What killed the stag beetles?

Britain’s largest beetle is the Stag beetle Lucanus cervus, a lucanid beetle that can reach 55 mm in total length. Yes I am aware that beetles aren’t tetrapods, but bear with me here. This evening I was surprised to discover, in my mother-in-law’s garden, the mangled remains of at least six adult male stag beetles, all of them clearly predated by a tetrapod, and all of them exhibiting distinct puncture marks on their elytra. Most individuals consist of an undamaged head, thorax and limbs, and it’s the abdomen that’s been eaten. All the corpses were scattered about an area of about 3 square m of lawn, with five of the six being discovered in the well-vegetated borders surrounding the lawn. So the mystery is: who dunnit?

I’ll start by stating that the garden where the beetles were found is smack in the middle of suburbia. We are not talking here about a place in the countryside, or anywhere that is adjacent or even close to any wilderness areas. It’s all built up, with various busy roads. Wildlife is actually thin on the proverbial ground.

At this time of year stag beetle males are out and about, flying around in search of mates. They are not clambering around on trees or spending much time on the ground. Could an airborne predator therefore have been responsible? Bats are out, as the only bats in the area are pipistrelles Pipistellus*, and they are way too small to tackle stag beetles (if you like bats see the previous posts: one on Greater noctules, one on Ghost bats).

* There are three species in Britain: Nathusius’ pipistrelle P. nathusii, the 45 kHz pipistrelle P. pipistrellus and the 55 kHz pipistrelle P. pygmaeus.

So what about birds? I am hard pressed to work out whether the damage present on the beetle elytra looks like it was caused by mammalian teeth or by an avian bill. Some of the specimens have a distinctly ‘chewed’ look, with punctures and dents matching the damage I’ve seen on bones and other objects chewed on by carnivorans. But others (see the close-up photo*) have puncture marks that just might have been caused by a bird. If it was a bird, it was a big, greedy one, and one that did all of its hunting during the evening or at night (as this is the only time when stag beetles fly). This makes corvids, woodpeckers and raptors unlikely to have been the predators, and in fact the close proximity of the dead beetles makes a bird predator unlikely.

* Nope. Can't get blogger to upload it.

I suppose some owls might eat stag beetles: the only own in the area is the Tawny owl Strix aluco. It’s difficult to think that an individual would consume so many big beetles in the same area, or leave the half-eaten bodies in borders around a lawn. Furthermore, there are no overhanging perches at all (there are no big trees at all in the vicinity), nor is there evidence for owls in the form of pellets or droppings.

Finally we come to terrestrial mammals, and I think this is where the true culprit must be found. Rodents are out: it’s feasible that Brown rats Rattus norvegicus would eat stag beetles, but rodent gnaw marks do not resemble the dents and punctures seen on the elytra in the least. Mustelids are out as, again, there are none in the area at all, and this goes even for badgers Meles meles which will eat stag beetles given the chance. Hedgehogs Erinaceus europaeus are definitely in the local area (go here to see how I know this: we live less than 1 km from my mother-in-law’s house), but I find it hard to imagine that they might catch so many individuals of a beetle that is not blundering around at ground level during the night. Hedgehogs can climb, but they aren’t in the habit of leaping 2 m off the ground or jumping from garden fences to catch airborne insects.

So this only leaves domestic cats Felis catus and foxes Vulpes vulpes. Both animals are big enough and agile enough to track and catch big flying insects. Cats will eat anything, including stag beetles, but I find it hard to accept that even the greediest cat would catch and consume six large beetles in the same small area. So we come to foxes. Neither the fox literature (Lloyd & Hewson 1986, Mcdonald 1987) nor the more general literature I have to hand on British mammals (Pitt 1944, Lawrence & Brown 1973, Freethy 1983, Matthews 1989, Macdonald 1995) mentions the stag beetle as a possible prey item of the fox, but then foxes will eat pretty much anything. They are also enterprising enough to exploit a new and locally abundant food source, are easily large enough to process even giant beetles, are mostly nocturnal, and will bring prey to the same place to eat it.

So that’s my conclusion. I’m not committed to this hypothesis but feel it best fits the evidence. I’d welcome any comments or better ideas, and if stag beetle predation like this turns out to be a novel observation I’ll see if I can get it published. The beetles have been retained in my personal collection of dead stuff.

It’s funny how much stuff there is to find if you only go out and look for it. For the latest news on Tetrapod Zoology do go here.

Refs - -

Freethy, R. 1983. Man & Beast: The Natural and Unnatural History of British Mammals. Blandford Press, Poole.

Lawrence, M. J. & Brown, R. W. 1967. Mammals of Britain: Their Tracks, Trails and Signs. Blandford Press, Poole.

Lloyd, H. G. & Hewson, R. 1986. The Fox. HMSO, London.

Macdonald, D. W. 1987. Running With the Fox. Unwin Hyman, London.,

- . 1995. European Mammals: Evolution and Behaviour. HarperCollins, London.

Matthews, L. H. 1989. British Mammals. Bloomsbury Books, London.

Pitt, F. 1944. Wild Animals in Britain. B. T. Batsford Ltd., London.