Think of a region of general mammalian anatomy. You’re probably not thinking of the tail. We’re mammals and yet have reduced ours to a puny coccyx embedded in muscle and fat. It’s an alien organ to us. Let’s face it, the tail gets the short shrift when it comes to morphological, functional and evolutionary studies in tetrapod vertebrates. There are notable exceptions such as in studies of prehensile tails or the role of the tail in cetacean locomotion, but broadly we know far less about the caudal vertebrae of mammals than we do about heads or limbs or some other bits. It is timely to coddle caudals: to talk about tails, not turn tail and run rostrally. This post wags its tail affectionately at the topic.
No blog post on tails is complete without a photo of the business end of Euoplocephalus’s (Ankylosauria) caudals.
Thagomizer of Stegosaurus. But of course!
Stomach-Churning Rating: well, there are some rancid emu butts, so I’m giving a 7/10; but otherwise mostly just line drawings.
My team has done a bit of work trying to better appreciate the tail as part of our expanded emphasis on the evolutionary morphology and biomechanics of the vertebral column; not just limbs as I was used to. Former PhD student Michael Pittman did his whole thesis on the evolution of tail form and function in dinosaurs, publishing a nice PLOS paper on theropod tail evolution, with other cool stuff in the pipeline. Pittman visited our lab recently with collaborator Heinrich Mallison to gather new data on the tails of archosaurs, focusing on Nile crocodiles and emus as an initial phylogenetic bracket. In my team’s prior work, we’d dabbled with theropod tails, focusing on the caudofemoralis muscle of theropod dinosaurs like T. rex, and on the size of the tail and its effects on the body’s centre of mass, following up on super-duper classic work by my mentor Steve Gatesy. So we have some caudal street cred, but we don’t have the tiger by the tail. (Hang in there, I’m going to milk the tail puns here!)
Don’t be afraid to touch the tail pics- they lead to bigger versions.
Mallison and Pittman know how to coddle caudals!
“Emu butt”- the tail is hidden in the smelly bulb of fat on the left side.
Bending an emu tail to measure its mobility.
Emu tail bones: our collection
I was inspired to write this post because of Michael’s visit, which gave me the opportunity to shoot some deliriously disgusting images of “emu butts” during the dissections and CT scans, but also got me thinking more about tails. And as usual, I poked around the literature looking for tall tales of tails.
I ran across one of those great review papers that is fodder for a hundred or more research projects: “The mammalian tail: a review of functions” (1979) by Graham C. Hickman (Mammal Review 9(4): 143-157. The rest of this post reviews his review.
Hickman, like I do here, starts off by reminding us of the tail’s neglect in science; e.g. “modifications of caudal vertebrae such as lengthened zygapophyses and neural spines are not as striking as the flexibility shown in the changing length and fusing of limb bones.” True that, but Hickman adds the great turn of phrase “A rodent chewing off its leg to escape a trap seems much more of an extreme than chewing off the tail, though it has four legs and but one tail.” Then he runs through a general overview of the diversity of tail forms and functions in mammals, with plenty of citations of older literature (there’s bound to be much to find in the tailings from the goldmine of 1800s German morphology papers, too).
What would a giant anteater look like without its tail? Odd indeed.
Mammalian tails range from four caudals in us freakish humans (does no mammal naturally have fewer, or have truly lost the tail? I wonder if anything has been missed) to fifty in pangolins (huzzah!). Breeds of dogs seem not to vary as much in terms of tail bone count as I’d expect: 20-23. But Hickman’s mention of Thorington’s (1970) study showing that mouse embryos raised at higher temperatures develop longer tails grabbed me… and reminded me of groundbreaking work that RVC PhD student Andrea Pollard is doing with temperature effects on bird and crocodile limbs (stay tuned).
Figure 2 in Hickman (1979) was what grabbed me most, depicting tail disparity in mammals. It’s a figure that gets your tail thumping. Check it out:
Anatomical disparity of mammalian tails! A, Black Rat; B, 9-banded Armadillo; C, Grey Squirrel; D, Horse; E, Fallow Deer; F, Wooly Spider Monkey; G, Coatimundi; H, Beaver; I, Bottle-nosed Whale; J, Manatee; K, Flying Squirrel; L, Fat-tailed Gerbil; M, Scaly-tailed Squirrel, N, Plains Pocket Gopher; O, Porcupine; P, Grey Kangaroo; Q, Naked Sand-rat; R, Big Brown Bat; S, Merriam’s Kangaroo Rat; T, anonymous Glyptodont; U, Ceylon Shrew.
Boom!
Hickman continues on to consider tail functions and behaviours, commenting that most bipedal mammals have long tails whereas humans buck the trend. Pangolins and anteaters get due mention here, but I really liked the factoid that “Beavers occasionally walk bipedally with an armload of mud” (p.145).
Mammals, like other vertebrates, that have substantial tails tend to use them for locomotor support at least when moving slowly, and Hickman lists kangaroos+kin, anteaters, pangolins and beavers as examples of mammals that thus use their tails as “fifth limbs”. But there are stranger tail functions in mammals than this ancestral tail-prop role. The bat Nycteris has a singular tail that ends in a “T”, bracing the uropatagium (tail-leg membrane).
Lovely kangaroo sculpt/skeleton from the incomparable comparative anatomy museum in Paris.
However, some mammals also don’t use their tails the way we might expect- the platypus (Ornithorhynchus) doesn’t power its swimming with its tail so much as it uses it for stabilization, according to Hickman; paddling with the limbs seems more important (but this could use some modern scientific study using proper hydrodynamic testing). Yet they do use their tails to tamp the earth of their burrows and, curling them up to their belly, to bring in vegetation and such to provision their nests, as well as using their tails as energy stores (like many animals do). In contrast, beavers don’t transport much with their flat tails, whereas the more prehensile tails of pangolins may be used for carrying their babies.
Hickman notes how few mammals use their tails as weapons to harm others, although he properly brings up glyptodonts as a counter-example. And then comes the striking description of how, by a “grinding motion of the tail against the body” a pangolin “almost severed the fore paw of a dog.” (p.148) And then, other mammals do the opposite of tail weaponry: Hickman cites that some 15 species of rodents can shed their tails (autotomy) as a defense, and like salamanders or lizards, regenerate them. Autophagy (self tail-cannibalism), however, Hickman rightly infers is a pathological, desperate condition, not a normal adaptation in mammals.
Big Glyptodon tail club!
More glyptodont tail clubs! Neosclerocalyptus
Giant armadillo Priodontes, showing glyptodont-lite version of the tail.
Need to motivate a rat to solve a maze puzzle or eat food? Pinching the tail had been shown to help, Hickman explains. This fits with the more obvious role of the tail in mammalian communication, including scent-marking. Here, Hickman notes that rather than using scent glands, hippos take the feces way out and just whip their tails around while pooping to spread their perfume. Which the internet knows well…
And then, finally, Hickman gets to the Rat Kings, which had me incredulous at first… but there are a bunch of references, so… What’s a Rat King? A “ball” of rats (from 3 to 32 of them!) with their tails tangled together for “group cohesion”, fabled in European stories for centuries but possibly “a frequent phenomenon” (p.152). An explanation for this phenomenon, Hickman explains, is confinement of rat in enclosed spaces where their tails do get entangled, only to be “found during a cold part of year, usually as a result of loud squealing noises which drew attention to the hide-away.”(p.153) In surveying the amusing range of explanations through history for Rat Kings (“itchy tails”?), Hickman relents and concludes “perhaps the tails of Rat Kings function best as cocktail discussion.” I concur—and append blogging discussion to that!
Tails you win, pre-caudals you lose, but Hickman’s review article is full of win! There’s plenty more of interest in there. I hope you enjoyed the look back at this classic paper, and at the tales that tails tell. This is the tale end.
I’ll let Ray get your caudals shakin’ as we depart:
What's In John's Freezer? 
Ah, the rat king. I’d always been inclined to view it as an urban legend myself, but there is one in the Otago Museum: https://www.travelblog.org/Photos/840905
Ah, rat kings – a great staple of the fortean literature. There are also squirrel kings and even bird kings on record. Short tails: guinea-pigs sometimes have as few as four coccygeal vertebrae.
[…] you win. Diversity in mammalian posterior appendages. Cool overview, by John […]
Thanks for the post and link to a great article – really interesting!
Re: short tails, it’s probably just fusion differences, but I have seen a report for the normal number of coccygeal vertebrae for Barbary macaques being 2 – 4, and for some gibbons 0 – 6!
Apparently although Barbary macaques appear to have remnant external ‘tails’ (albeit tiny ones), the coccyx of some individuals simply runs past this structure, which I guess means it wouldn’t count as a tail at all.
Not a bad bit of diversity for a group that seems to have largely eschewed tails from their bauplan. That there are mammals that have caudal autotomy is just cool. Does anyone know if they have fracture planes in their caudals like some lizards? I’d love to see a follow-up post on tail diversity in diapsids, though I suspect that would be a much larger undertaking (as far as I can tell no one has done a survey of tail morphology in diapsids, or even just lepidosaurs)
“Not bad diversity…” point- yes, definitely what I was thinking, too, when writing this. I don’t know if there are autotomy fracture planes; was wondering that too. Yeah, tails in diapsids would be a monumental task to undertake! Bird/archosaur tails alone are tough!
Kangaroos have two, radically different, ways of using their tails in locomotion. At high speeds (when they are moving by bipedal hopping), the tail has a balancing function. At low speeds… (It’s been a few years since I’ve seen this, so I may be misremembering some details.) Imagine a kangaroo proceeding quadrupedally, nose down, looking for edible snacks in the grass. (They do this.) Because the hind legs are so much bigger than the forelegs, a normal gait is impossible. So. Step 1: lean forward, advancing the forelegs in much the way abnormal quadruped advances them while walking. Step 2: with body more or less stationary, support body on forelegs AND TAIL (not, of course, the tip of the tail: the tail contacts the ground maybe a third (?) of its length from the base, so at a point where it is still thick enough tone weight-supporting), and (one at a time, I think, but maybe both at once: as I said, it’s been some years since I watched one) lift hind legs from ground and advance them.
It’s a very clumsy looking way of walking. I’ve read somewhere that (due perhaps mainly to using the elastic tendons of the hind legs to store energy when jumping) high-speed kangaroo locomotion is more energy efficient than high-speed antelope locomotion. So maybe, seeing as how the “design” is optimized for high speeds, it’s not surprising that they’re not as good at low speeds!
Hi Allen, that’s right- the tail of many kangaroos/macropodids acts as a “fifth limb” during slow locomotion. Its role in faster hopping is less clear but probably is balancing. Good recent story + paper on the “pentapedal gait” of slow kangaroos: http://news.sciencemag.org/biology/2014/07/amazing-five-legged-kangaroo
I had a laughing fit a rancid Emu Butts and lost track of what brought me to your blog. Great post by the way, fond memories of the rat king from one of my undergrad bio classes.
Welcome, and thanks, and stick around for more bad jokes! 🙂