Estimating small mammal prey size based on owl pellet remains in the Arctic

Schmidt, E., Fauteux, D., Therrien, J.‐F., Gauthier, G. and Seyer, Y. (2020), Improving diet assessment of Arctic terrestrial predators with the size of rodent mandibles. Journal of Zoology, vol. 311, pp. 23-32. https://doi.org/10.1111/jzo.12756

 

Understanding food webs and how resources are shared among consumers can be difficult due to the number of complex species interactions that occur within ecosystems. Food web interactions can be especially difficult to interpret within ecosystems such as the Arctic where migrants create energy fluxes upon their arrival and departure. However, a number of bird species (e.g. snowy owls Bubo scandiacusrough-legged hawks Buteo lagopus, long-tailed jaegars Stercorarius longicaudus) in the Arctic help make understanding their food webs a little easier, by leaving behind clues as to what they have been eating. Predators often leave behind regurgitated pellets. These pellets are often filled with information, such as mandible bones and skulls that are particularly common, that can be used to identify prey species.

Small mammals compose the majority of prey items for a wide range of avian and mammalian consumers in the Arctic, however few prey species are ever found in high numbers at any given site. Under these conditions where a wide range of consumers are supported by few prey species, resource partitioning is likely to occur based on prey features such as body mass (size). For example, birds of prey in western Poland were found to eat preferably large voles, while tropical terns shared fishes and squids based on the size of the prey. By examining prey remains found in pellets left behind by predators, ecologists can begin to determine how few different prey species are shared across a wide range of consumers.

Elliott Schmidt blog Fig 1
Figure 1. Brown lemmings (Lemmus trimucronatus) serve as a common prey item for Arctic predators. Photo by Dominique Fauteux

Mandibular bones of small mammals often survive the digestion process and are therefore easy to retrieve from pellets. Dental patterns among small mammals are unique and can be sufficient to differentiate between species with a high degree of confidence. However, the mandibular bones of prey species may be able to provide additional information to ecologists in the field aside from just species identification. The focus of this experiment was to determine if it was possible to gain information regarding the body mass of small mammals that different predators are feeding upon based on different mandibular measurements. To answer this question, we used the large small-mammal collection of the Canadian Museum of Nature, which is home to thousands of lemming and vole skulls and mandibles. Using these vast collections allowed us to obtain a massive sample size and study the relationship between body mass and mandible size in multiple populations across the Canadian Arctic.

To conduct our experiment, we obtained four mandibular measurements: length of the mandibular toothrow socket, ramus-molar toothrow distance, condylo-incisor distance, and angulo-incisor distance, from six different small mammal species (red-backed vole Myodes rutilus, tundra vole Microtus oeconomus, meadow vole Microtus pennsylvanicus, Ungava lemming Dicrostonyx hudsonius, brown lemming Lemmus trimucronatus, and collared lemming Dicrostonyx groenlandicus) in the Arctic. The sex and mass of each specimen (taken when initially captured) were also recorded by the collectors. For each mandibular measurement taken, we were able to determine a best fit model and then develop an equation for each species to allow ecologists to estimate the mass of prey items based on different mandibular measurements. Since location was found to influence the relationship between mandible size and mass, different equations were developed for different geographic locations for each species on a province-by-province basis.

Elliott Schmidt blog Fig 2
Figure 2. Long-tail Jaegar (Stercorarius longicaudus) commonly feed on small mammals in the Arctic, however clues as to what species have been preyed upon can be determined by examining the remains found in regurgitated pellets. Photo by Dominique Fauteux

The use of pellets to gain information about Arctic food webs using the equations developed by this study provides ecologists with a non-invasive and inexpensive methodology to determine species and mass of prey that predators are feeding on in the Arctic. This information can allow ecologists to gain better insight into resource partitioning and biomass influxes, and better understanding of food web interactions within Arctic ecosystems.  

Elliott Schmidt and Dominique Fauteux

Contact information: Elliott Schmidt (elliott.a.schmidt@gmail.com)

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