Despite their creepy, crawly appearance, some species of spiders make for exceptionally caring parents. Even more impressive, the jumping spider nurses its young much like mammals do, feeding their spiderlings nutritious milk packed with four times the amount of protein found in cows milk, according to a study published today in the journal Science.
Surprised? You’re not alone. Researchers at the Chinese Academy of Sciences first became interested in the odd behavior of Toxeus magnus when they noticed the spiders nest in the same way ants do, creating a space home to several spiders at a time.
"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 test it," said author of the study Chen Zhanqi in a statement.
The mother spider wasn’t seen to bring food back to the nest, but her babies continued to grow. So, the team did what any responsible scientist would do and grabbed a few microscopes. Upon closer observation, they could see droplets of nutritious fluid “leaking from the mother’s epigastric furrow" – a specially designed sexual organ found on the abdomen. Mum would deposit these milk droplets on the nest, where her babies could then come and suck them up. After the first week, she stopped depositing the droplets and instead allowed the spiderlings to suck directly from her. When researchers blocked milk production, the spiders stop developing and died, showing their “complete dependence on the milk supply.”
This behavior was observed both in a laboratory and field setting until the spiders were at least 20 days old, at which point spiders are able to forage for themselves, and generally lasted until the spiders reached their sub-adult age around 40 days. During this time, mom was also seen taking care of the nest and helping her babes shed their exoskeletons (aw, cute).
When given both maternal care and milk, more than three-quarters of the hatched offspring survived to adulthood and reached a normal body size. Though mum treated all her babes the same, she only allowed her daughters to return to the nest after reaching sexual maturity. Adult sons were attacked if they tried to come home, probably to reduce the likelihood of inbreeding.
Lactation is a trait associated with mammals – so why would arachnids develop such a trait? The researchers hypothesize it likely evolved in response to predation risk, uncertain food access, and a way to survive in harsh living environments. And if spiders do it, what else?
"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 reevaluation of the evolution of lactation and extended parental care and their occurrences across the animal kingdom."