The 2021 AAVN conference was filled with a variety of insightful presentations. In continuing with our previous post, which looked at Dr. Dzanis’s presentation on the evolution of pet food regulation, we will now give a recap of Dr. Power’s presentation on the wonders of milk!
Michael L. Power (PhD) entitled “The Evolutionary Biology of Milk; A brief review of the past 300 million years”
When you think of milk you may think of a beverage that is part of your daily diet, nothing more than a food. However, Dr. Power took this time during AAVN to divulge the hidden power of milk to his audience, describing it as “the means by which mothers are biochemically signaling to their offspring to guide growth and development”.
As illustrated in the picture above, milk is composed of so much more than nutrients, it also contains important molecules for the immune system and vital signaling molecules. Despite the integral role milk plays in today’s society, Dr. Power emphasized how little we know about its origins and gave a hypothesis on the possible genesis of milk/lactation. Lactation first began not as a means of delivering nutrients but rather to maintain a water balance within the first terrestrial vertebrate eggs. The animals that performed this action are referred to as synapsids and the only remaining descendants of synapsids are those that developed the ability to lactate. Located on their ventral end, synapsids possessed a gland that would secrete a moist solution to coat their eggs. This coating was vital as their eggs contained a parchment shell instead of a calcified shell and thus, water was free to move back and forth. In effect, during dry days, water would seep out of the eggs leading to reproductive risks. However, by coating the egg and keeping the surface moist, water would stay inside. Even at its earliest stages, lactation played a role in immunity. Since the process of keeping the eggs moist would create an environment perfect for microbial growth, the liquid secreted by synapsids also possessed antimicrobial agents perfect for keeping the egg safe.
Skeleton of a Synapsid
Dr. Power then explained that the maternal behaviour of coating the eggs with this liquid eventually extended past hatching and into a method for continuing to nurture, and eventually nourish, the newborn. However, the evolution of milk was not linear and thus its components vary significantly between species. For example, the sugar molecule lactose is found in many milks and is formed when glucose and galactose join to produce a disaccharide. This process is quite unique as glucose and galactose do not come together in free solution but require the enzyme lactalbumin, exclusively found in the mammary gland, to facilitate their synthesis. Despite some species containing the specialized genes required to produce lactalbumin, not all species possess this gene, and thus lactose is not a universal component of milk.
The variability of milk between species is a part of what makes this liquid so special. Dr. Power emphasized that the most variable component of milk between species is the fat content. Furthermore, the fat content of milk can provide a lot of information about the milk, such as the sugar content (high fat means low sugar and vice versa). Although not quite as variable, the protein content of milk also differs between species. What does this all mean practically? Dr. Power explored the idea of milk variability in terms of calf growth in the Asian Elephant and the African White Rhino. The Asian Elephant produces milk that is high in fat but lower in sugar and water. Whereas the African White Rhino produces a very dilute milk high in sugar and water but low in fat. However, both of these species produce calves of similar size and growth rates. Evidently, the variability of milk between species simply demonstrates that there is no one solution to the needs of life/development and that milk can vary between species to accomplish very similar goals. Dr. Power also mentioned another cause for variability in milk and that is the stage of lactation. The milk from Asian Elephants, as Dr. Power pointed out, continues to decrease in sugar and increase in fat and protein as the newborn progresses through lactation.
Dr. Power concluded his presentation by discussing research into the milk produced
from xenarthrans as a means of tracing back the origins of milk.
Two Xenarthrans; Armadillo and Sloth
The early synapsids were small furry nocturnal insectivores that closely resemble the xenarthrans of today. Therefore, what can species like the armadillo, the sloth, and the anteater tell us about the first milks? Something peculiar in the composition of the milk of these species is that it is quite high in protein. Dr. Power quickly explained that since the armadillo contains a boney carapace, this is of no surprise as their milk would require extra casein in order to prevent the calcium and phosphorus in the milk from reacting, thus keeping it available for carapace bone formation. However, as Dr. Power pointed out, this does not explain why the milk of sloths and anteaters would also be high in protein. It is thought that the low energy and protein rich diet of these animals allows for their high protein milk. Moreover, the high protein content of their milk is consistent with the high protein required for these animals’ fast growth rates. Dr. Power’s current research seeks to explore the secrets of milk more thoroughly, hopefully bringing us closer to understanding its origins.
Keep an eye out for future posts and conference updates!
Written by: by: Cristian Mastrangelo, OVC Pet Nutrition Summer Student
Edited by: Shoshana Verton-Shaw, RVT, VTS (Nutrition)
Dr. Adronie Verbrugghe, DVM, PhD, EBVS Specialist in Veterinary and Comparative Nutrition® (Dip ECVCN)