Happy Sunday everyone! I have another installment of The “Inside” Scoop for y’all, this time on the Nile crocodile! The timestamps are based on this episode of Inside Nature’s Giants: Crocodile and are meant to help you navigate around the dissection if you are squeamish or just short on time. Viewer discretion is advised for the video. Have fun!
The crocodile died unexpectedly in a conservation and breeding center in France. The body was taken to the Royal Veterinary College for dissection, allowing veterinary students the opportunity to observe the necropsy. The crocodile was 17-years-old and had been swimming strangely for a few days before dying. First off, since I also get these types of questions a lot with seals vs. sea lions, here is a link to a video explaining the differences between crocodiles and alligators. Now, onto the necropsy!
3:00 – The external analysis begins with the biologists examining the crocodile’s mouth to investigate the mechanisms behind its incredible bite force. (4:15) They begin removing the skin under the mouth/neck. Because crocodiles are ambush predators, they require an extremely strong bite force to hold onto their prey and subsequently drown/kill them. They showed a clip at 5:18 of an alligator wrestler getting a little too ambitious and his head being caught in the jaws of an alligator (his own fault, but not sure why so many of the veterinary students initially laughed, kind of disturbing…moving on!). At 6:12, they showed really interesting footage of a researcher measuring the bite force of a live alligator. The biologist’s bite force was 78.5 lbs of pressure, but the crocodile chomped down at an incredible 1413 lbs! This is more than lions and great white sharks. If you are interested, Inside Nature’s Giants also did an episode on great white sharks and compared bite forces. You can follow the Bloggerhead link here: The “Inside” Scoop: Great White Shark.
8:45 – Back to dissecting! The scientists uncovered the massive muscles responsible for the crocodile’s bite. The main muscles are located on the bottom jaw and are very pale in color. The faded color indicates that these muscles receive low blood oxygen levels, making them useful for only short bursts of time. The muscles on the top of the head are smaller, but also darker because of more blood flow. Consequently, crocodiles can hold their mouths open over longer periods of time without their jaws getting tired. Interestingly, by having the larger muscles on the lower half of the jaw, crocodiles are also better adapted to maintain a low profile in the water. (10:00) Animation of the muscles a crocodile uses for biting.
10:42 – Crocodile jaw. Not all crocodile/alligator jaws are created equal, but they are created efficiently. Richard Dawkins explains at 11:35 that crocodile/alligator jaws are adapted to different environments. Large saltwater crocodiles need huge jaws for crushing their prey, but gharial crocodiles in India are smaller, with skinny snouts that are adapted for catching fish. The sleeker jaw means that the gharial sacrifices power for speed. As Dawkins aptly states, “Evolution is filled with compromises.”
13:08 – Moving onto hunting and the teeth! The teeth are cone-shaped and not actually that sharp. They are really only meant to hold onto the prey so that the crocodile can drown them. The teeth also come in useful when crocodiles begin their “death rolls,” where they rapidly rotate around in an effort to tear off chunks of meat. How do they thrash around so much without breathing/drowning themselves? – Evidently, they have re-sealable nostrils to help. However, the most interesting part is that they can consciously move their larynx/back of their mouth to cover up their airway and prevent water from entering their lungs. The cool thing is that when they close the back of their mouth, they, in turn, connect their larynx airway to their nostril airway. This is kind of difficult to explain in words, but they provide a great animation at 15:35.
16:39 – Tail anatomy. A crocodile’s tail is almost completely muscle. Paired with their fin-like scutes, crocodiles can swim about 20 miles an hour! (17:35) Crocodiles are not only apt swimmers, but they can also move well on land. They have the ability to slide on their bellies or walk on all four legs. In addition, small crocodiles can even bound like rabbits, which may stem back to ancient times when they might have had to chase down their prey on land.
18:10 – Using fossils to investigate crocodile evolution. Crocodiles are reptiles that date back about 320 million years. Although amphibians may have been the first to initially move to land, reptiles were able to thrive because their eggs do not require moisture to succeed. This segment includes a neat cladogram animation and elaborates slightly on Megacrocs, which could grow up to 12m, or the length of a bus! (Thankfully, perhaps?) these Megacrocs died out, and modern crocodiles thrived. Modern crocodiles have thrived so well that they have not really had to evolve in about 100 million years!
20:15 – Back to the necropsy to check out digestive anatomy! This is a relatively short clip that shows how small the digestive tract of a crocodile is. In this case, the crocodile seemed to be eating odd foods, like leaves, which gave the pathologist clues about why the animal might have died. Normally, a crocodile can produce about 10X the amount of stomach acid as a human, which is why they can digest such difficult food.
26:06 – Chest cavity. The esophagus is stretchy so it can accommodate the oddly-shaped prey items that crocodiles consume. Because of the expandable esophagus, the trachea is reinforced so that it is not crushed while the animal is feeding. The biologists artificially inflate the lungs of the animal. Normally, crocodiles can expand their lungs 4X the size of a human lung and stay submerged for ~30min. Crocodiles even have a special muscle that allows them to pull back their liver, thereby creating more room for the lungs. Moreover, the animals can continually utilize this special muscle to manipulate the position of their lungs, which allows them to adjust their body position in the water! A cool animation and research footage are shown at 28:25.
29:18 – Circulatory system! This might have been my favorite part of the video. Crocodiles possess a highly sophisticated circulatory system. Although humans have only one aortic arch, crocodiles have two! The mystery of the second arch has boiled down to a matter of DIGESTION! Crazy, right? At 31:10, the camera crew goes into a research laboratory for American alligators. If an alligator has a blocked aortic arch, their digestion is drastically inhibited. The reduction in digestion capabilities is because the extra aortic arch can divert blood from the body, which is high in carbon dioxide, to the stomach. Carbon dioxide is a vital component for the formation of stomach acid, so once blood is diverted, the alligator can increase acid production and decrease indigestion! (Animation at 32:50)
35:40 – Short section on reproduction. Apparently, crocodile penis’s have fingerlike projections at the end that help channel sperm. Anatomists note that the testes should be bigger in this crocodile and speculate further on the cause of death.
37:30 – Bisecting a frozen crocodile! I really liked this clip because they sawed a small, frozen crocodile in half, and you could see the pathway that food and air would take in the body.
39:04 – The fight for survival. Skin is extremely important for a crocodile’s survival. It is armor plated on both the stomach and back. However, the plates on the back are much thicker, bumpier, and tougher than the ventral scutes. The scutes on the back of the animal lock together to form an armor without chinks. Apparently, “back in the day,” people even dried crocodile skin to use as armor. But, armor is heavy and stifling, so how do crocodiles thermoregulate? There are actually channels within the scute bones that allow blood vessels through, thereby allowing the blood to accumulate heat before returning to the rest of the body. The process is tantamount to personal solar panels for each crocodile. (Animation at 42:00)
43:40 – A pathologist analyses the cause of death. A stone was found in the digestive tract, which is common because it aids in digestion. However, it was clear that the animal had not eaten real food in a long time. The primary cause of death was a severe respiratory infection. There were abscesses on the lungs and the infection likely spread to cause symptoms in the liver, stomach, and testes.
I hope you liked this episode! I leave you with a picture of a small black caiman that I saw in the Amazon last year. The big ones were crazy! Glad I was in a canoe!