The Reluctant Earth Mother

Baby 2 has just walked into the kitchen with his bat costume on and demanded milk by flapping his wings. Got me to thinking, how do bats breastfeed? Turns out they can feed on the wing. Baby bat clings to Mum and sucks from her armpits; the AI overview also suggests that some male bats have been observed to lactate but I’d need to put more effort than I’m willing to to verify that particular titbit. 

Milk is thought to have originated as a fluid to prevent eggs from drying out long before mammals were a thing. Antimicrobial properties were an advantage because of the warm, moist environments that the soggy mess I’m imagining would have fostered. These immunity properties were far more important and developed much earlier than any of the nutritional properties it’s so important for now. 

Breast milk is made of up of fat, sugar and protein at different ratios depending on how the mammal parents. It also contains immunity factors and a ton of mysterious mechanisms for signalling and communication between mother and baby allowing them to modify the milk to suit baby’s needs hour by hour. 

Human milk is a bit odd in comparison to other milks as it contains a ton of sugar to power our oddly big brains. The amount of protein and fat is also as low as it can go though that is in keeping with us being categorised as carry mammals. Carry mammals stay with baby, or carry them around, so they feed often and the milk doesn’t need to be formulated in a way that keeps baby full for long amounts of time. Primates, unsurprisingly, are similar but so are kangaroos who have a literal baby pouch to keep them in. You’ll be pleased to learn that this lower energy milk also seems to mean that we feed our babies for longer than other mammals but does also reduce the toll of making it. 

I know I would have given a lot to be any other type of mammal in those early days (and even now sometimes) when it felt like baby was never off me. You’ve got cache animals who hoard their babies returning something like 12 hours later with their high protein and high fat milk keeping their kids full and content with no reason to cry out and alert predators to their whereabouts. Nest animals, like cats, tend to be born fairly immature and in litters. Mum has to return fairly frequently but can still be away for a few hours due to the highish levels of protein and fat in her milk. Follow animals have even less protein and fat in their milk but the lambs, calves, kids, are mature enough to keep up with mum and help themselves. These ones feel like a good plan to me but it was hard enough to birth a 3.5kg human, nevermind a 16kg one.

Fewtrell, M S. Shukri, N H M. Wells, J C K. (2020). ‘Optimising’ breastfeeding: what can we learn from evolutionary, comparative and anthropological aspects of lactatio. BMC Med. 18(4).

Haight, J. (2016). Why our babies are more like kangaroos than cows – and what that means for your breastfeeding experience. [Online]. KAMLOOPS Breastfeeding services. Last Updated: 7th October 2016. Available at: https://www.kamloopsbreastfeeding.com/articles/why-our-babies-are-more-like-kangaroos-than-cows-and- [Accessed 18 November 2025].

La Leche League Canada. (2022). Mammal Milk Composition and Mothering Styles. [Online]. La Leche League Canada. Last Updated: August 2022. Available at: https://www.lllc.ca/mammal-milk-composition-and-mothering-styles [Accessed 18 November 2025].