Got Milk? These Spiders Do

Scientists have discovered a species of jumping spider that is capable of producing milk for its offspring, a trait previously thought to be unique to mammals.

AsianScientist (Dec. 11, 2018) – Scientists in China have discovered a spider that produces milk, a characteristic that is highly unusual in non-mammalian organisms. They published their findings in Science

Lactation refers to the production and secretion of milk for the young. This attribute is one of the defining traits of mammals, which include dogs, monkeys and humans.

However, in this study, scientists led by Dr. Chen Zhanqi of the Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, has discovered a jumping spider—Toxeus magnus—that produces milk for its offspring. The researchers observed that this species of jumping spider built breeding nests within which two or more adults, or one adult female and several juveniles, resided.

“It’s a puzzling observation for a species assumed to be noncolonial. It’s possible that the jumping spider might provide either prolonged maternal care or delayed dispersal. We decided to study [this behavior] more deeply,” said Chen.

The researchers assessed how offspring developed and behaved under maternal care both in laboratory conditions and in the field. They found that no spiderlings left the nest for foraging until they were 20 days old. Closer observation revealed that the female parent provided a seemingly nutritious fluid to the spiderlings.

According to the scientists, milk provisioning in T. magnus involves a specialized organ that functions over an extended period. They noticed droplets of milk leaking from the mother’s epigastric furrow, near its abdomen, where the spiderlings were suckling.

The spiderlings would ingest these nutritious milk droplets until the subadult stage (around 40 days). If prevented from obtaining milk, the newly emerged spiders would stop developing and die within ten days, indicating to the researchers that the mother’s milk is indispensable for offspring survival.

Milk provisioning after 20 days did not affect adult survivorship, body size, sex ratio or development time, but the mother’s presence played a key role in assuring a high adult survival rate. Although the mother apparently treated all juveniles the same, only daughters were allowed to return to the breeding nest after sexual maturity. Adult sons were attacked if they tried to return. This may reduce inbreeding among the spider population.

“Our findings demonstrate that mammal-like milk provisioning and parental care for sexually mature offspring have also evolved in invertebrates,” said Chen. “We anticipate that our findings will encourage a re-evaluation of the evolution of lactation and extended parental care and their occurrences across the animal kingdom.”

The article can be found at: Chen et al. (2018) Prolonged Milk Provisioning in a Jumping Spider.


Source: Chinese Academy of Sciences; Photo: Chen Zhanqi.
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