Hippos and Pezosiren: Walking Under the Water

Have you ever tried to walk along the bottom of a pool while fully submerged? It isn't easy. Keeping your feet on the bottom is enough of a task, and you would probably need a weight belt to take an underwater stroll. Hippos (Hippopotamus amphibius), though, walk and even prance along the bottom of lakes and rivers with ease. How do they do it?

When compared to a whale or even a manatee (the latter of which I will address a bit later on) a hippo does not look especially well-adapted to life in the water. It has a low, squat body and lacks a broad tail, flippers, or any other broad surface to help propel itself through the water. Neither is this amphibious mammal well-suited to quick movements on land. Hippos can trot a bit, but they are so cumbersome that while walking on dry land they always keep three of their feet in contact with the ground at a time.

The hippo is obviously a cumbersome animal, but it is not just because of their tubby physique. Parts of the appendicular skeleton of hippos are osteosclerotic, meaning that their bones are extraordinarily dense due to the replacement of porous bone with more compact bone. (Hippos have extra, lighter bone material in the large medullary cavities within their limb bones, too.) This means that their bones act as a kind of ballast to help them achieve neutral buoyancy underwater. Without this added weight they would have to actively expend a lot of energy to remain underwater (and, conversely, too much "bone ballast" would make it harder to surface when they needed to come up for air).

This adaptation to aquatic life allows hippos to exhibit a greater range of locomotion under the water than on land. Since they do not have to actively hold themselves down, and the watery environment buoys them up, they can walk, prance, and even "fly" underwater. This range of movement was recently described by biologists Brittany Coughlin and Frank Fish in the Journal of Mammalogy after observing the two female hippos kept at Adventure Aquarium in Camden, New Jersey.*

*[As a side note, these individual hippos often harass and even attempt to eat the birds kept within the enclosure. Many captive birds have died from stress as a result. This raises questions about the ethics of attempting to create a "natural" setting for aggressive animals in zoos and aquariums.]

Unfortunately the size of the hippo enclosure and the amount of time selected for study limited the observational data, but Coughlin and Fish still recorded 102 sequences of hippos moving underwater (of which 32 were selected for analysis). The hippos did not perform their full locomotive repertoire, but they did often walk by the viewing window along the pool bottom. When they did so they moved in a sort of slow-motion gallop with only one foot (vs. three on land) in contact with the ground at any one time. It almost goes without saying that the hippos would not be able to move the same way on land as they did underwater.

Could Pezosiren, an Eocene relative of modern manatees from the rock of Jamaica and described by D.P. Domnig in 2001, have moved in a similar fashion? Today manatees and their relatives within the Sirenia are adapted to a fully aquatic lifestyle; their forefeet are stubby flippers, they lack hindlimbs, and they propel themselves with up-and-down oscillations of their broad tails. Like whales, however, sirenians evolved from terrestrial ancestors, and Pezosiren is a startling creature that helps illustrate how sirenians became adapted to aquatic life.

If you only saw the skull of Pezosiren you might be fooled into thinking it was a fully aquatic form like modern sirenians. While it's skull differs in some subtle ways, it is easily identifiable at first glance as being a relative of manatees and dugongs. The strange thing is, however, that Pezosiren had four limbs and almost certainly was an amphibious creature. It could still walk on land and did not have the swimming adaptations seen in its later relatives.

Indeed, Pezosiren looked very hippo-like. It was short and barrel-bodied, and it also had osteoslerotic bones. This was not an animal that was just dipping its toes into the water but one that was already semi-aquatic. There is little doubt that it was spending considerable time in an aquatic environment. I do not know of any studies seeking to reconstruct the locomotion of this animal, but it would seem a fair inference to suggest that while its movements would be restricted on land it would have exhibited a wider range of movements under the water. It may have moved in ways analogous to, but not exactly like, the hippos studied by Coughlin and Fish.

For more on aquatic mammals see:

• 
  Puijila darwini: A Significant Seal

• 
  Maiacetus: The Good Mother Whale

• 
  Elephants Out of the Ooze

• 
  Indohyus: Almost Like a Mouse Deer?

Coughlin, B., & Fish, F. (2009). Hippopotamus Underwater Locomotion: Reduced-Gravity Movements for a Massive Mammal Journal of Mammalogy, 90 (3), 675-679 DOI: 10.1644/08-MAMM-A-279R.1

Domning, D. (2001). The earliest known fully quadrupedal sirenian Nature, 413 (6856), 625-627 DOI: 10.1038/35098072