Well, it is and it’s not. The basics are similar – all carbon-based life forms on this planet have a body with some kind of structural support and some sort of internal goo that performs various life functions. Consider insects, who look nothing like us: they have an exoskeleton to provide structural support (albeit on the outside of their body) and internal organs, blood, and rudimentary systems for nerve conduction and sensory transmission. They breathe, eat, locomote, and defecate – much like we do. They build houses for themselves (i.e., spider webs) like we do, but we are slightly better at that than they are (though I defy you to find a house that is more amazing than a spider web).
How are our skeletal and circulatory systems similar to or different from those of the animals? Well, let’s jump out of the realm of invertebrates (not the realm of vets anyway) and start talking vertebrates. All vertebrates have a skull, a spinal cord, ribs (usually fewer than 20, but snakes have a ton of ribs and their x-rays are hilarious), and some sort of appendages (i.e., legs and arms, but they aren’t called that in quadripeds). Interestingly, every vertebrate has 7 cervical (neck) vertebrae – even giraffes! Imagine that long, long neck with just 7 bones supporting it on the inside…if you ever get a chance to see a giraffe vertebra or model of one in person, you should. Coolest thing ever.
Another cool aspect of comparative anatomy is that there are really only 3 foot structures across all species that have feet: plantigrade, digitigrade, and unguligrade (which is the best word EVER). Each structure has advantages and disadvantages in terms of speed and stability. Generally the more you walk towards the end of your toes, the faster you run, but the more trouble you have with injuries. This is why leg and foot injuries in horses are so dire. Plantigrade essentially means flat-footed. This refers to us, bears, squirrels, and anything else that makes for good children’s reading about interspecies partying through a snowstorm inside a sleeping bear’s den. Digitigrade is essentially a foot structure in which the balls of your feet contact the ground. This is the structure of dog and cat feet. Unguligrade means walking on your tiptoes, generally referring to any hooved animals (e.g., horses) and ballerinas on toe shoes. Incidentally, “unguligrade” might be the BEST WORD EVER. I had to work it into my ATA presentation for precisely that reason.
The other example I want to discuss pertains to blood. Some non-mammalian vertebrate species (birds, reptiles, amphibians, fish) have nuclei in their circulating red blood cells (or “rbc’s,” as they are known in the biz). Nucleated rbc’s in mammalian circulation (including that of people) are, in contrast, typically a sign of serious illness. Why? Let’s backtrack a little and talk about how blood cells are born. The bone marrow produces blood cells and then releases them into the circulation. Every rbc is born with a nucleus but kicks out that nucleus as it ages, usually before the cell enters the circulation – unless the cell is malfunctioning or the circulatory demand for cells outstrips the supply of mature cells. Thus, nucleated rbc’s are immature ones that are being prematurely released from the marrow for some reason. Usually that reason is an underlying blood disorder (e.g., leukemia) or other illness. But take a look at a bird’s blood smear and, well, those nucleated rbc’s are no big whoop. But getting that blood sample from a bird in the first place can be quite dangerous (that’s a topic for another post).
In vet school we had t-shirts that said “REAL doctors treat more than one species.” Great way to troll MD’s, if nothing else.