additionally, for example, roe deer that are arguably (because there is not a scientific consensus on this assumption) mostly avoided by wolves relative to their abundance, in some areas, tend to forage on items that are high in secondary metabolites and their large liver is adapted to metabolizing these compounds that cause digestive problems in other ungulates (Minder, I., 2015).
Roe deer might even be using essential oils and tannins (such as found in hedge woundwort, herb-Robert, belladonna, ivy) to induce natural parasite control.
If some of these substances do not cause damage to the deer but alter the deer’s ‘chemistry’ otherwise, the roe deer might be self-medicating against predators, as well, albeit probably not on purpose.
What if roe deer are avoided also in areas where they consume greater amounts of plants rich in secondary metabolites? (While less avoided in areas where they consume moderate amounts of tannins and SM.)
Perhaps the browsing ungulates can withstand different rates of chemical compounds than grazers or intermediate feeders but the effects do not disappear altogether and are not beneficial for wolves.
In fact, I have another strange theory than in places where ungulates have been overabundant and where wolves have recolonized (including Yellowstone National Park), initially, the excessive feeding on ungulates that have been consuming overly high rates of certain woody species (aspen, willow, fruit and berry shrubs) can cause upset in wolves that show, e.g., through increased aggression and somewhat unbalanced behaviour which might be related to testosterone.
I suppose I will dedicate a separate post to that theory.
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For example, Himalayan wolves appear to prefer smaller wild ungulate species to larger species (Werhahn, G. et al., 2019) while Himalayan wolf is larger in size than Eurasian grey wolf although the body mass is rather similar between the two (sub)species.
While there are important habitat difference considerations, this implies that large wolves do not necessarily rely on large prey preferentially.
Himalayan wolves also compete for prey with other carnivores, including snow leopards that have a high dietary niche overlap with the Himalayan wolves.
I note this because lynx in Europe predominantly prey on roe deer and competition with lynx might be referred to in order to explain the niche partitioning although I find it unlikely that wolves would adapt to lynx hunting strategies because wolves appear to be the dominant predator of the two.
However, selectivity by Himalayan wolves could have been influenced by the habitat use of prey species where, for example, the selected-for Tibetan gazelle can be found on plains while other wild ungulates dwell on a more rugged terrain which might impede hunting.
White-lipped deer that constitute the larger ungulate prey in at least two regions where they are sympatric with the Himalayan wolves, were not abundant in wolf diet and it is possible that wolves do not select for them because these deer might be more difficult to encounter and to kill (but they also have larger amounts of browse in their diet compared to gazelles).
Notes by Ieva 