Hi all. Been a while since the last time I was here. How are you doing? Well, I'm currently wondering about swimming animals. See, when I see (in videos or real life) animals swimming, Chordata animals especially (fish up to mammals), their methods of swimming seems to differ depending on the animals. However, as far as I've observed and remembered, ectothermic (or should I say poikilothermic?) Chordata such as fish and reptiles tend to swim in sideways waving (sinestral-dextral, left to right), though in case of legged reptiles, this is also the way they walk. In contrast, aquatic mammals (endothermic Chordata) such as dolphins tend to swim in vertical waving (dorsal-ventral, up to down), and as far as I know I've yet to see humans who swim with sideways waving either. Well, admittedly, considering the morphological differences between fish and dolphins, their different methods of swimming may be because of that, since fish are often flat sideways (that is, shorter left-to-right/sinestral-dextral body size) while dolphins are not, and humans are flat back-to-front (dorsal-ventral) and generally swim the same way as dolphins. However, reptiles, like humans, have a shorter dorsal-ventral body distance, and yet they tend to swim like fish. Why is this so? As disclaimer, I haven't really taken note of swimming birds' swimming methods (I tried checking on penguins on Youtube, but their main bodies rarely exhibit any significant waving movement while their flippers fo all the work), and I understand that some fish are more tubular in appearance, not to mention some vertically-flat fish (such as rays) who swim totally differently from their kin. PS: Sorry if I sound a little too smarty with all those latin words ^_^". It's just that I'm trying not to confuse the notion between up/down, front/back and up/down due to involving humans in this question. PS2: And thanks in advance for any replies. Please Register or Log in to view the hidden image!
Remember that dolphins and whales evolved from land animals, while fish evolved in the water. That's a large part of the solution to the puzzle.
The reason is the all fish-related species a flattened in the the axis such that it sideways movement while mammals, originally being quadrupleds, developed a up and down movement for land locomotion. I think this is one of the reasons why mammals rather than replites came to dominate the earth.
Specifically from primitive hippopotamus-like creatures. Artiodactyla has been renamed Cetartiodactyla and Cetacea has been demoted to a clade within that order, which it shares with the hippopotamuses. Apparently some of their river-dwelling ancestors swam all the way out into the ocean and liked it there. Warm-blooded air-breathers, with their much more energetic metabolism, always dominate cold-blooded gill-breathers of comparable size, so when they move into an aquatic ecosystem they just take over. Surprisingly enough, although the ancestors of the cetaceans and all surviving artiodactyl species are completely or primarily herbivores, often with huge multi-chambered stomachs that host a bacterial culture for digesting cellulose very slowly, the cetaceans themselves have evolved into carnivores. The Mysticeti or baleen whales could be called grazers with an asterisk, since they survive by sifting the plankton out of seawater. But the Odontoceti or toothed whales (the sperm whale and all dolphins) are true predators, hunting fish, often in packs. Nearly unique among predatory endotherms, they still have their eyes on the side of their heads. The orcas (the largest dolphins) even hunt mammals (seals) and birds (penguins) although remarkably there are no reliable reports of attacks on people and they have even been known to assist in rescuing drowning humans. Perhaps they plan on petitioning for U.N. membership. The mammalian spine has evolved for mammalian locomotion. Mammals walk forward, so their spines flex in a front-to-back motion, allowing one pair of legs to be brought forward while the other pair provides support. This is especially critical for mammals that jump or merely gallop, moving both front legs or both back legs in unison. Few mammals need very much sideways flexibility in their spines. Primates do, so perhaps our spines have more of it than other mammals. I've never paid close attention to a climbing monkey. When they use their tails, do they bend them in 3-D?
Film of dolphins and penguins underwater are amazing. You see a lot of similarity in their adaptations, yet they arrived at this from such different evolutionary causes. And then you have some of the more specialized versions, like Pleuronectiformes (e.g. flatfish) or Myliobatidae (rays) that adapted their geometries a little differently. I remember watching a manta ray strike at me from quite a distance across shallow water. It moved with an incredible burst of speed, then stopped on a dime, as if only curious, then turned and cruised away without any discernible swimming motion. And they are built like Klingon warships (reverse actually). Your observation is a good one. Just to consider how much complexity is found in the vertebrate structure, to shield the cord and provide sufficient flexibility, amazes me. Also, don't forget the invertebrate chordates. You have Craniates (having skulls), Cephalochordata (lancelets, which are long like eels) and the sessile oddity, Tunicates, who only possess a chord and swimming ability in the larval stage: Please Register or Log in to view the hidden image!
I remember reading something about group theory and locomotion in vertebrates. The example covered the way horses can walk (which is similar to the way reptiles walk), trot (not many reptiles can do this), or gallop (no reptiles do this). The math was about how different groups of neurons cooperate through signalling each other so that the different motions are always coordinated. Locomotion is apparently about a kind of symmetry in neural communication. Presumably fish were the first chordates to develop a synchronised swimming action. With the evolution of limbs this undulatory motion becomes redundant, although reptiles still move this way because the upper part of their limbs projects sideways, whereas more evolved animals like horses and other ungulates have upper limbs that project downwards. A lizard at full gallop weaves left and right as its legs alternately contact the ground, presumably to maintain balance between the weight of each leg (since it can't rotate any of them downwards).
1) Looks like I mixed up Chordata and Vertebrates (or just being too general)... oops ^_^" I didn't mean to ask about tunicates and friends actually, just as I didn't ask about the rest of the animal kingdom yet. 2) Regarding sea mammals evolving from land mammals, that reminds me; When I look at running mammals, such as big cats and horses, they do flex their spine in a dorsal-ventral motion. So yeah, it makes sense that sea mammals also flex their spine the same way. 3) Most importantly though, I guess I have to read further about evolution. I mean, when and where did the phenomenon of sinestral-dextral (left-right) spine flexing for locomotion transform into a dorsal-ventral flexing? I still occasionally get the terms of reptiles, synapsids, etc mixed up, so I wonder about things, such as which prehistoric animal was the first one to develop the dorsal-ventral flexing locomotion.
