Though there are many social animals who live in groups, or protect their growing children, we still aren't sure how social behavior really began. Now we may have a clue — and it comes from observing the way a humble earwig shares food with its brothers and sisters.
Photo via Mathias Kölliker
What is it that drives humans to make great movies together, or to collaborate across great distances to discover a new planet in a distant star system? And what evolutionary process caused crows to come together in small groups, to help each other find food? It all has to do with being social animals, creatures whose lives depend (at least some of the time) on other members of the same species for survival.
There are many ways to be social, ranging from simply taking care of young to forming intense, hierarchical societies. The question is how all of these behaviors arose. What is the smallest, most primitive unit of social behavior? Zoologist Joël Meunier and his colleagues are trying to find out, and they recently published the results of their work in The American Naturalist
Forficula auricularia, known as the European earwig, was their research subject. F. auricularia has what the researchers believe is very primitive social behavior. The mothers in this species care for their young, called nymphs, for several weeks after they hatch — you can see this behavior in the picture above. In fact, as the authors of this new study point out, many evolutionary biologists trace social behavior in all animals back to parental care.
You might say that a mother caring for her offspring could be the seed that evolved into a broad range of social behaviors, which range from dogs hunting in packs, to birds sharing a nest and humans conducting science experiments collaboratively. But our intrepid earwig researchers believe they've found another model for social behavior: sibling cooperation.
Earwig siblings are known to kill and eat each other, especially when food provided by the mother is scarce. But, as Meunier and his colleagues observed, earwig nymphs also share food with each other, especially when their mother isn't around.
Here we show experimentally that sibling interactions also reflect cooperative behaviors in the form of food sharing in nonderived families of the European earwig, Forficula auricularia. Food ingested by individual offspring was transferred to their siblings through mouth-to-anus contacts and active allo-coprophagy. These transfers occurred in both the presence and the absence of the tending mothers, even though the direct contact with the mothers limited sibling food sharing.
In case you had any doubts about what the researchers are describing, I'll spell it out for you. One of the most fundamental acts of sharing and sociality could be giving poop to your siblings to eat. Indeed, most of this food sharing was done via "mouth-to-anus contacts," which gives the movie Human Centipede a strangely altruistic new gloss.
Meunier and colleagues suggest that social behavior developed out of these simple food-sharing practices. What's important is that our earliest models for sociality may not be just parents sacrificing for the next generation, but also siblings cooperating to keep the whole family fed.
Conclude the researchers:
[Because F. auricularia probably reflects] an early intermediate level in the transition from solitary to highly developed forms of social life, our findings also provide evidence that the evolutionary forces promoting the emergence and the persistence of parental care and social life do not (necessarily) rely only on the benefits of parental care for offspring but may also involve the benefits of cooperative sibling interactions—possibly even before the evolution of parental care.
One of the most beautiful parts of human life — our ability to cooperate and create amazing things together — may have started long before we evolved, with a little poop sharing between brothers and sisters.
You can read the full study at American Naturalist