The fascinating connection between Whales, hippos, and camels: high fructose

What is a common denominator for all those animals? Interestingly, all the animals listed above have either the epitheliochorial or the very similar syndesmochorial type of placenta (1, 5,  6).

The development of an Epitheliochorial placenta was a common evolutionary adaptation of larger mammals. This type of placenta separates maternal blood from fetal blood by 3 layers of tissue. Having such a placenta allowed the mammal to have a long gestation period, smaller litters, and precocial offspring (born in an advanced state, able to feed themselves and move independently almost immediately after birth) (4). 

Clearly, this gave the mother and her offspring a survival advantage. They could escape to safer grounds soon after delivery. Whales, hippos, and camels fit this pattern of placentation (4). 

What is also unique among the animals with this type of placenta is that the placenta changes glucose to fructose.

Take a mother pig! Her baby piglets can have a sweet drink any time they want during the mother’s pregnancy!  It’s as if they are swimming in high fructose corn syrup!

All ungulates (the clade that groups hoofed large mammals together) - for example, camels, cows, deer, goats, hippos, llamas, pigs, horses, rhinos, goat, and sheep - have one of the two kinds of placenta listed above (1, 2, 3, 4, 5) . And in all cases, their fetus has high fructose in their blood and in their amniotic fluid.

Curiously, mammals in the clade Cetacea - whales, dolphins, porpoises - also have an epitheliochorial placenta that fills the entire uterus. And whales - based on their molecular biology (7)  - have been classified with ungulates. What is even more fascinating is that whales, hippos, and camels - even though they look and act very differently - have all been classified as even-toed ungulates.

In fact, whales and hippos are cousins, sharing a common ancestry from ~50 million years ago. There is molecular data demonstrating that hippos and whales share an evolutionary line. In fact, this two groups of mammals share genetic sequences that are not found in other mammals (7). 

And to make an even stronger case linking ungulates and whales, in 2011 a 43 million-year old fossil of a four-legged whale was found off the coast of Peru. The giant fossil was 41 meters long (that’s 3 schoolhouses end to end) and the whale had tiny hooves on its feet!   

Remarkably,  even though whale, hippos, and camels are very different, they share a common physiology before birth and through their ancestry: 

•  They are ungulates (belonging to the hoofed mammal group) (not only that, they are even-toed ungulates)
• They have similar placentas that are epitheliochorial, and diffusely covering the uterus
• Their placenta converts glucose to fructose such that the fetal blood and amniotic fluid have high fructose levels.

Thus, even though those three mammals adapted to quite different environments (aquatic, terrestrial/aquatic, and desert), they share an incredibly similar genetic, biochemical, and developmental biology.

What about us?

As simple primates. a human placenta just transfers glucose from the mother to the baby (except in the first trimester when the fructose is actively being produced in the fetus - 8).   

More than 50 millions of years ago, primates chose an evolutionary tree that had rabbits, rats, and mice as their cousins. They chose to dissociate from the group of animals which had hoofed mammals as their ancestors (see this and this).

I think primates lost out when they made that choice. It would have been nice to have a sweetened beverage whenever we wanted as we developed in our mother’s uterus.