Sunday, May 07, 2006

When animals die in trees










I like living animals as much as the next person, but I also can’t help but find dead ones just as interesting. Corpses, decomposition, burial… all of it I find fascinating. Most palaeontologists do, and in fact there are whole books written about what happens to animals after they die. This area of study is termed taphonomy, and part of my palaeontological training has involved taphonomic interpretation of fossils as well as actualistic work on modern bodies and bones. I’ll be posting about some of this actualistic work in the near future.

Many tetrapods that climb or rest on, in, or under branches, cave roofs and so on have evolved structural adaptations that allow their digits to grip automatically, and with only a minimal expenditure of energy. If we humans want to grasp firmly to a branch overhead, we have to use a lot of muscular effort to keep our digits flexed. But bats, passerines and members of other groups have evolved tendon locking mechanisms. If I start talking about these adaptations, I’ll easily add another 1000 words to this blog, so I’m going to avoid doing that and will direct you to the literature instead. For the tendon locking mechanisms of bat feet see Schutt (1993) and Quinn & Baumel (1993), for bats as well as dermopterans see Simmons & Quinn (1994), for rodents see Haffner (1996) and for birds see Quinn & Baumel (1990) [thanks to Matt Wedel for help in getting hold of these].

But here’s a thought. If your digits automatically secure a purchase to a branch, or the roof of a cave, then what happens when you die? Answer: your corpse stays there, dangling for all eternity.

Some years ago I spent time collecting literature on accidental deaths. A Cape buffalo collides with a tree and dies of a broken neck, a giraffe gets hit by lightning, a sparrowhawk gets stuck in foliage while pursuing a passerine... that sort of thing. Numerous accounts describe death by choking (particularly among snakes, predatory lizards, aquatic birds and raptors). Poisoning is not uncommonly reported (usually where inexperienced young herbivores eat toxic plants). But what of deaths in trees? To start with, there are the animals that die after getting their necks caught in forked branches, as has been recorded for both giraffes and deer. Honestly. Then there are animals that get tangled up in thorny or spiky vegetation. This is particularly well reported for bats, with Hill & Smith (1984) discussing cases of bats dying after being entangled in burdock burs, after getting impaled on the spines of cacti, desert shrubs and rose thorns, and after getting strangled by Spanish moss fibres.

But I haven’t just been collecting reports of strange deaths, I’ve been collecting corpses too, and lots of them. A fox that died on a bed of straw in a stable. Pressed toads. Road-killed deer. Finches, wrens and bats that were killed by cats. A shrew that died on a railway platform. Moles, voles, newts, frogs, snakes, pigeons, thrushes.

Two of my favourite corpses died in trees (or, at least, big shrubs). The first is a European robin Erithacus rubecula. Already highly decomposed, and consisting only of a feathered skeleton with some ligaments, it was discovered within a 2 m high privet hedge, and within the hedge the bird was about 1.2 m off the ground. Not only was its little corpse still stuck in the hedge where it died, one of its feet was still curled tightly around a branch, and in fact the bird was hanging upside down by this one foot. My interpretation is that the bird died while sleeping in the hedge, and that after death its foot remained clenched around the branch. I carefully removed the bird, and the branch it was attached to, and I still own them today (see photo above left). As you can see, the corpse still hangs from the branch by a single foot.

Given that so many bird species hide, nest and roost in trees, we should expect them to die in trees quite often. As discussed in one of my previous blogs (Little birds in crevices), small passerines even secrete themselves deep into tangled vegetation and into cracks and fissures in bark. If they then die in their sleep, they’re just going to stay stuck there. While I’ve checked and collected quite a lot of literature on mortality in birds and other tetrapods, I have yet to read of any other instances of birds being found dead in trees. Surely other instances of this have been reported. Anyone? Squirrels are often found dead in their dreys (Gurnell 1987).

My second favourite corpse is a Grey squirrel Sciurus carolinensis, and it also died in a tree. But this case is a little odd, as the corpse was discovered wedged in thin branches about 3 m off the ground. Stuck up there, it began to decompose. Its skin ruptured. Its guts fell part-way out of its abdomen. But the soft tissues didn’t all rot away: they became wind-dried, and the corpse (with the hanging guts) became mummified (see photo above right). Today, the corpse has a large U-shaped concavity on its ventral surface, caused by a supporting branch that was under the body while the rest of it sagged down toward the ground. Ordinarily, any squirrel that dies in a tree falls to the ground. Gurnell (1987) reported on a 1985 mortality event in England where squirrels were mysterious dropping dead, and ‘one was actually seen dying and falling off a branch’ (p. 135).

So, thanks to its tendon locking mechanisms, the dead robin still clings to its perch. Tendon locking mechanisms also mean that bats that die while hibernating also stay fixed to their point of purchase. As Schweitzer et al. (2003) wrote: ‘They may hang all night long, during hibernation or still after they have died’ (p. 70). In fact, even when the dead bat’s body has rotted and the bones have fallen to the cave floor, the feet can remain secured to the roof. Moving now to cases that don’t involve tendon locks, I’ve heard of sloth carcasses that have been seen lodged up in trees, and Knott (1998) reported a case where an orangutan died slumped on a horizontal log. Frank Buckland owned a mummified marmoset that had been discovered within the hollow branch of a tree (he took it with him when he went to have dinner with Bishop Wilberforce one day), but I’m not sure if the marmoset actually died in the branch or was placed there.

I wonder if I should publish this stuff.

Coming soon... Mystery birds of the Falkland Islands. For the latest news on Tetrapod Zoology do go here.

Refs - -

Gurnell, J. 1987. The Natural History of Squirrels. Christopher Helm (London).

Haffner, M. 1996. A tendon locking mechanisms in two climbing rodents, Muscardinus avellanarius and Micromys minutus (Mammalia, Rodentia). Journal of Morphology 229, 219-227.

Hill, J. E. & Smith, J. D. 1984. Bats: a Natural History. British Museum (Natural History) (London).

Knott, C. 1998. Orangutans in the wild. National Geographic 194 (2), 30-57.

Quinn, T. H. & Baumel, J. J. 1990. The digital tendon locking mechanism of the avian foot (Aves). Zoomorphology 109, 281-293.

Quinn, T. H. & Baumel, J. J. 1993. Chiropteran tendon locking mechanism. Journal of Morphology 216, 197-208.

Schutt, W. A. 1993. Digital morphology in the Chiroptera: the passive digital lock. Acta Anatomica 148, 219-227.

Schweitzer, A., Frank, O., Ochsner, P. E. & Jacob, H. A. C. 2003. Friction between human finger flexor tendons and pulleys at high loads. Journal of Biomechanics 36, 63-71.

Simmons, N. B. & Quinn, T. H. 1994. Evolution of the digital tendon locking mechanism in bats and dermopterans: a phylogenetic perspective. Journal of Mammalian Evolution 2, 231-254.

11 Comments:

Blogger Karen McL said...

Well, at least more *interesting* than *sail animal* corpses. (You know...the flattened remains ya can pick up and *sail* like a frisbee into the air!)

Gross... I know, but I couldn't resist as a follow up to your piece.

;-D

9:07 PM  
Blogger Gary S. Hurd said...

I have a large collection of carcasses left in trees by young hawks. They are mostly ground squirrels and Audobon's cottontail rabbits, but also a gopher snake and a few voles. I also collected the cast pellets from below the roosting tree. I got a couple of abstracts out of it so far.

A neighbor recently had me rescue a oppossum out of a tree. It had gotten forked between two branches. I would have observed it, but she asked very nicely and was determined to "DO Something."

Best of luck with you collection.

1:23 AM  
Blogger TheAlphaWolf said...

I find that squirrel picture very puzzling. what in the world is that thing between its legs? Sorry, had to ask.

2:06 AM  
Blogger Webs said...

My favourite course of my short VP career was Andrew Hill's class in taphonomy.

It's so important, and a shame that it only developed as a paleontological science so relatively recently.

3:08 AM  
Blogger Mark said...

Dying in trees is common because of their environment...

So I've got to ask, is there fossilized evidence of critters that died in a tree or something? Perhaps a prehistoric animal in a hole in a tree?

7:47 AM  
Blogger Darren Naish said...

Thanks to everyone for their comments, especially Karen on the use of corpses as frisbees. Excellent.

Response to Gary: thanks for the info, very cool. While writing the blog I toyed with the idea of introducing animals that hadn't necessarily died in trees, but rather had been cached there by predators. Think impala and deer put into trees by leopards; passerines, rodents and lizards impaled on thorns by shrikes and butcherbirds; and the bits of rabbits and so on that litter the nests of some large raptors. In the end I decided not to cover this area though.

You've published abstracts on this subject? Is there any chance I could see them?

Thanks again, all the best.

10:21 AM  
Blogger Darren Naish said...

Oh yeah.. in response to Alphawolf (err, nice name) the thing between its legs is (honest) its lower intestine. As mentioned in the blog, these dangled from the corpse during decomposition but then got wind-dried.

And this is the sort of question I'd have expected from Matt.

4:21 PM  
Blogger Gary S. Hurd said...

Here are some titles from my CV, but I have not found the file with abstracts prior to 2001 (which is a bit disturbing).

1996 "Raptor Prey Bone Accumulations from a Nest Area." M. S. Pyatt and G. S. Hurd. Southern California Academy of Science, Annual meeting, May.

* In this one we focused on the differences in MNI, and species from the carcass and the cast pellets. More individuals and more species were identified from carcuss than pellet data, however it is possible we did not find all the pellets. None the less, most habitat and forage data are from pellets and may be seriously under estimated.

1996 "Deer Bone Accumulations from Mountain Lion Kills." R. Travis, G. S. Hurd. Southern California Academy of Science, Annual meeting, May.

* Parts and marks.

1998 "Carnivore Modification of Deer Bone" Steve McCormick, G. S. Hurd. Society for Californian Archaeology.

* Mostly about secondary scavengers, particularly coyotes.

1998 "Bone Modification and Deposition by Raptors" Mike Pyatt, Melissa Pryor, Gary Hurd. Society for California Archaeology.

* In this one we focused on perferential consumption, sequence of consumption, plus marks such as bone fracturs punctures etc...

1998 "Rockshelter Deposition of Insect Remains By Fox and Mouse" Matt Ritter, Gary Hurd. Society for California Archaeology.

* Channel Island fox eat a great number of crickets. Their scat are collected and stored by mice.

1998 "Fish Bone Deposition by Coyote" Karl Allwerdt, Gary S. Hurd. Society for California Archaeology, March.

1998 "Primary and Secondary Predation Patterns of Avian Bone," Ken Riddell, G. S. Hurd, Society for California Archaeology.

* bobcats killing and eating ducks, scavenging by coyote and racoon.

1998 "Arthropod Succession on a Small Mammal Carcass" Southern California Academy of Science Annual Meeting. Poster [G. Hurd, Maria W. Garrity, Steven Mc Cormick]

* This is not too relevent to your topic, but we took some great maggot photos. Truly grotesque!


All and all, these papers are useless without the photos, and they were all "pre-digital" so I will need one day to scan them all. I wish I still had students wanting "extra credit" assignments. Some of the lion killed deer are shown at Faunal Taphonomy.

5:43 PM  
Blogger Caio de Gaia said...

Darren, your two last posts do not allow for comments. Was it on purpose?

I wanted to ask after your post on anguids, if you know any good method for looking for amphisbaenids, since I'm planning to take some pictures of local herps during my summer vacations (south Portugal). The snakes, lizards, geckos and chamelons, are relatively easy to find, but I've seen amphisbaenids very rarely (one every two or three years only).

3:29 AM  
Blogger Darren Naish said...

Response to Caio de Gaia's last post.. sorry, somehow the 'do not show comments' box got clicked (though I don't recall doing it). I've fixed it, so you can now comment on slow-worms and the avifauna of the Falklands as much as you like.

Amphisbaenians: I am going to blog on them one day actually (see VERY FIRST blog post). I have no field experience at all with amphisbaenians, but the two European species are reported to be most easily found under stones or rock piles in areas where there are sandy soils, little grass or herbage, trees providing shade, and some leaf litter. Apparently, stone walls are a good place to look, but I imagine that you'd have to dismantle the wall to find any hiding animals. See Martin et al. (1991) for a detailed look at habitat choice.

European amphisbaenians are also active on the surface sometimes, even on hot, sunny days, so it is at least possible to just stumble across them. See Busack (1978) where the discovery of surface-active amphisbaenians in Spain was reported.

Refs - -

Busack, S. D. 1978. Diurnal surface activity in the amphisbaenian, Blanus cinereus (Vandelli) 1797 (Reptilia, Lacertilia, Amphisbaenidae). Journal of Herpetology 12, 428.

Martin, J., Lopez, P. & Salvador, A. 1991. Microhabitat selection of the amphisbaenian Blanus cinereus. Copeia 1991, 1142-1146.

11:43 AM  
Anonymous Randy said...

Something that I mentioned to Darren in an email:

On a recent fieldtrip, I visited a man-made reservoir. At the dock/pier/boat-ramp, there was a colony of cliff swallows nesting beneath the pier. There were hundreds of individuals including quite a few nests. Interestingly, beyond the usual swallow nesting material, the nests also included a variety of man-made items. One common item was fishing line, which often dangled freely out of the nests. This had the tragic consequence that it was a hazard to the flying swallows. As you can guess, there were several examples of a swallow that had caught in the line and were suspended dead in mid air by the fishing line that they had incorporated into their nests.

Taphonomy rocks.

5:43 AM  

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