The Ideal Milk: Why Is There Such a Difference Among Mammals?

Michael Chitty · December 27, 2022
This article has been written and endorsed by biochemist Luz Eduviges Thomas-Romero
Understanding the differences in different species' milk can help people better understand how human breast milk is the ideal milk for child development and growth.

The ideal milk should have certain requirements and contain a mix of nutrients tailored to babies’ developmental stage. It should also have a mix of protective factors that help the vulnerable young fight harmful microbes and adapt to their environment.

So, what is the ideal milk? In this article, we’ll see why the milk of each species is the ideal food for their respective young.

Nature designed the ideal milk

It’s important to keep in mind that breast milk isn’t an exclusively human characteristic. All mammals produce milk. Nature made sure that each mammal species produced its own “special blend,” adapted to the needs of its offspring.

Discovering how and why milk differs between species can help people better understand how human breast milk influences infant growth and development. By using this knowledge, increased breastfeeding rates can be encouraged.

The World Health Organization (WHO) recommends exclusive breastfeeding for six months. This should be followed by the introduction of age-appropriate and safe foods, breastfeeding at the same time, for up to two years or more.

The Ideal Milk: Why Is There Such a Difference Among Mammals?

The ideal milk for pigs (Sus scrofa domesticus)

Pig’s milk is fattier than cow’s milk – about 8.5% compared to 3.9%. It also has higher amounts of protein and lactose, 5.6% and 5.5% compared to 3.2% and 4.8% compared to cows.

The composition of pig’s milk makes the piglet double its weight at birth, in only 8 days. This is in comparison to the 36 days it takes for the cow calf and more than 110 days for the human baby.

Why, then, do we drink cow’s milk but not pig’s milk? The answer comes down to a physical limitation: sows are very difficult to milk.

It’s important to note that sows have about 14 small teats, compared to the four large teats on a cow’s udder. Also, in the postpartum period, sows aren’t very willing to be touched by humans.

Moreover, sows expel milk from their piglets in bursts that last only a minute, while a cow’s may last up to 10 minutes. For this reason, you’d have to wait a long time to collect even half a liter of milk.

The case of the black rhino (Diceros bicornis)

In contrast, the black rhino produces skimmed milk; its milk is watery and has only 0.2% fat. Experts believe that the production of low-fat milk is related to the animal’s slow reproductive cycle.

In the case of black rhinos, they reach sexual maturity after four or five years. Also, this species has long pregnancies that last more than a year and they give birth to only one offspring at a time. Once they’re born, the parents spend almost two years caring for their young.

Hooded seals (Cystophora cristata)

This species’ milk is fascinating. It’s the milk with the highest fat content known to man. To illustrate this fact, let’s remember that human breast milk contains between 3% and 5% fat. In contrast, the hooded seal produces milk with more than 60% fat.

The Ideal Milk: Why Is There Such a Difference Among Mammals?

This very high fat content is combined with the exceptionally short nursing period of only four days. It’s important to realize that such a high fat diet is a matter of life or death for the seal pups, who must brave the icy waters of the North Atlantic.

It’s a matter of adaptation and survival. Mother seals give birth to pups on floating ice, an unstable and unreliable environment. Therefore, the mother seal feeds her pups for only four days, in which they gain about 7 kilograms (15 lbs) per day.

The fattening that this breastfeeding allows makes it easier for the pup to survive for several weeks, until it’s able to swim and fish on its own.

Cotton-tailed rabbits (Sylvilagus floridanus)

The milk of this species is the milk with the highest protein content known to man – about 15%. According to researchers, milk with a high protein content are associated with species that leave their offspring unattended for long periods of time. This is the case with rabbits where the mother is absent for long periods in search of food.

Cotton-tailed rabbit mothers, for example, return to their terrestrial nests to nurse their young only once or twice a day. The hatchlings double their weight every week and are breastfed for 28 to 35 days.


  • McClellan, H. L., Miller, S. J., & Hartmann, P. E. (2008). Evolution of lactation: nutrition v. protection with special reference to five mammalian species. Nutrition research reviews, 21(2), 97-116.
  • Skibiel, A. L., Downing, L. M., Orr, T. J., & Hood, W. R. (2013). The evolution of the nutrient composition of mammalian milks. Journal of Animal Ecology, 1254-1264.