Underwater nursing: how marine mammals feed their babies

I haven't written many posts lately because I've been busily growing and now nursing a baby girl. As I was breastfeeding the other day, I wondered how marine mammals like whales and dolphins feed their babies milk while underwater. I'm not a marine mammal specialist, so I've never seen their mouths up close - but I couldn't imagine how baby whales would form a seal around the mother whale's nipple so that they could drink the milk instead of drinking seawater and/or letting the milk simply leak out into the ocean.

My Google Scholar and You-Tube searches turned up these generalizations:

Whale and dolphin babies take relatively short dives underneath their mothers to drink. The mother's nipples are normally inverted but upon stimulation by the baby, the nipples come out and milk is ejected into the baby's mouth, while the baby holds onto the nipple with its mouth, or holds its mouth and/or tongue around the nipple. Here are some videos of a Beluga whale, a dolphin, and a Humpback whale nursing. Wow! Those babies are competent.

Some researchers suggested that sperm whale babies might actually receive milk through their blowholes (perhaps because their mouths are shaped even less conveniently for suckling), but others found evidence for oral suckling. This sequence of high-quality photographs also shows a sperm whale baby nursing orally; the high-fat content of the mother's milk is also apparent from the slow rate that undrunk milk mixes into the seawater.

In a way, nursing underwater is therefore similar to nursing above water: the baby stimulates the mammary glands to eject milk, and then it drinks the milk. However, the mechanism for stimulating the milk ejection reflex must be somewhat different - in humans the babies create a seal and suction on the nipple stimulates the reflex. In whales and dolphins, it seems that the reflex is likely stimulated when the babies bump the mammary glands; for example, milk ejection was also observed in Beluga whales bumping into the bottom of a tank.

References:

Drinnan, R. L., & Sadleir, R. M. F. S. (1981). The suckling behavior of a captive beluga (Delphinapterus leucas) calf. Applied Animal Ethology, 7(2), 179-185.

Gero, S., & Whitehead, H. (2007). Suckling behavior in sperm whale calves: observations and hypotheses. Marine mammal science, 23(2), 398-413.

Johnson, G., Frantzis, A., Johnson, C., Alexiadou, V., Ridgway, S., & Madsen, P. T. (2010). Evidence that sperm whale (Physeter macrocephalus) calves suckle through their mouth. Marine mammal science, 26(4), 990-996.