Sometimes, it is a bit confusing when people use human terms – anatomical but also vernacular – for animals. “dorsally” comes to mind, when people use it interchangeably with “cranially”. In humans it’s the same direction, in most animals it’s a 90° difference, which is why it is important to be utterly anal about using terms accurately. “dorsally” means “towards the backbone”, and is the opposite of “ventrally” (“to the belly”). “Cranially” comes from cranium, and thus is the direction the skull is in.
Equally confusing is much of the terminology in the biomech of locomotion. Let’s see what you’d say the bird in the picture below is doing: standing? sitting? something else?
Bush Thick-knee (Burhinus grallarius)
in the Stuttgart zoo ‘Wilhelma‘.
Well, what is it?
The spontaneous reaction of most people is to say “why, it’s sitting down!” Whereas in the next picture, it is “standing”.
Is this correct?
Depends on what you really mean. Does “sitting down” here refer to “resting a bit”, as it often does in humans? When we “sit down”, it doesn’t really matter how much we fold our legs, what exact posture we adopt. Rather, the word refers to the act of adopting a resting posture that does not involve a horizontal trunk position (which would be “lying down”). On the other hand, as a biomech guy, I look at both pictures and see the animal standing! Plantigrade in the first picture, digitigrade in the second, which is obviously the more commonly used stance.
Plantigrade means that the entire lower surface of the foot, tarsals, metatarsals and toes, contacts the substrate. That’s what humans normally do when we walk. When we run, however, things can change. Most people jog using their complete foot, but some keep the heel off the ground – and this automatically keeps the posterior (proximal) end of the metatarsus off the ground as well. Sprinting as fast as possible is always done digitigrade – on the toes. And if only the last phalanx of the toes is used, it’s called unguligrade.
So the bird at the top has its looooooong metatarsus on the ground (in fact it’s a tarsometatarsus, as it consists of fused tarsals and metatarsals), and is thus standing in a plantigrade position, which just happens to be a sort-of-resting pose for it. The second photo shows it in its normal standing pose, from which it can start walking or running without delay -an alert, digitigrade standing pose.
What’s causing this confusion?
Mostly, the large difference in leg proportions between humans and many other animals. Cursorial animals (those adapted to rapid and/or long-distance running) typically have long feet, whereas humans still have the short feet of our arboreal ancestors, despite our excellent endurance running abilities. If one now copies our proportions to an animal like an ostrich, one is always tempted to call the toes “foot”, the metatarsus “shank”, and the shank “thigh” – and end up wondering why the knee is “backwards”! The fact that birds hide their thighs so well doesn’t help. In fact, all is normal: the ankle points back, the knee forward.
For many dinosaurs, there has been and in some cases still is extensive debate on how digitigrade or even unguligrade they were. Plateosaurus comes to mind….. but I’ll stop here, before derailing another theropod post into one on my pet basal sauropodomorph 😉