“If I’m going to have a high-calcium, high-phosphorus milk, I have to have a high-protein milk,” Dr. Power said, “because a lot of that protein is a calcium-phosphorus delivery device.”

Olav Oftedal, now an emeritus researcher at the Smithsonian Institution, and his colleagues sought to estimate milk consumption rates among black bear cubs as they nursed in a den. The researchers were startled to detect in the mother bear’s blood and milk traces of the distinctively labeled water samples they’d earlier given to the cubs as part of the experiment.

There was only one way the water isotopes could have ended up in mother’s milk. “She’s lactating in a den,” Dr. Oftedal said. “She’s not eating or drinking. But she is consuming all the excreta of her young, which she then puts back in her milk.”

Small wonder, then, that the amount of milk the denning mother produced exceeded the weight she lost. “She’s recycling everything,” Dr. Oftedal said.

Expensive lactation

Biologists warn against the wanton use of the word milk — sorry, almond “milk” really isn’t — and some mammalogists would like to restrict the term to the secretions of a dedicated mammary gland, which only their study subjects happen to possess.

But many scientists concur that if a parent synthesizes or highly modifies a substance on which its offspring’s life then depends, that parent is making a milk. By this measure, predigested food alone may not count, but if the parent first adds essential ingredients to the bolus, the regurgitate can fairly be deemed a milk.

Sandra Steiger of the University of Bayreuth in Germany and her colleagues recently reported on an American species, Nicrophorus orbicollis — a handsome, inchlong burying beetle with orange and black stripes — in which both parents care for their young. A parent beetle will eat a bit of carrion, predigest it and, on being tapped on the mouth by an offspring’s front legs , transfer the morsel into the little supplicant’s mouth.