Beetles in the beech marten Martes foina and pine marten Martes martes diet

1. What they did

Studies of marten, weasel and fox diets usually record beetles and other invertebrates only as a group. The authors wanted to know which beetle species martens actually eat, and whether the prey species could show where martens search for food, since some beetles live in leaf litter, some on trees and some in open country.

They collected marten scats in the small forests around Rogów in central Poland. They searched forest paths and stands, and checked places martens use for resting and shelter: young spruce thickets, pole-stage pine stands, tree holes, raptor nests and the attics of buildings next to the forest. Some scats were collected while following marten tracks in winter. Each scat was bagged, numbered and dated, then washed through sieves and sorted into bones, feathers, insect remains, fruit stones and skins, hair and other items. Beetle remains were identified as far as possible, to species where they could.

Each scat was also assigned to one of three zones: forest interior (more than 100 m from the edge), forest edge (the 100 m strip bordering open land) or buildings (mainly attics). The authors compared the beetles found in each zone.

2. What they found

• Of 766 scats washed, 225 held invertebrate remains. Beetles were the most common invertebrates.
• Other invertebrates turned up only in single scats (ants, true bugs, moths or butterflies, bumblebees, even a millipede), except wasps, which were found in more scats.
• The cockchafer was found most often (47 scats). Usually there was not one beetle but a clump of remains from 5 to 20 individuals.
• Next came the dor beetle Geotrupes stercorosus (42 scats, one or a few at a time), then the ground beetles Carabus auronitens (36) and C. nemoralis (32), unidentified ground beetles (12), the dor beetle G. vernalis (10) and C. hortensis (9).
• Eight Carabus species were found. Of the Carabus species known from these forests, only C. glabratus and C. cancellatus were missing.
• C. auronitens remains were also found in winter scats.
• Beetles in scats from the forest interior and the forest edge were closely alike (r = 0.76, p < 0.001). Neither matched the beetles in scats from buildings. In the figure, forest ground beetles are most common in forest and edge scats, while cockchafers and dor beetles make up the largest shares in attic scats.

3. What they concluded

• Beetles are an occasional extra food for martens, not a staple.
• Martens are 50 to 150 times bigger than the beetles they eat, and they do take some small ones, so they are capable searchers. But small beetles were not common in their diet. The authors suggest there is a prey size, about 12 to 14 mm, below which martens find beetles in leaf litter much less often. Weevils may be an exception in other studies because they live in the open on leaves and trunks.
• Cockchafers are plentiful and easy to see and catch in their mating season. Because martens ate mostly ground-dwelling beetles, the authors think they picked cockchafers up from the ground. Dor beetles are common, slow, easy to spot on paths and roads, and rustle the dry leaves as they dig.
• The movement and rustling of dark beetles in dry leaf litter probably helps martens find them, as with the nocturnal, dark C. nemoralis and C. hortensis.
• Martens probably forage not only in leaf litter but also on tree trunks and in tree holes. C. auronitens and C. hortensis climb trees, and the tree-dwelling Calosoma inquisitor was also eaten.
• C. auronitens often spends the winter under tree bark, so its remains in winter scats suggest that martens search under loose bark in winter.
• The difference between forest and attic scats may reflect what lives around the den: forest ground beetles near forest dens, and cockchafers, easy to catch in the open ground around homes, near attic dens.

4. How tracking was or was not used

Scats were collected along forest paths, inside stands and at marten resting places, and some were collected while following marten tracks in winter. The paper does not say how marten scats were identified, or how beech marten and pine marten scats were told apart (the results treat them together).

5. Tracker or observer certification

Not reported.

6. Other technologies, skills or innovations

• Identifying beetles to species from the broken pieces of their hard outer shells in scats, which the authors say is rarely attempted.
• Using the known habits of each prey species (where they live, when they are active, whether they climb trees, where they spend the winter) to work out where a predator was foraging.

7. Implications for conservation (from the paper)

• Knowing which beetle species vertebrate predators eat can help judge how much those predators threaten the beetles.