All mammals are capable of producing milk; such animals like platypuses, kangaroos, rabbits, tigers, hippos, monkeys, dolphins, whales and the rest. But because different species have different needs and live in different environments, it turns out their milks are very different as well; though the ingredients are similar, it is in proportion that they are different.

Take a look at some of the examples given below, as provided by BBC Future

Seals

hooded seals

Hooded seals for example live in very cold regions, after a mother seal gives birth, her offspring need to build up a layer of fat very quickly, which helps them keep warm and survive the harsh conditions of their environment while learning to swim and hunt. Hence, it is not surprising that their milk contains about 61% fat, 5% protein and 1% milk sugars.

Since the seals give birth on unstable ice sheets to avoid the threat of polar bears, they have a very short time in which to give birth and nurture their babies for survival. Over the course of just four days, hooded seal moms must transfer up to seven kilograms of milk fat to their pups each day.

Land Mammals

gorrila breastfeeding

By contrast, land mammals are generally free of the need to transfer as much energy to their infants as possible in a brief period of time. Instead, these mothers nurse their infants over many weeks or months, delivering similar amount of nutrition, but in smaller doses.

Animals like rhinoceroses, goats, sheep, zebras, giraffes, tigers, gorillas and others fall in this category. They only offer milk to their young when they demand it. Perhaps that explains why a black rhinos’ milk contains just 0.2% fat, or why a gorilla’s milk contains about 1.5% fat.

Humans and Zebras

black family - mother nursing son

Humans, being mammals also, produce milk for their young. Because human infants require a very long period of parental care following their birth, this is the reason mothers produce more watery, diluted milk (as you may have noticed).

If the milk produced were were too energy-dense, like that of the hooded seal, our mothers would not have enough energy to chase their kids around since they will grow up so fast. So human milk contains just 4% fat, 1.3% protein, and 7.2% lactose, with about 90% of it being water.

Zebras also produce very similar milk, with 2.2% fat, 1.6% protein, 7% lactose, and 89% water. Hence, human and zebra milk are both characterised by high water content and low energy density, with more of that energy coming from lactose than from fat. 

Some Quick Facts

  • Mammals that eat meat, like tigers, have more fat and protein in their milk compared with mammals that eat mostly leaves like gazelles or giraffes.
  • Hooded seals nurse for just four days, meaning they need energy-dense milk, while bottlenose dolphins nurse over the course of 18 months. 
  • Breastfeeding with milk serves as a means of boosting infants’ immune systems. Antibodies passed between the two might allow infants protection from parasites to which their mothers were exposed, and which could be a threat to the babies.