Time allocation is important in species’ survival strategies.
For example, many species have to consider trade-offs between foraging and vigilance or foraging and reproductive activities.
Both trade-offs are observed, e.g., in deer species.
However, scavengers also have to allocate time to tracking their commensal predators and this might impose a trade-off on their other feeding activities.
There is a gradient of scavenging frequency in species from, e.g., ravens that heavily rely on large predator kills and that have elaborated intrinsic systems of tracking their commensal predators (e.g., wolves) to the point that it is thought wolves have adjusted their sociality to raven scavenging (Vucetich, J.A. et al., 2003).
Some species might be considered specialist scavengers (possibly, Corvidae species, Aegypiinae spp., Gypaetinae spp. etc.).
Most species are probably opportunistic scavengers and some species (e.g., wolverines, coyotes, foxes etc.) can switch between predatory and scavenging diets where large predators are sympatric or absent.
Some species are probably adapted to tracking particular large predators which is why I used the term ‘target predator’ in the title because scavenging in a high risk activity and not only the scavenger has to learn indicators to inform the individual on the time and location of the kill but also the scavenger has to avoid the large predator that made the kill (which is why, e.g., wolverines appear to have developed a system of foraging on wolf kills adapted to visiting carcasses after a certain period has elapsed from the departure by wolves from the carcass and they delay use of carcasses that are frequently revisited by wolves – see, i.a., Nordli, K. & Rogstad, M., 2016).
Coyotes, as well, seem follow wolf tracks (i.a., Paquet, P.C., 1991), moreover, choosing the direction where the wolves were headed rather than backtracking the wolf trail presumably to increase the probability of discovering a fresh kill (instead of an old kill) which has a greater amount of biomass and greater quality biomass that is more easily handled (choice pieces vs. chewable bones).
Other species (pine martens etc.) might not have developed a strategy of finding carcasses to scavenge on and they might happen upon them opportunistically because many kills occur in their core habitat.
Due to these strategies and non-strategies, I have begun wondering about certain aspects of scavenging activity adjustments.
1.
Are there specific scavenger-large predator species associations that are dependent on similar prey use or similar habitat use (with respect to kill sites by predators and foraging/den sites by scavengers) in order to reduce the time trade-off on behalf on the scavenger?
If the scavenger has to face a trade-off between the time invested in tracking the predator to its kill (or otherwise detecting the presence of a carcass), it might be more beneficial to form associations with a predator that pursues similar prey species (because these prey species might also be predated by the scavenger that switches between predatory and scavenging tactiques), or with a predator that kills prey in habitats and microhabitats where the scavenger is likelier to occur anyway.
That way, the scavenger would be able to perform its own daily activities and supplement the basic foraging strategy with opportunistic scavenging only if the opportunity arises on the individual’s own routes that are otherwise largely unaltered (the scavenging opportunities are not purposefully sought out but come across).
If this is true:
- Scavengers might pick out specific target predators (some scavengers might scavenge on carcasses left by specific predators because of the correlation of the aforementioned use of space patterns);
- There might be species that have evolved scavenging strategies largely because they live in habitats where these opportunities are more abundant compared to similar species that could scavenge, in theory, but they do not because rarely kills are made in habitats where they forage and den on daily basis;
- Far roaming, mobile species might be likelier to scavenge than short-distance travellers due to exposure to a greater variety of habitats and opportunities (also the case of scavenging birds);
- Species whose diet resembles that of large predators might be likelier to scavenge than species whose diet is different and perhaps constrained to, e.g., aquatic or arboreal environment etc.
For example, bird species that live in shrub thickets might be likelier to scavenge on lynx prey because lynx cache their prey in microhabitats with greater structural diversity and concealment levels (Podgórski, T. et al., 2008).
On the other hand, bird species that live higher above ground in tree cavities whence they can oversee the forest floor, might be likelier to scavenge on wolf kills that are left relatively in the open.
The same consideration might be relevant when comparing deep forest species and edge/open field species.
Also, the scavenging behaviour gradient (specialist vs. frequent opportunist vs. rare opportunist) might be determined by the sensory abilities of the scavenging species (visual detection, olfactory detection, auditory detection).
This could particularly affect the relationship between typical large predator kill sites and typical large predator behaviour (e.g., howling in wolves) and the evolution of specialist scavengers that might be better equipped than other, opportunist scavengers to detecting kills which is why they rely on scavenging as their dominating strategy while others use it as an alternative food acquisition method.
It might be possible to identify the ‘perfect scavenger’s properties’ and to compare this set with partial scavengers and rare scavengers.
The foraging method might also bear influence on scavenging probability in a species.
As mentioned before, a species that forages in water or high in the trees might find it more difficult to come across predator kills or the species might face greater trade-offs with respect to investing time in scavenging rather than other types of foraging.
It would be interesting to study whether there are more scavengers in regions where predators exhibit a greater variety of prey killing and caching behaviours.
E.g., in Europe wolves, lynx and bears leave or cache their carcasses close to forests or in the forests on the open ground level or slightly concealed, even covered with soil.
Meanwhile, leopards and other felids cache their kills in trees which offers an opportunity for a greater variety of arboreal species to find the carcass.
Similarly, wolves and lynx inhabit mountain ranges but show preferences regarding altitude and ruggedness while other predators (e.g., aerial predators) might prey higher above tree level or on steeper slopes.
In such areas, the scavengers might be represented by species that exhibit a more diverse habitat use behaviour.
There might even be closely related subspecies in different regions where, in one region, the subspecies demonstrates scavenging behaviour but in the other – it does not due to the predation/caching behaviour specifics by large predators in the two areas.
Notes by Ieva 