Found: a lactating spider

A new study reveals a species of jumping spider in which newly hatched young are entirely dependent on a maternally excreted milk-like substance. Suckling continues for 40 days – well beyond the point at which the young can forage for themselves.

The feeding method, write researchers led by Zhanqi Chen from the Chinese Academy of Sciences in Yunnan, “compares functionally and behaviourally to lactation in mammals”.

Toxeus magnus is a very small black spider that mimics ants.

The species aroused the interest of Chen and colleagues because it is often found in nests comprising several large individuals, including grown juveniles, yet is not known to form colonies. The researchers wondered if the nests indicated extended maternal care – unusual among arachnids.

The mystery deepened when the researchers set up a spider nest in their laboratory and noted that for their first 20 days spiderlings did not leave the nest – and their mother was never seen bringing food in.

Closer examination revealed that the mother T. magnus was producing a white fluid from a groove on the underside of the abdomen, known as the epigastric furrow. For the first 10 days of their lives, the spiderlings consumed droplets of this fluid deposited on the nest wall, and thereafter suckled directly from the mother.

Providing milk for young is one of the defining features of mammals, but the provision of specialised food sources for offspring is found in a surprising variety of animals. Parent fish feed their young their own epidermal mucus, some amphibians and invertebrates produce unfertilised eggs for consumption, and types of excreted liquid have been used by parent cockroaches and birds including pigeons, flamingos and penguins.

“However, these other foods are very different from mammalian milk in both duration of provisioning and degree of parent-offspring interaction,” the researchers note in a paper published in the journal Science.

Chen’s team could see that milk-provisioning in T. magnus was remarkably similar to mammalian lactation. Unlike other specialised food sources, spider milk is supplied to the young from hatching until near adulthood, and is accompanied by a high degree of maternal care. The team carried out a series of field and laboratory experiments to find out just how crucial milk provision and parental attention is to the species.

When provided with milk and maternal care, including nest repair and clean-up, 76% of spiderlings survived to adulthood. When the epigastric furrow of the mother was blocked with correction fluid just after hatching, the spiderlings stopped developing and died at approximately 10 days.

Clearly the milk produced is essential for the survival of young jumping spiders at the early stages. The white liquid is highly nutritious, with four times the protein of cow’s milk. But why continue suckling after 20 days, by which time the juveniles are foraging outside the nest?

A 40-day lactation is approximately double that seen in mice and rats, which wean at about 21 days. Extended lactation periods and parental care of offspring until sexual maturity is most often seen in long-lived mammals such as elephants, chimpanzees and humans.

To test the importance of care and milk in juveniles, the team removed the mother spider at 20 days, or left her in the nest with a blocked epigastric furrow.

The authors found that while nursing was not critical to offspring survival after becoming independent, their mother’s presence greatly assured their overall health and adult survival. The provision of milk reduced the time spent outside the nest. The presence of the mother and her nest-care also seemed to reduce the parasite load on the spiderlings.

The mother’s presence also conferred a genetic advantage. When she was there, newly adult male spiders were more likely to disperse from the nest, thus avoiding inbreeding.

“These findings demonstrate that mammal-like milk provisioning and parental care for sexually mature offspring have also evolved in invertebrates, encouraging a re-evaluation of their occurrence across the animal kingdom, especially in invertebrates,” the researchers conclude.