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.