Fructose is an essential molecule to many mammals
In a previous article, I described how fructose is a commonly found molecule in nature. It is also essential during the development of the offspring of many mammals.
For example, whales and dolphins have their fetuses swimming in amniotic fluid that has high fructose levels. The primary sugar in their fetal blood is fructose, not glucose.
And not only whales, but also, pregnant camels, cows, deer, goats, hippos, llamas, pigs, and sheep all have their babies floating in an environment that is enclosed, warm, nurturing, and sweet (1, 2, 3, 4, 5)!
A placenta that meets the needs of large animals
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).
A placenta that transforms glucose to fructose
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.
And whales follow suit
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!
Linking whales, hippos, and camels
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 human mammals?
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.
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- Goodwin RF. Division of the common mammals into two groups according to the concentration of fructose in the blood of the foetus. J Physiol. 1956 Apr 27;132(1):146-56. doi: 10.1113/jphysiol.1956.sp005509. PMID: 13320379; PMCID: PMC1363546.
- Bazer F, Wu G, and Johnson G. Pregnancy recognition signals in mammals: The roles of interferons and estrogens. Animal reproduction 2017 Jan 14(1):7-29. DOI:10.21451/1984-3143-AR888.
- Carter AM and Mess AM. The evolution of fetal membranes and placentation in carnivores and ungulates(Ferungulata). Anim. Reprod 2017 Mar 14 (1): 124-135.
- Kim J, Song G, Wu G, Bazer FW. Functional roles of fructose. Proc Natl Acad Sci U S A. 2012 Jun 19;109(25):E1619-28. doi: 10.1073/pnas.1204298109. Epub 2012 May 23. PMID: 22623530; PMCID: PMC3382482.
- Furukawa S, Kuroda Y, Sugiyama A. A comparison of the histological structure of the placenta in experimental animals. J Toxicol Pathol. 2014 Apr;27(1):11-8. doi: 10.1293/tox.2013-0060. Epub 2014 Apr 30. Erratum in: J Toxicol Pathol. 2016 Jan;29(1):74. PMID: 24791062; PMCID: PMC4000068.
- Shimamura, M., Yasue, H., Ohshima, K. et al. Molecular evidence from retroposons that whales form a clade within even-toed ungulates. Nature 388, 666–670 (1997). https://doi.org/10.1038/41759
- Jauniaux E, Hempstock J, Teng C, Battaglia FC, Burton GJ. Polyol concentrations in the fluid compartments of the human conceptus during the first trimester of pregnancy: maintenance of redox potential in a low oxygen environment. J Clin Endocrinol Metab. 2005 Feb;90(2):1171-5. doi: 10.1210/jc.2004-1513. Epub 2004 Nov 23. PMID: 15562012.