There are many animal species that hunt in packs to kills their prey, especially when the prey is large. Examples of animals that hunt in packs include wolves, lions, dogs, hyenas, and ants.
Pack hunting is also called cooperative hunting because each individual animal in the pack cooperates with the rest of the pack for the same purpose.
A new 2021 research study shows that some spider species also practice cooperative hunting to help each other to catch their prey. Researchers at the University of Toulouse in France observed two colonies of a social spider species called Anelosimus eximius. The researchers copied the actions of prey entering a spider web – they created vibrations in different parts of the webs – and filmed the responses of the spiders.
The researchers already knew that not all spider species are solitary. They knew that there were social spiders that lived in groups of thousands of individuals on enormous webs. Of the 50,000 known spider species, just one or two species of them can hunt as a group.
The Social Spider (Anelosimus eximius) is an arachnid in the Theridiidae family of social spiders. The colonies are found in South America in countries from Panama to Argentina.
After studying the social spiders, the research team, led by Raphael Jeanson, found that social spiders synchronise their movements to catch prey hundreds of times larger than themselves and up to 700 times heavier than each individual spider.
The webs of social spiders are not sticky, so the web does not trap an insect. The spiders have to move quickly to catch the insect in the web.
They found that spiders (arachnids) that hunt in packs use web vibrations to coordinate their attacks. When an insect (the prey) lands on a web, the spiders synchronise their movements to attack the much larger insect.
The synchronisation included the spiders moving and stopping, and starting again, all at the same time. The stopping time corresponded to the amount of ‘noise’ in the web, according to computer models of the spiders’ movements. The spiders stayed still for as long as they had to so that they could feel the vibrations of their prey and their fellow spiders. The pack of spiders could tell the difference betweent the movement of other spiders and the movement of the insect.
Jeanson said, “It’s like when there are lots of people talking in a crowded room and then there’s this other noise, like a telephone that rings, and everyone has to hush to find the source of the noise. Of course, the louder the telephone is ringing, the less people have to be quiet to find the phone. It’s the same thing with these social spiders.”
He said, “Depending on the size of the prey and the vibrations that the prey creates on the web, the spiders have to be more or less quiet and be still in order to localise the prey without getting disturbed by the vibrations of other spiders that are moving around.”
The scientific researchers did not notice a leader in the pack of spiders. When all of the spiders stopped, the group starts moving again when one or two individuals moved. “We don’t know how it works exactly, but when one of them moves, it sets them all moving. It’s really a snowball effect,” said Jeanson.
The pack of spiders, during cooperative hunting, caught butterflies, grasshoppers, and other flying insects. Because the web was not sticky, the spiders acted quickly to stop the flying insects from escaping.
The researchers said that if the social spiders all arrived at the same time to the vicinity of the insect then there is strength in numbers, which is more beneficial that random individual spiders trying to catch a large insect. “There is a clear advantage of synchronisation despite the costs of the waiting time,” said Jeanson when discussing the time that the spiders wait for each other to synchronise.
Another interesting observation was that the social spiders also synchronised the timing of ejecting an immobilising ‘glue’ from their hind (back) legs to trap the insect, which allows the spiders to bite their prey to inject venom to kill it.
Journal reference: PNAS, DOI: https://doi.org/10.1073/pnas.2115103119
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM