Diet of sympatric pine marten (Martes martes) and stone marten (Martes foina) identified by genotyping of DNA from faeces

1. What they did

Pine martens and stone martens live side by side across most of Europe. They are so alike in size, colour and habits that their scats cannot be told apart by shape and look, so earlier diet studies in places where both live either lumped them together or used only scats from radio-collared animals. The authors used DNA from scats to sort them by species and then compared what each species ate.

• Collecting scats (2001 to 2004). Near Rogów in central Poland, they collected 1375 scats that looked like marten scats: along forest paths and inside forest stands, at likely marten spots (spruce clumps, young pine stands, tree hollows and nests of birds of prey), during snow-tracking, and in the lofts and attics of 5 buildings near the forest.
• Identifying the species. They genotyped two microsatellite loci (Mel10 and Ma18), whose allele lengths differ between the two martens, in 365 forest scats and 20 building scats. A scat was assigned only when both loci gave the same result twice.
• Analysing the diet. They washed the scats through a sieve, sorted the undigested remains (bones, teeth, hair, feathers, insect parts, fruit skins and stones) into food groups, and estimated biomass eaten. Small mammals were identified from teeth or from hair under the microscope.
• Comparing the species. They compared diets by frequency of occurrence and by biomass, across the year and by season, and measured niche breadth and overlap.

2. What they found

Identifying the scats

• Of the forest scats, 155 were pine marten, 78 stone marten, 1 polecat and 1 American mink.
• 11 of the 20 building scats were stone marten; the rest gave no result. No pine marten scat was found in a building.
• 36% of samples could not be assigned, most likely because the DNA had degraded, although some may have come from other carnivores such as the red fox.
• Further genotyping showed the scats came from at least 34 pine martens and at least 25 stone martens.
• Snow-tracking showed that martens living in buildings regularly visited the nearby forest and foraged there, so the authors combined the 78 forest and 209 building stone marten scats (287 in total).

Diet

• Both species ate a wide range of foods. Small rodents, fruits and birds were the three most important for both.
• Diet overlap was high: 0.83 by frequency of occurrence and 0.70 by biomass.
• Pine martens ate rodents and birds more often; rodents made up about 56% of the biomass they ate. Stone martens ate fruits and insects more often; fruits made up about 47% of their biomass.
• Bank voles were the pine marten’s main rodent (about 59% of identified rodents); stone martens took more voles (Microtinae) and a wider range of rodents.
• Apples made up about 79% of the fruits pine martens ate and 41% for stone martens. Garden fruits (pears, plums, cherries, grapes) made up about half of the stone marten’s fruits but only about 8% of the pine marten’s.
• Pine martens ate more of the forest ground beetle Carabus auronitens; stone martens ate more dung beetles (Geotrupes stercorosus) and cockchafers.
• Bird prey did not differ significantly between the two.
• The stone marten scats from forests and from buildings were similar, except that building scats held more insects and forest scats more shrews.

Seasons

In every season pine martens ate rodents and birds more often, and stone martens ate fruits and insects more often. Stone martens ate fruit all year, even in winter, and both martens ate many insects in spring (in 2002 there was an outbreak of cockchafers). For pine martens, birds were over 40% of the biomass eaten in spring. Diet overlap was highest in summer.

3. What they concluded

• Both martens are generalists with highly overlapping diets, but they differ clearly in what they eat most: rodents and birds for the pine marten, fruits and insects for the stone marten.
• The stone marten used a wider range of foods. Its diet suggests it finds food both in forests and in open farmland and settlements, while the pine marten forages mainly in forest.
• Differences in habitat use and foraging may limit competition between the two in this mosaic landscape, and the high overlap may come from both exploiting a rich, varied food supply.
• Genetic identification of scats works well for studying the diets of closely related carnivores living together. Where traditional ways of telling scats apart by size, shape or smell fail, genetic analysis may be the best way to identify the species.

4. How tracking was or was not used

Tracks and signs were the starting point. The team searched forest paths, stands and likely marten spots for scats and picked out 1375 that looked like marten scats, but the paper stresses that pine marten and stone marten scats cannot be told apart by their appearance, so species was confirmed by DNA. Scats were also collected during snow-tracking, and snow-tracking showed that stone martens living in buildings travelled into nearby forests to forage, which the authors used to justify combining forest and building scats. The paper gives no further detail on how the snow-tracking was done.

5. Tracker or observer certification

Not reported. Who collected the scats and did the snow-tracking is not described; students from Warsaw Agricultural University are thanked for help with the research.

6. Other technologies, skills or innovations

• DNA extracted from a 1 cm piece of each scat (QIAamp DNA Stool Mini Kit), with the two microsatellite loci analysed on an ABI Prism 377 DNA sequencer. Each locus was run twice, with each locus acting as a check on the other.
• Scats were picked up with a new stick each time and frozen at minus 70 °C to avoid contamination.
• Small mammal hairs were identified with keys to the cuticle scale and medulla patterns of hairs.

7. Implications for conservation (from the paper)

Not discussed directly. The authors note that the method allows diets of similar, coexisting carnivores to be compared across a whole population, rather than only from the few animals that can be radio-collared.