Wednesday, June 14, 2006

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.


Blogger cpbvk said...

Very neat. They look to me very much like the remains of a mammal's meal, not a bird's. Where do the male Lucanus find their females? Could all six males have been trying to get to the same ensconced female when they were killed by a housecat?

10:40 PM  
Blogger Dr. Vector said...

You seem to have extraordinary good fortune in finding all kinds of weird stuff. Have you gone looking for scat? Fox scat with beetle parts in it would be pretty conclusive.

10:44 PM  
Blogger Darren Naish said...

Response to Carel: I regret I know little of Lucanus sexual behaviour.. hey, it's not a tetrapod after all. Given use of pheromones, it's likely that multiple males would be attracted to a female, but this is a possibility I couldn't test.

Response to Matt: I searched the garden for scat (hoping to find fox dung) and all I found was Wood pigeon Columba palumba poop on the lawn. Four nestling passerines, with their remiges still in pin, were dead at the bottom end of the garden. But they didn't seem to have been chewed up and still had intact legs, feet and heads, so they likely weren't killed by a carnivoran.

11:44 PM  
Blogger Mike Taylor said...

"All I found was Wood pigeon Columba palumba poop on the lawn" would make a good title for a Modern Novel.

10:35 PM  
Blogger Webs said...

I'm betting on the housecat.

Can you get cat/fox jaws to match to bite patterns?

4:54 AM  
Blogger Matt Mullenix said...

What about a shrew?

5:00 AM  
Blogger Darren Naish said...

No shrews at all in the area (at leats according to decades of surveys), not one.

9:12 PM  
Blogger Darren Naish said...

I should menton I've spend all day in the pub.

9:13 PM  
Blogger Matt Mullenix said...

Good on yer, mate!

1:44 PM  
Anonymous Emma White said...

Having read your post re the stag beetles, I have this morning had a similar thing - we have discovered 10 stag beetle heads on our lawn, some of which are still moving and alive - no bodies to be seen. We have no idea what has done this or if it's some sort of very strange mating behaviour (seems a bit extreme!) Anyone got any ideas? It's quite disturbing to find something like this in your garden. They were all flying around last night.

10:03 AM  
Blogger nickp said...

Hi there we have lots of stag beetles in our garden also in the suburbs of surrey we have a large tree stump in the back garden and they live-in there. I have found many stag beetle heads on the ground and have been wondering what has been eating them i thought birds but haven’t seen any trying to eat them thay are more interested in our grass seed. We do however have bats flying around the garden that we have recently found live in the tile hung front of our house. I am beginning to suspect that the bats are eating the stag beetles the parts we have found do look munched rather than pecked ? i think a bat could easily take down a stag beetle in flight.

10:39 AM  
Anonymous Bob Armitage said...

Live in SW France near Pyrenees and on this am's walk have just seen the remains of several stag beetles mutilated in exactly the way you describe. Googled to try to satisfy curiosity as to cause and up your link came.
The location of the remains was in wooded countryside / farmland which may serve to exclude the domestic cat as the culprit.

10:51 AM  
Anonymous Anonymous said...

I found around 10 half eaten stag beatles in my garden in berkshire over the past two weeks. Its very disturbing as they seem to die a very slow death. Any further insights into this "tragedy"?

10:07 AM  

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