They hunt as a group, taking coordinated action to subjugate the prey, but when it comes to eating, they prioritize their offspring. When attacked by a predator, they form a circle around the defenseless young. Though this description might remind one of a group of lions, the animals in question lack the notoriety of the large felines. They are the pseudoscorpions of the Paratemnoides nidificator species, arachnids that live in colonies with as many as 30 or 40 individuals – sometimes more than 100 – beneath the bark of trees on the Cerrado savannas. For the last seven years, Kleber Del Claro and Everton Tizo-Pedroso, from the Federal University of Uberlândia, in Minas Gerais, have investigated the complex social behavior of these animals. They proposed, in an article published in 2009 in Acta Ethologica, that the extreme maternal act of offering the mother’s own body as a meal to the famished young was an essential step in the evolution of the society of the pseudoscorpions, going as far as to question the generally accepted definitions of social systems.
Pseudoscorpions are arachnids some five millimeters long, with two pincers that make them look like miniature scorpions. However, they lack the tail with a sting on the end. A colony starts whenever a female expels her offspring from the web nest where she raised them in order to produce her next brood. These youngsters, the nymphs, then build their own cocoons right there to finish growing and, together, they maintain the order of that small universe within the tree bark, to which similar but unrelated insects may also move. After hunting, whether they are parents or not, the adults stand back and allow the young to eat first.
“These animals share the same area, hunt as a group, share their food with their young and defend them collectively,” explains Del Claro. “It is undeniable that they have achieved an advanced level of sociology.” He disagrees with the definition of the apex of sociology, historically based on bees, whose peculiar system of sex determination essentially results in colonies of females – all of them very similar genetically. “There is no reason for us to uphold this definition,” he argues, as he explains that the Paratemnoides nidificator have an inherently gregarious nature. “They have no personal space restrictions; there is no aggression, even among non-related pseudoscorpions,” he tells us. According to him, no other arachnid has such characteristics, given that even social spiders maintain separate spaces in the community web and attack their neighbors when they come too close.
Just a few years ago, virtually nothing was known about the behavior of these discreet arachnids. Everything changed when Tizo-Pedroso, an undergraduate student with higher than average curiosity, started looking for a promising project in order to learn from Del Claro how to be a researcher. Though his search was not limited to scientific publications, which is where one finds what is already known, he did not have to go far: just up to the sibipiruna tree standing in front of the Biology Institute of the Federal University of Uberlândia. Beneath the bark of this tree (Caesalpinia peltophoroides) ,which has yellow flowers and is common on the Cerrado savanna, and which is also planted in cities such as São Paulo, the young aspiring biologist found a group of invertebrates with which he was unfamiliar and which produce round silk cocoons. He then discovered that he was not the only one who knew nothing about the pseudoscorpions. This is how the project arose that will lead to Tizo-Pedroso’s PhD and that has already given rise to a series of scientific publications.
The behavioral observations were conducted in the laboratory, where the two researchers managed to set up colonies inside glass chambers, covered by the tree bark that they inhabited, along with a set of mirrors designed to allow observation of what was going on. The major discovery, published in 2005 in the Journal of Arachnology, resulted from an experiment in which they left a mother and her brood for some days with nothing to hunt. When the famished nymphs started to attack each other, some mothers left the silk chamber, knocked their pincers on the ground and then extended them upward, whereupon the young promptly started eating the mother, who did not react in any way. Such a meal is sufficient to do away with the rivalry between the siblings: the group starts to hunt together and forms a stable society. However, this supreme dedication does not always materialize. The younger females, which presumably still have plenty of time left to reproduce, actually consume their own young when there is nothing to eat.
According to the observations that are currently being analyzed for publication, these pseudoscorpions have an organized distribution of tasks – another indication of an advanced society. The females look after the young and hunt; the males hunt, defend the nest and do the cleaning; the larger nymphs also help with the cleaning, removing waste from the chambers. “If the work is validated by the journal’s reviewers, it will be yet one more feature that makes these pseudoscorpions stand out from other arachnids,” comments Del Claro.
Another curiosity identified by the two researchers from Uberlândia was one of the means whereby the P. nidificator gains new ground: by getting a ride on their own prey, as the researcher described in 2007 in Insectes Sociaux. When they detect prey that is far larger than themselves, such as a beetle, a bedbug or a wasp, the pseudoscorpions undertake a mass attack. Several of the small arachnids hang onto the prey, holding the victim’s legs with their venom-injecting pincers. After a terrified flight, the debilitated prey lands on another tree and dies. This is where a new colony is established, with the first meal already guaranteed.
Kleber Del Claro and Everton Tizo-Pedroso have barely started to explore the world that they found under the sibipiruna tree bark. They continue to research this species’ society, including genetic analyses that reveal that the groups do not only hold relatives. They have also tried to find new pseudoscorpions to study and have identified some in the Caatinga scrub. Now they plan to collect them in the Canastra hills, in a region where the vegetation grows on the rocks, an environment known as rupestrine. They are almost maternally proud of these animals they found by chance, but do not expect exclusivity. They have been seeking collaborators to expand knowledge. This includes an Argentinean group that specializes in the (systematic) classification of arachnids. “Everton will also become a pseudoscorpions systematist,” his advisor forecasts.
DEL CLARO, K. e TIZO-PEDROSO, E. Ecological and evolutionary pathways of social behavior in Pseudoscorpions. Acta Ethologica. v. 12, n.1, p. 13-22. abril 2009.