|Dear UCSB ScienceLine: Despite that cats are four-legged and walk on their toes, shouldn't the fact that cats retain a large foot indicate descent from a bipedal (upright) ancestor? As the elongated foot is a hallmark of bipedalism, my understanding of evolution tells me that kitty would never have evolved an elongated foot if her ancestors had never walked upright. Please help!|
|Question Date: 2020-07-30|
Thanks for your interesting questions about cat posture and foot evolution. Cats have developed a foot posture in which only the toes touch the ground--this is called a digitigrade stance. It's illustrated in the middle figure below. (Dogs walk in this fashion too.)
The feet of humans are arranged as shown in the figure on the left--the so-called plantigrade stance--where the toes (digits)(shown in pink), plus the metatarsals (yellow), plus the ankle bones (blue) contact the ground during walking. In cats and dogs, the ankles are held above the ground, and the metatarsals are held vertically, and are elongated (this makes the hindlimbs longer, and facilitates faster running).
So, I'm not sure if cat feet are truly big--compared to what? (And how, exactly, is "foot" defined--the portion of the hind limb that strikes the ground--or the tip of the longest toe to the ankle?) An elongate foot (tip of the toe through the ankle) is a "hallmark" of digitigrade and unguligrade (right figure above, in which animals walk on the tips of their digits, on hooves) mammals, rather than of bipedal ones.
I'm also not sure whether the feet of bipedal primates (humans), are truly bigger (scaled to the size of the animal) than those of quadrupedal primates, e.g., lemurs, chimps, New World monkeys, gorillas, etc. Perhaps research has been done on this, but I'm uncertain.
In short, bipedalism is quite rare among mammals. To my knowledge, a bipedal ancestry of cats has never been proposed. It also seems highly unlikely. Still, you've given yourself plenty of food for thought. Digitigrady has evolved multiple times among mammals (and other terrestrial back-boned animals). It's often ascribed to cursoriality (running) and a predatory lifestyles. I encourage you to think about these interesting questions more.
Interesting question. As with many good questions, there are a few parts to look at.
If you look at most mammals, they are either up on their fingernails/toenails (hooved animals) or on their "fingers/toes" (almost everyone else). Those of us who walk on what we humans consider the soles of our feet are pretty limited (some great apes, bears, and such). Take a look at the toenail and toe walkers and they seem to have a knee that bends backwards. This is actually their ankle. They have a knee, it's just higher up, next to their bodies. I think you recognized this because you are talking about the elongated feet of cats (which are toe-walkers). If you want some fancy vocabulary, we're plantigrade (plantar = sole of foot), cats are digitigrade (digits = fingers and toes), and horses are unguligrade (from the root that means hoof).
One part of your question is whether elongated feet are an adaptation for bidepalism. Relative to body size, we bipedal humans have rather short feet compared to cats. My cat's "foot" (measuring from the end of her heel bone to the tips of her toes) is about 12.5 cm (about 5 inches). My foot is about 28 cm (11 inches) long. But my body length is many times her body length.
So what's up with the elongated feet of cats and so many other animals? One answer is speed. Muscles can only contract so fast. If only one joint is moving, it is limited by that speed. Two ways to increase speed are to lengthen the limb and increase the number of joints involved in the movement. Think about a fly swatter. It works a lot better than swatting with your hand because it moves faster. The trade-off is less power. So we see long limbs on running and jumping animals and short ones on diggers that need a lot of force, but not a lot of speed. Getting up on fingernails/toenails adds to the number of joints that can be involved in the motion to add to the speed.
Does that mean that elongated feet can't be a bipedal adaptation? No. If you look at ancestral primates, they walked on the soles of feet that were more like hands--good for gripping branches. But more upright walking evolved along with longer feet like we humans have today. Bipedalism is one thing that makes elongated feet beneficial, but not the only thing.
Another part of your question is whether cats had a bipedal ancestor. When we're trying to figure out whether a trait is a leftover from ancestors (ancestral) or a more recent adaptation (derived), we look at the fossil record. The earliest mammals walked on the soles of their feet, but were definitely not bipedal. Here's a sketch of the oldest mammal found in South America so far:
oldest mammal .
The ancestors of mammals, cynodonts also walked on the soles of their feet, but were quadrapedal. So walking on the sole of the foot, is the ancestral trait for all mammals. The branch of mammals that cats belong to (Order carnivora). The earliest ones were rather weasel-like. Definitely not bipedal. Here's some more information on this group: mammal carnivora.
Don't be discouraged though. You asked a question that shows good analytical thanking. Some areas of study that might interest you are morphology (why organisms and their parts have certain shapes) or paleontology (the evolutionary history of life).
Here's a question for you. Why do you think horses run on only one toenail/fingernail per limb instead of 4 or 5?
Thanks for asking,
Bipedalism has independently arises in many animal lineages. This means that bipedalism is not a trait that passed down from a common ancestor, but instead a trait through convergent evolution.
There are far more mammals with elongated hind feet than bipeds. Ungulates (pigs, hippo, dear, horses, zebras, tapirs, and rhinoceros), carnivores (cats, wolfs, bears, and raccoons), rodents, rabbits, and primates all have elongated hind feet. Interestingly, those animals belong to the clade Boreoeutheria and descent from a common ancestor. Therefore elongated hind feet is a trait pass down by the common ancestor yet bipedalism is not.
You sound like a cat lover, but you need to work on your understanding of evolution.
My foot is 9 cm wide and 23 cm long:
Do you think cats are more related to 4-legged animals or to humans?
An elongated foot is not an indicator of bipedalism in mammals. It is an indicator of bipedalism in primates, but this is because primates are normally climbing animals that live in trees, and the ancestors of apes had opposable toes, more like hands. However, mammals other than primates, who never lived in trees, often walk on their toes with long feet, and there is no evidence that the ancestors of mammals were bipedal (in fact there is a lot of evidence that they were quadrupedal).
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