Do hippos and whales have the same aquatic ancestor?

Do hippos and whales have the same aquatic ancestor?

Violet Field inPlanetJan 26, 2021 7 min read24 views

Discover, every day, an analysis of our partner The Conversation. Today, a paleobiological researcher explains what links and differentiates two of the most massive mammals

Artist's impression of an Anthracotherium - © Dmitry Bogdanov / CC BY-SA 3 & Republica / Pixabay
  • Molecular studies have recently established that certain anatomical characters of whales bring them closer to hippos, according to a study published by our partner The Conversation.
  • The question is whether the common ancestor of these two species was aquatic or if they independently adapted to a semi-aquatic way of life for one, aquatic for the other.
  • This research was carried out by Alexandra Houssaye, a researcher in Paleobiology / Functional Morphology at the National Museum of Natural History (MNHN).

It is well known today that whales are mammals. Molecular studies as well as certain anatomical characters have brought them closer to artiodactyls, ungulates with an even number of fingers (cows, camels, pigs, sheep, giraffes).

These organisms are particularly characterized by the shape of one of the ankle bones, the astragalus, which has a double pulley (see below). But how were researchers able to observe this particularity in cetaceans since they do not have a hind limb? If this is the case in current cetaceans, this is not the case with the first cetaceans (for example in Pakicetus ) which had hind limbs and in whom this character has been observed.

Astragalus of Pakicetus (left), one of the first cetaceans, showing the double pulley typically seen in artiodactyls (pig, deer) © (via The Conversation)

We then had to find which group was closest to cetaceans within the artiodactyls. Since the 1990s, molecular analyzes have suggested that these are hippos. However, between modern hippos and cetaceans - even the first - it is difficult to visualize such proximity.

There are currently only two species of the hippopotamus: the common hippopotamus (about 1,500 kg) and the pygmy hippopotamus (about 225 kg). The common hippopotamus lives in rivers and streams in sub-Saharan Africa while the pygmy hippopotamus, whose way of life is very poorly known because it is little observed, lives in forest environments close to rivers in Africa. the west.

Common hippos (left) and pygmy hippos (right) © Wikimedia CC BY-SA

It was only recently that it was found out who the Hippopotamidae root group was. While hippopotamids were compared to pigs and peccaries, recent paleontological discoveries have confirmed their relationship with anthracothères, a group of herbivores of various sizes (from about thirty kilos to several tons) and presenting terrestrial and semi-aquatic, which have diversified since the end of the Middle Eocene, around 40 million years ago, and are represented in the fossil record by just under a hundred species.

A common semi-aquatic ancestor?

The question is whether the common ancestor of hippos and whales was aquatic or if these two lineages independently adapted to a semi-aquatic way of life for one, aquatic for the other.

Anthracotherium and Elomeryx, two anthracothères, close relatives of modern hippos and cousins of cetaceans © Apokryltaros / Wikipedia, CC BY-SA

Indeed, many forms are considered as semi-aquatic within anthracothera but their lifestyles remain very uncertain, the sedimentary and morphological data being ambiguous. Various types of complementary approaches seek to clarify the way of life of these extinct forms: the study of their sensory capacities, the type of wear of their teeth reflecting their diet, isotopic analyzes (different values in particular according to the environment of the 'water drunk).

The postcranial skeleton (body) of these organisms has been relatively poorly studied from a functional point of view. However, the adaptations of the postcranial skeleton can clarify the paleoecological hypotheses. And why not look inside their bones? Indeed, the distribution of bone tissue in the bones varies according to the constraints imposed on the skeleton, and therefore according to the mode of locomotion and the living environment of the organisms.

Our recent study (in the process of being published) therefore focused on the internal structure of the bones of the limbs in hippopotamoïdes (hippopotamids + anthracothères). The researchers put humeruses (arm bones) and femurs (thigh bones) through an X-ray microtomography which makes a kind of 3D X-ray of the bones and compared the structures observed to those of present-day animals.

Pygmy hippopotamus humerus X-rayed to observe its internal structure ©, Alexandra Houssay

These comparisons show that almost all terrestrial quadrupedal mammals, such as the wild boar, exhibit a tube structure, with a compact bone tube surrounding a free medullary cavity (like a marrow bone; see bone sections below. ). In fairly massive animals, this tube is thicker, as in the buffalo. In very massive animals, such as rhinos, the organization is completely modified: a spongy tissue occupies the medullary space, in order to better withstand the very heavy constraints linked to weight. As for aquatic animals showing fairly limited swimming abilities, such as the first cetaceans, they tend to have very compact and therefore very heavy bones, which allows them in particular to control their buoyancy.

Cross-sections of hippopotamoid humerus (bottom) and other mammals for comparison. Cuts made at the location of the red line in the previous figure. In black the bone tissue. In white: modular space. T: Terrestrial; L: Heavy; SA: semi-aquatic; A: aquatic; +: strongly © Alexandra Houssaye

As for semi-aquatic animals, they usually show thickening of the compact bone tube and slight filling of the medullary cavity. The importance of this change depends on their degree of adaptation and dependence on the aquatic environment. Thus it is quite low in the pygmy hippopotamus and the tapir, which can both swim below the surface, but much higher in a sea otter which spends most of its time in water and whose terrestrial locomotion is very difficult.

An increase in compactness may thus be associated with massive weight or a semi-aquatic lifestyle. If between an otter and an elephant the adaptive constraint in play seems obvious, things are not so simple for the semi-aquatic massive animals like the common hippo, which are both heavy and semi-aquatic! It is thus necessary to take into account the stature to infer the living environment of these organisms.

Our "Animals" file

The common hippopotamus is more massive and more closely linked to the aquatic environment than the pygmy hippopotamus. His humerus shows a very thick tube and spans in the medullary space.

The study of these osseous structures made it possible to suggest a terrestrial way of life in Microbunodon, slender form not exceeding 20 kg, a slight degree of adaptation to a semi-aquatic way of life in Brachyodus, massive form exceeding 2 tons, and a greater degree in Hexaprotodon, a massive form also exceeding 2 tonnes. These results suggest that, in hippopotamus, no form shows a stronger dependence on the aquatic environment than the common hippopotamus. The adaptation to a semi-aquatic way of life is done differently from in cetaceans, which suggests two convergent returns to the aquatic environment in these lines. There is therefore surely no one and the same semi-aquatic origin to these two lines.

This analysis was written by Alexandra Houssaye, a researcher in Paleobiology / Functional Morphology at the National Museum of Natural History (MNHN).
The original article was published on The Conversation website.