A Cat on a Cow’s Back and Exploding Paper Bags Showed How Fear Can Shut Off Milk Flow

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The milk that had already collected in the udder drained out. After that, the flow practically stopped.

The startling setup was part of a serious investigation into how cows “let down” their milk, and it helped establish that milk release depends on a hormone in the blood rather than direct nerve signals. The researchers, Fordyce Ely and William E. Petersen, received the 2024 Ig Nobel Prize in Biology “for exploding a paper bag next to a cat that’s standing on the back of a cow, to explore how and when cows spew their milk.” Ely’s daughter and grandson attended the ceremony.

Ely worked at the Kentucky Agricultural Experiment Station and Petersen at the Minnesota Agricultural Experiment Station. They first reported their findings in a 1939 abstract in the Journal of Animal Science and later published the full study, titled Factors Involved in the Ejection of Milk, in the Journal of Dairy Science.

The 1939 abstract laid out the core result. Frightening a cow, or injecting adrenaline into a vein, prevented milk ejection. Injecting extracts of the posterior pituitary gland, the source of the hormone oxytocin, brought ejection back about 30 seconds later.

The cat turned out to be unnecessary. According to the full paper, it was later dropped from the protocol, and the exploding paper bags alone did the job.

The experiment’s most important design choice was less theatrical. The main cow in the study had the nerves to the left half of her udder surgically cut. If milk release depended on direct nerve signals, the two halves should have behaved differently.

They did not. The denervated half ejected milk as readily as the intact half and responded the same way to fright. The researchers concluded that letdown is a conditioned reflex driven by a hormone carried in the bloodstream.

After the fright test, workers hand-milked 11 pounds from the cow 30 minutes later. Her next regular milking came in 3 to 5 pounds above normal, suggesting the milk had been held back rather than lost.

Adrenaline produced a similar block. An injection stopped milk ejection, and an hour later workers could remove milk by hand only with considerable effort. A saline injection had no such effect. The cow trembled, refused her feed, and her udder became hard, and the researchers capped the dose in later trials. A second Jersey cow responded in much the same way. When a cow was given a pituitary extract after a normal milking, she released extra “strippings” unusually high in fat, about 17% fat in one test compared with 4% in her regular milk.

Later research refined how stress interferes with milk flow. A 1993 study by Rupert Bruckmaier and colleagues in the Journal of Dairy Research milked eight cows in an unfamiliar operating room and in their home barn. Only 9% of the milk was removed in the strange setting, compared with 79% in familiar surroundings. Oxytocin stayed at baseline levels in the unfamiliar room, which the authors attributed to the brain inhibiting oxytocin release.

That points to a second mechanism beyond the one Ely and Petersen highlighted. Stress can act in the brain to keep oxytocin from being released, in addition to adrenaline’s effects in the udder itself. The 1941 authors were candid about the limits of their own evidence, describing their case for muscle fibers around the milk-producing sacs of the udder as circumstantial but convincing.

For dairy farmers, the lesson is practical. Calm, predictable milking routines help cows release their milk fully.

The same hormone runs the letdown reflex in people. Oxytocin released when an infant suckles causes muscle-like cells around the milk glands to contract, as described in the National Library of Medicine’s StatPearls review of lactation physiology.

Stress may interfere with that reflex in humans too, though the evidence is more mixed. A 2022 narrative review in Clinical Therapeutics summarized an older study in which 22 women exposed to noise or mental arithmetic five days after giving birth had reduced and delayed oxytocin release during breastfeeding. Studies of depression, anxiety and oxytocin levels have produced inconsistent results, and the review concluded that the mechanisms remain unclear.

Clinicians still note that stress can make letdown harder. “Stress can prevent your oxytocin release, so finding ways to manage stress can be helpful,” Dr. Heidi Szugye, a pediatrician and breastfeeding medicine specialist, told the Cleveland Clinic in 2023. Parents who have ongoing difficulty with milk letdown or supply should talk with a pediatrician, obstetric clinician, or certified lactation consultant.

More than 80 years later, a cat, a cow and a paper bag remain a memorable illustration of a basic biological rule: the body can hold milk back when it senses danger.

What did the cat-and-cow experiment show?

Frightening a cow with a cat on her back and exploding paper bags stopped milk ejection once the milk already in the udder had drained.

Why did researchers cut nerves to half the udder?

To test whether milk release depended on nerves. Both halves responded the same way, pointing to a hormone in the blood instead.

Oxytocin. In the experiment, posterior pituitary extracts restored milk ejection about 30 seconds after injection.

The researchers linked it to adrenaline. Later work showed stress can also keep the brain from releasing oxytocin.

Does stress affect breastfeeding in humans?

Some studies suggest stress can reduce oxytocin release during nursing, but the evidence is mixed, and the mechanisms are not fully understood.