Whales are mammals, and like all mammals they nurse their young with milk produced by mammary glands. The twist is that they do it entirely underwater, without lips that can form a seal around a nipple the way a land mammal’s can. Instead, whale mothers have evolved a system of retractable mammary slits, muscular milk ejection, and calves that learn to latch at precise angles, all while holding their breath and drifting through open ocean. The mechanics are genuinely unlike anything on land, and the details researchers have captured on video and with biologging tags reveal a process that is fast, calorie-dense, and surprisingly quiet.
How the Milk Gets Delivered
A whale’s mammary glands sit inside paired slits on the underside of the mother’s body, just ahead of the genital area. These slits stay flush with her skin during normal swimming, reducing drag. When a calf is ready to nurse, the mother’s teats protrude slightly from the slits, and the calf presses its mouth against the opening. Because a whale calf cannot purse its lips and suck the way a terrestrial mammal does, the mother uses muscular contractions around the mammary gland to actively pump milk into the calf’s mouth. The result is less like a leisurely suckle and more like a pressurized squirt. Researchers have even documented milk visibly clouding the water column near mother-calf pairs during or shortly after nursing bouts.
The milk itself is nothing like the thin, watery milk most people picture. Whale milk is extraordinarily rich in fat, often ranging from about 30 to 50 percent fat depending on the species and stage of lactation. For comparison, whole cow’s milk hovers around 3.5 percent fat. That extreme richness serves a clear purpose: calves need to build a thick blubber layer as quickly as possible, both for insulation in cold water and as an energy reserve. The consistency of whale milk has been described by researchers who have handled it as closer to toothpaste or soft butter than to anything you would pour into a glass.
Positioning and the 30-Second Latch
Underwater video of humpback whale calves nursing shows the calf approaching the mother and angling itself at roughly 30 to 45 degrees relative to her midline, then pressing its mouth against the mammary slit. These nursing events are strikingly brief. In one study of humpback whale mother-calf pairs filmed underwater, the average duration of a single nursing bout was about 31 seconds, with individual bouts ranging from 15 to 55 seconds.1PubMed Central. A Note on Suckling Behavior and Laterality in Nursing Humpback Whale Calves from Underwater Observations That brevity makes sense when you consider that calves have to surface to breathe. Each nursing attempt is essentially a dive, and the calf has limited breath-hold capacity, especially in the first weeks of life.
What the calves lack in duration they make up for in frequency. A separate study that used suction-cup tags on humpback whale neonates in breeding grounds found that sucking dives accounted for roughly 21 percent of the total tagging time, with mothers either resting at the surface or hovering at shallow depth while the calves hung motionless beneath them.2Functional Ecology. High suckling rates and acoustic crypsis of humpback whale neonates maximise potential for mother–calf energy transfer That is a remarkably large fraction of the day spent either nursing or in position to nurse, and it reflects just how much energy the calf needs to grow during its first months.
A Preference for One Side
One of the more unexpected findings from underwater observations is that humpback whale calves may have a preferred side when nursing. In the same study of filmed nursing events, all five observed suckling bouts had the calf positioned on the mother’s right side. On four of those five approaches, the calf swam in from the right. In the one case where the calf approached from the left, it repositioned itself to the right side before latching on.1PubMed Central. A Note on Suckling Behavior and Laterality in Nursing Humpback Whale Calves from Underwater Observations Five observations is a small sample, so researchers are cautious about drawing broad conclusions, but laterality preferences have been documented in many other contexts in whales and dolphins. It is plausible that a consistent approach side helps mother and calf coordinate in low-visibility water without wasting time or energy on awkward repositioning.
The Energetic Cost to Mothers
Producing that rich, fatty milk comes at an enormous cost. Many baleen whale species nurse while fasting or eating very little, because their breeding and calving grounds are in warm tropical or subtropical waters where food is scarce. The mother is essentially converting her own body reserves into milk. A study of humpback whale mothers and calves found that mothers lost an average of about 97 kilograms per day while fasting and nursing, an energy expenditure equivalent to roughly 2,641 megajoules per day.3PubMed. Maternal investment, body condition and calf growth in humpback whales To put that in perspective, that daily energy output is roughly equivalent to what the mother would get from consuming over 800 kilograms of krill. She is burning through her blubber and muscle at a punishing rate so her calf can grow fast enough to survive the migration to colder, food-rich feeding grounds.
This is why body condition matters so much for whale mothers heading into the breeding season. A mother who arrives at the calving grounds with insufficient fat reserves may not be able to sustain lactation long enough for her calf to build the blubber it needs. Climate-related shifts in prey availability at the feeding grounds can have cascading effects on calf survival months later and thousands of kilometers away, because the entire nursing period runs on energy the mother banked beforehand.
Baleen Whales vs. Toothed Whales
Not all whales nurse the same way, and the biggest divide is between the two major groups: baleen whales and toothed whales. Baleen whales (humpbacks, blue whales, right whales, gray whales) tend to have intense but relatively short lactation periods. Their calves grow at explosive rates and are typically weaned within about six to twelve months. The mothers endure a punishing fast-and-nurse cycle, pouring energy into the calf as fast as they can.
Toothed whales and dolphins take a very different approach. Their lactation periods are much longer, typically one to three years, and the mothers continue to feed throughout.4PubMed. Lactation in whales and dolphins: evidence of divergence between baleen- and toothed-species This makes intuitive sense given their ecology: many toothed whale species live in stable social groups, hunt cooperatively, and have complex learned behaviors that take time to develop. A longer nursing period provides not just nutrition but an extended window for the calf to stay close to the mother and learn foraging techniques, social rules, and navigation.
Some species push this even further than researchers expected. Northern bottlenose whales, a deep-diving beaked whale species, were originally thought to wean their calves within the first year based on a single stomach-contents observation. But a more comprehensive study using chemical analysis of tooth layers from 50 individuals found that weaning does not typically finish until the calf is between three and four years old.5PLoS ONE. Prolonged maternal investment in northern bottlenose whales alters our understanding of beaked whale reproductive life history That extended maternal investment was a surprise for a species in a family (beaked whales) where almost nothing about nursing had been documented, and it suggests that prolonged nursing may be more common among deep-diving toothed whales than the sparse data had indicated.
Why Nursing Mothers Go Quiet
Nursing is a vulnerable time. The mother is moving slowly or resting at the surface, and the calf is small and clumsy. Predators, especially killer whales and large sharks, are a real threat on calving grounds. One adaptation researchers have documented is that mother-calf pairs become acoustically cryptic during this period, meaning they get dramatically quieter.
A study of North Atlantic right whale mothers on their calving grounds found that mothers produced a much higher proportion of quiet, low-amplitude calls compared to pregnant females or juveniles in the same habitat.6PubMed Central. Acoustic crypsis in communication by North Atlantic right whale mother–calf pairs on the calving grounds The interpretation is straightforward: loud calls travel far through water and could alert predators to the presence of a vulnerable pair. By whispering, in effect, mothers reduce the risk that their calls will be eavesdropped on by killer whales patrolling the area. This acoustic crypsis has been documented in humpback whales as well, and it fits a broader pattern seen in other species where vulnerable young are kept quiet to avoid detection.
The calves themselves also seem to vocalize less. The tag-based study of humpback neonates noted that both mothers and calves minimized sound production during nursing, keeping the acoustic environment around the pair as muted as possible. For the calf, which has not yet developed the speed or stamina to outrun a predator, staying quiet is one of the few available defenses.
Nursing From a Non-Biological Mother
In most whale species, a calf nurses exclusively from its own mother. But allosuckling, where a calf takes milk from a female other than its biological mother, does occur. It had been documented in various land mammals and in some dolphins, but until recently it had not been quantified in baleen whales. A study of southern right whale calves off Australia observed allosuckling during social interactions between multiple mother-calf pairs. One calf nursed from both of the non-biological mothers present in the group. The time spent allosuckling was relatively small, averaging about 4 percent of interaction time, compared to 8 percent spent nursing from its own mother.7Mammalian Biology. Allosuckling in southern right whale calves
Interestingly, the calf that allosuckled was the largest in the group, from the largest mother. This runs against the idea that allosuckling is a sign of desperation or poor maternal care. It may instead reflect opportunistic behavior by a healthy, bold calf exploiting close-quarters social situations. Whether the non-biological mothers tolerated it willingly or simply did not resist is harder to determine. Allosuckling in whales remains poorly understood, but its documentation in a baleen whale species is one more reminder that whale social dynamics are richer than they appear from the surface.
What Passes Through the Milk Besides Calories
The fat-rich composition of whale milk is critical for calf growth, but fat is also where many industrial pollutants accumulate. Persistent organic pollutants, including compounds like PCBs and certain pesticide residues, are lipophilic, meaning they dissolve in and concentrate within fatty tissues. When a mother whale converts her blubber into milk, those stored contaminants come along for the ride.
A study of gray whales off the eastern North Pacific confirmed this process for the first time in that species. Researchers found that pollutant concentrations in nursing calves were statistically similar to or even higher than those in their mothers for 15 of the modeled compounds, confirming that significant contaminant offloading occurs through milk.8PubMed Central. Influence of Life-History Parameters on Persistent Organic Pollutant Concentrations in Blubber of Eastern North Pacific Gray Whales (Eschrichtius robustus) Since all the calves in the study were sampled during nursing season and expected to be feeding exclusively on milk, the mother’s milk was effectively the sole source of these chemicals.
This creates a pattern where firstborn calves tend to receive the highest pollutant loads, because the mother has been accumulating contaminants for her entire life up to that point and has not yet offloaded a portion through a previous pregnancy and lactation. With each successive calf, her stored burden decreases slightly. For populations already stressed by habitat loss, noise pollution, and ship strikes, the additional burden of industrial chemicals passed through nursing adds another layer of pressure. It is a grim irony that the very mechanism designed to give calves the best possible start in life also serves as a delivery system for some of the most persistent toxins in the ocean.
Why This Has Been So Hard to Study
Almost everything described above has been painstakingly pieced together from brief glimpses. Whale nursing happens underwater, often at depth, between animals that can be 10 to 15 meters long, in water that may have limited visibility. Researchers cannot exactly set up a nursery camera. Early understanding of whale lactation came largely from whaling-era examinations of killed animals, which could reveal mammary gland anatomy and milk composition but nothing about behavior.
Modern tools have changed the picture considerably. Suction-cup biologging tags, which attach temporarily to a whale’s skin and record depth, orientation, and sound, have allowed researchers to track the dive profiles and body positions associated with nursing without needing to see the animals at all. When a tagged calf repeatedly descends to the same shallow depth, hangs motionless with near-zero roll and pitch, and surfaces again after a consistent interval, that pattern maps to nursing with high confidence even if no human witnessed it. Underwater video from divers and remotely operated cameras has provided direct visual confirmation, including the footage of calves angling in at 30 to 45 degrees and milk clouds appearing in the water.
Drone technology has added another dimension, allowing researchers to monitor surface behaviors and body-condition changes over time. Photogrammetry from drones can track how much a mother’s body shrinks over the nursing season and how much the calf grows, connecting behavioral data to the energy-transfer story. Chemical techniques have also advanced: the tooth-layer isotope analysis used to determine weaning age in northern bottlenose whales, for instance, let researchers reconstruct a nursing timeline from animals that had been dead for years, bypassing the near-impossibility of observing deep-diving beaked whales nursing in real time.
Even with all these advances, sample sizes remain small for many species. The five filmed humpback nursing events that revealed laterality preference are five events, not five hundred. The gray whale contaminant study relied on biopsy darts fired from small boats. Every data point in whale nursing research represents hours or weeks of fieldwork in remote, expensive, and physically demanding conditions. The picture is filling in, but much of what we know about how whales nurse comes from a handful of species in a handful of locations, with the expectation that many species-specific variations remain undocumented.