Researchers in China's Yunnan province made a surprising discovery last year while studying the jumping spider species Toxeus magnus — nursing moms were feeding their offspring what looked like milk well into their teens. And when the supply was cut off, spiderlings younger than 20 days died.
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“Though other cases of milk-like secretions have been found, this combination of ongoing maternal care has largely been considered a uniquely mammalian trait,” behavioral ecologists Chen Zhanqi and Quan Rui-Chang wrote in the study published Friday in the journal Science.
Researchers suggest the liquid is made from unviable eggs prematurely passed through the birth canal and turned into food for surviving offspring.
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According to the study, females of the species secreted the milk, which contains fat, sugar and 123.9 mg/ml of protein (four times more protein than cow’s milk), as soon as eggs hatched and continued supplemental nursing until they were sexually mature. That’s about 40 days of life.
"She had to invest so much in caring for the baby," Chen told Science Magazine.
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When he cut off the milk supply by painting over the mothers’ birth canals, Chen found all spiders younger than 20 days died. Removing the mother from the nest altogether led to an increase in deaths among older spiders who hunted outside the nest, but still fed on their mother’s milk.
This kind of long-term care between parent and young doesn't typically exist among "long-lived social vertebrates" such as elephants or humans, Quan said. "The extended maternal care indicates that invertebrates have also evolved [this] ability."
While lactating milk is technically a mammalian concept, Quan and Chen believe this “compares functionally and behaviorally to lactating.”
Explore more from the study, currently behind a paywall, at science.sciencemag.org.
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