Understanding the different methods of locomotion in mammals relies on an understanding of the evolution of mammals so I’m starting this post with a very brief overview of that.
The lineage that lead to mammals diverged from reptiles about 325 million years ago. This evolutionary line is referred to as synapsids. It’s thought that the first true mammals arose some 250 million years ago and split into marsupials and placentals around 160 million years ago.
66 million years ago Earth was struck by an asteroid that wiped out not only the majority of dinosaurs but also many species of mammals. It‘s thought that larger mammals and mammals with a specialised diet went extinct whereas smaller mammals and mammals with a more diverse diet survived. Most marsupial species went extinct at this time.
It didn’t take long for those surviving mammal species to get bigger and diversify, probably in only a few hundred thousand years.
Today there are an estimated 6,000 + species of mammals.
Because mammals have evolved from the same line, they have more or less the same skeleton. That might not be immediately obvious, but essentially the bone structure is the same in all mammal species, it’s just that some of those bones are longer or shorter in some species, thicker or thinner, but they’re in the same basic order.
This is useful for us to know when it comes to tracking or trailing wildlife, especially in regards to the footprints they sometimes leave behind.
There are 3 means of locomotion within mammals, plantigrade, digitigrade and unguligrade. Knowing which is which can help us to identify which animal has made a particular track.
The earliest mammals were plantigrades. In common with their reptilian ancestors they had 5 toes on each of their four feet and walked on the entire sole of their feet. In technical terms, plantigrade locomotion means walking with the digitals (toes), metatarsals (the bones in between the heel and toes) and tarsals (heel) flat on the ground.
Often (not always, take humans for instance) plantigrades have short legs and move around at a relatively leisurely pace.
Examples of plantigrades found in the British Isles include:
This type of locomotion isn’t particularly efficient as there’s a lot of foot in contact with the ground. Over time, as predators became faster, and consequently so did prey species, the amount of foot in contatc with the ground changed.
The first change was the arrival of digitigrades. The name itself comes from Latin where digit means ‘finger’ and gradior means ‘walk’. A digitigrade is an animal that walks on its toes so that the heel isn’t touching the ground.
Digitigrades are often animal species that travel at a run or who can jump. As well as walking on their toes, digitgrades have evolved extended toe bones and long slender limbs. They’ve also evolved to have less toes, typically the equivalent of the thumb has disapeared completely or has changed it’s position (think of the dew claw on a dog). This allows the animal to move faster.
Examples of digitigrades found in the British Isles include:
- Walking birds
Unguligrades (or ungulates) took the concept further. They are animals that walk on the very tips of their toes and include all hooved animals. They’re often big and typically herbivores.
Some species walk on the tips of two toes (even-toed ungulates), such as:
Sometimes they walk on one toe (odd-toed ungulates), such as
Technically speaking, cetaceans are ungulates but are generally excluded as they don’t have many of the characteristics shared by other ungulates.
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