(CN) – Mother jumping spiders lactate and care for their young into adulthood – behaviors previously associated only with mammals and long-lived vertebrates like whales and elephants, according to a study published Thursday in the journal Science.
Female Toxeus magnus, a jumping spider common in Asia, feed their young a “nutritious fluid” researchers dubbed “milk.” The substance contains four times the protein of cow’s milk, and spider mothers give it to their offspring until they’re “teenagers” at 80 percent of their adult size. Mothers also protect, teach and house their young until adulthood, with adult females sometimes continuing to live with mom as adults.
The findings challenge accepted ideas about the evolution of lactation and give an almost mammalian maternal cast to a creature many regard with an instinctive fear and disgust.
A team of researchers led by Zhanqi Chen at the Center for Integrative Conservation in Yunnan, China, wanted to know the degree to which the lactating mothers improved the survival rates of their young, and also whether their survival was increased by the spider mothers’ lengthy parental role.
Researchers collected 36 adult jumping spiders and observed their breeding and parenting habits in the lab. Some spider mothers were allowed to proceed normally, feeding their young the milk they secrete from a specialized gland called an epigastric furrow, teaching them how to hunt and avoid predators, and protecting and cleaning them into adulthood. In one picture from the study, five three-week old spiderlings are shown clustered around their mother, suckling. It looks a little like a pile of puppies vying for their mother’s milk.
For other spider mothers, the researchers covered their epigastric furrow with correction fluid on the day the spiders hatched. Another group was prevented from lactating when their young reached 20 days old – the age at which the spiders first begin to emerge from the nest to hunt. And for a third group, the mothers were removed from their young entirely when the spiderlings were 20 days old.
The results showed that normal mothering meant a high level of survival – 76 percent of spiderlings whose mothers fed them milk and cared for them into adulthood survived. Without it, the young spiders didn’t do so well.
Spiderlings who were prevented from consuming their mothers’ milk at birth soon died. Young whose mothers were unable to feed them milk after 20 days old died at a much higher rate, grew more slowly and left the nest more often. In the lab that meant venturing into a large plastic box to eat fruit flies. But in the wild, that change would expose them to predators at a younger age.
Those without mothers after 20 days died at higher rate and grew to be smaller adults.
Interestingly, spiderlings who made it to adulthood tended to be female, at a rate of 84 percent, according to the group’s findings. Although mothers seemed to treat daughters and sons the same as they grew, only daughters were allowed to return to the nest at night once they reached adulthood. If sons tried this, the mothers would attack them, the group found. That behavior probably results in less inbreeding, the group speculated.
Prior to this study, parents caring for their offspring into adulthood had been recorded only in long-lived vertebrate animals like whales, monkeys and elephants.