A scale of scavenging
Submitted by editor on 31 January 2017.
Factors influencing the proportion of scavenging in a vertebrates’ diet. Each of the traits/ factors ranges from low on the left to high on the right. A high value for a given trait can either increase scavenging propensity (+ +) or reduce it (– –), the same is true for a low value of said trait. Silhouettes ids: basal metabolism – crocodile and puma, locomotion cost – albatross and five-lined skink, detection rate – shaggy frogfish and Gyps vulture, carrion availability – fish skeleton and dinosaur skeleton, food processing – insectivorous bat and hyena, competition – Gyps vulture and Dilophosaurus, facilitation – Gyps vulture and wolverine. All images from phylopic.org, <http://creativecommons.org/licenses/by-nc-sa/3.0/>.
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By Adam Kane
IRC Funded Postdoctoral Researcher at University College Cork, adam [dot] kane [at] ucc [dot] ie
While it may not come as a surprise that lions and hyenas are capable carrion eaters, what about squid-eating albatrosses or the long-extinct pterosaurs of the Mesozoic? Even hippos supplement their typically herbivorous diet with dead meat on occasion. So, although vultures are regarded as the canonical scavengers, nature is replete with other species who take carrion to a lesser or greater extent.
What this suggests is that there is no hard divide between predator and scavenger, rather species fit along a gradient or a scale. Without giving it much thought we’d expect vultures to exist at one end as the adept scavengers and something like a bat at the other. But why is this the case? We set out to address this by identifying the traits and environments that allow a species to flourish as a scavenger.
Our result was a ‘scale of scavenging’ (Kane et al. 2016). This scale has a number of features which we considered important to the evolution of a would-be scavenger but can be broadly divided into those traits that modify encounter rate and those that govern handling time.
Watch a video abstract of this paper here
If we focus on encounter rate we can see why vultures do so well, they tend to live in environments where carrion is readily available, they can move large distances at low energetic cost because of their soaring flight and they have high visual acuity.
Albatrosses have convergently evolved many of these features allowing them to survive on a diet of squid carcasses which they pluck from the ocean surface. Such is this convergence that they have been dubbed the ‘vultures of the seas’ (Grémillet et al. 2012).
In contrast, species that have energetically costly movement, have poor senses or live in areas with few carcasses fall at the other end of the scale. For instance, habitats that experience sub-zero temperatures can freeze a dead body solid, rendering it inaccessible to most interested parties.
The realisation that this feeding behaviour has a venerable history means we can even put our scale to work on extinct species. The diets of the gigantic carnivorous, theropod dinosaurs are often queried. But we suspect that moving their multi-tonne body masses around in search of carrion would have proved too costly for them to have been exclusive scavengers.
Handling time is equally important for the survival of a scavenger. This is perhaps best realised when it comes to direct competition for food; a slender cheetah stands little chance against its stockier Savannah rivals like lions, leopards and hyenas meaning it’s better off pursuing live prey.
The ability to actually process dead meat can also hinder a carnivore interested in carrion. The tough hides of ungulates require powerful jaws and the right dentition to tear open. This is why Marabou storks are often seen waiting in line for the more skilled species to get to work. Once the carcass is open they will try to snatch a morsel from the melee of vultures and hyenas.
Again we can apply this facet of our scale to extinct species, this time in the form of Necromantis (‘death-eater’), a large bat from the Eocene, whose powerful jaws were originally suggested as an adaption to scavenging. Quite a difference to the bats we encounter today.
What our review proposes is that the idea of scavenging being an all or nothing feeding behaviour is a false dichotomy. Indeed, this mistaken view looks like a textbook case of Richard Dawkins’ “tyranny of the discontinuous mind”; the very human compulsion to neatly categorise things into boxes even when reality is more nuanced (Dawkins 2011). We hope the scale we have constructed will allow researchers to identify likely candidates for scavenging among species, both extinct and extant, even if it’s yet to be observed in them.
References
Dawkins, R. 2011. The tyranny of the discontinuous mind. – The New Statesman, <www.newstatesman.com/blogs/the-staggers/2011/12/issue-essay-line-dawkins>, accessed 30 Jan 2017.
Grémillet, D. et al. 2012. Vultures of the seas: hyperacidic stomachs in wandering albatrosses as an adaptation to dispersed food resources, including fishery wastes. – PloS One 7, e37834.
Kane, A. et al. 2016. A recipe for scavenging in vertebrates – the natural history of a behaviour. – Ecography <http://onlinelibrary.wiley.com/doi/10.1111/ecog.02817/full>.