If you’ve ever watched ants, you’ve probably noticed
their tendency to “kiss,” quickly pressing their mouths together in
face-to-face encounters. That’s how they feed each other and their larvae. Now,
scientists report that the insects are sharing much more than food. They are
also communicating—talking via chemical cocktails designed to shape each other
and the colonies they live in.
The finding suggests that saliva exchange could play
yet-undiscovered roles in many other animals, from birds to humans, says Adria
LeBoeuf, an evolutionary biologist at the University of Lausanne in
Switzerland, and the study’s lead author. “We’ve paid little attention to what
besides direct nutrition is being transmitted” in ants or other species, adds
Diana Wheeler, an evolutionary biologist at the University of Arizona in
Tucson, who was not involved with the work.
Social insects—like ants, bees, and wasps—have long
been known to pass food to one another through mouth-to-mouth exchange, a
behavior known as trophallaxis. They store liquid food in “social stomachs,” or
crops, from which they can regurgitate it later. It’s how nutrients are passed
from foraging ants to nurse ants, and from nurses to the larvae in a colony.
Other research has suggested that ants also use trophallaxis to spread the
colony’s odor, helping them identify their own nest mates.
To see whether trophallaxis plays a role in ant
communication, LeBoeuf and her colleagues collected the trophallactic fluid of
Florida carpenter ants (Camponotus floridanus).
Using a range of chemical and biological methods, the researchers discovered
that this social fluid contains a diverse array of chemicals, including
growth-, digestion-, and immune-related proteins; nest mate recognition cues;
and a juvenile hormone, important for regulating ant reproduction, development,
and behavior.
The discovery of the juvenile hormone was particularly
unexpected. “[It’s] a big upset,” says LeBoeuf, explaining that previously
scientists thought the hormone was simply present in the insect’s bloodstream.
Instead, it appears that the adults are passing it “in a private communication”
to the larvae. “It suggests they’re deciding what the larvae are going to be
when they grow up.”
The scientists tested that idea experimentally. They
gave groups of 25 to 30 worker ants five to 10 larvae to rear, providing the
workers with food that was either supplemented with the hormone or a solvent.
Larvae that received the hormone-laced food were twice as likely as the
controls to mature into adults, and they grew into majors, the bigger of two worker castes. That
means that nurse ants must be selectively feeding larvae the hormone, depending
on what type of adult workers the colony needs, LeBoeuf says. In essence, the
nurses are “deciding the long-term shape and fate of the colony.” Adds Wheeler,
“It’s a way to integrate the colony,” to ensure it functions as a single
entity.
The researchers also analyzed the socially exchanged
fluid of three other species: two other ants and the honey bee. They found
similar and additional proteins connected to growth,
development, and maturation, they report today
in eLife.
Scientists used to think the molecules in social
insects’ regurgitated food were simply “byproducts,” says Ofer Feinerman, a
biophysicist at the Weizmann Institute of Science in Rehovot, Israel, who was
not involved in the study. The new work, he says, shows they instead play “an
active role in regulating the colony’s behavior, population structure, and
immune resistance.”
Ants, of course, aren’t the only animals exchanging
body fluids over a social network. “Sperm, breast milk, the saliva in a
kiss—there are so many social fluids that are so exciting and just waiting to
be explored,” LeBoeuf says. “Up to now, these have been private, privileged
forms of communication,” though she notes that semen is known to influence female physiology. Could
these fluids affect the receivers in other unexpected ways, influencing their
behaviors, relationships, and social standing? The ants’ answer is “Yes.”
