Modern spiders are known to take care of their eggs and young – and now we have evidence that the behaviour is ancient.
Researchers at Capital Normal University in Beijing recently acquired four specimens of spiders trapped in Burmese amber, dated to 98.8 million years ago and mined in the Hukawng valley of northern Myanmar. In collaboration with at the Natural History Museum in London, the team analysed the pieces using photography and micro-computed tomography scanning.
All four specimens feature lagonomegopids, an extinct spider family known especially for their large eyes – which are similar to those of modern Salticidae jumping spiders, says Selden.
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In one specimen, a 7.5-millimetre-long female lagonomegopid clutches an egg sac filled with embryonic spiderlings wrapped in silk. “I imagine this female guarding her egg sac would just remain completely silent and quiet while she was then engulfed in this resin,” says Selden. “That’s really very exciting in terms of showing fossilised behaviour.”
The three other specimens contain 24, 26, and 34 recently hatched spiders – or spiderlings – respectively. In two of these specimens, the researchers also identified adult spider legs presumably belonging to the mother, and one of these two specimens also contained silk strands wrapped around pieces of wood and other debris – potentially remains of a spider nest.
The findings hint that spiderlings in this species lived as a family in a nest guarded by the mother, says Selden. The preservation of the mother’s legs in two of the amber specimens suggest she would choose to stay with her offspring even as tree resin slowly immersed the nest.
“It’s essentially being altruistic, I suppose, in biological terms, [when you’re] doing something that could be a danger to you in order to protect your offspring,” says Selden.
Inside one chunk of amber the researchers also found a cockroach, and in another there was a wasp. Whether these insects were caught by the mother spider to feed to her spiderlings isn’t clear, however, says Selden.
Either way, the new find adds to evidence that female spiders have the potential to be devoted mothers. In fact, says Selden, many researchers believe spider silk evolved initially as a material to protect spider eggs, and was repurposed as a way to trap prey at a later date.
“Female spiders are nearly always tremendously maternal,” he says. “That’s clearly the way they’ve evolved.”
Proceedings of the Royal Society B
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