The Eavesdropper by Eugene de Blaas (1843-1932).
Image in the Public Domain due to age.
Human history is full of examples of eavesdropping, from neighbors snooping on the party line to the FBI scouring the Gmail account of Gen. David Petraeus. In a post on her blog Brain Pickings, Maria Popova takes a peek at John L. Locke’s book Eavesdropping: An Intimate History (Oxford University Press, 2010) for some explanation of its causes. Animals do plenty of eavesdropping, too, to avoid predators, defend their territories, and assess the fitness of their mates. Here are some ways that beetles, bats, and canaries snoop for information. (But don’t tell anybody you heard this from me.):
Margaridisa sp. Flea beetles normally like to chomp away on leaves from the pink flowering Cordobán tree (Conostegia xalapensis). But the presence of aggressive ants seems to put a damper on their appetite.
To test if these beetles eavesdrop on the communication pheromones of ants in order to avoid them, David Gonthier, an environmental sciences researcher at the University of Toledo, coated the underside of some leaves with “crushed ant liquids” from the species A. instabilis and dotted others with only water. His results appear in the open-access journal PLOS One.
The control leaves sustained 3.8 times more damage and had three times more beetles on them than the ant-marked leaves. In addition, two experiments comparing control leaves with leaves that had previously been “patrolled” by A. instabilis and another ant species, C. textor, found about twice as much damage among the control leaves.
The researcher found no significant differences in leaves that had been patrolled by a third ant species, S. geminata, but more experiments may be needed to understand those results.
Bats use echolocation, releasing short wavelength signals that bounce off other objects, in order to find food and orient themselves, as well as to socialize. By eavesdropping on these signals, bats can glean information from others. One study in Proceedings of the Royal Society Bby scientists from Germany’s University of Ulm and other institutions looks at the extent to which bats can gather social information, such as the sex of the caller, from these vocal signatures.
Greater sac-winged bat (S. bilineata).
Felineora. Creative Commons.
Their study involved the greater sac-winged bat (Saccopteryx bilineata). Even though they mate only during a brief time in December, males of this species defend their daytime roosting space from other males and try to attract females to it throughout the year.
After capturing bats with mist nets, researchers banded individuals and released them one at a time in order to record their distinctive echolocation calls. Then they recorded the responses of roosting males as individual bats were released, one by one, near the colony’s roosting site. All of the roosting males crooned courtship songs to approaching females and warned off approaching males with territorial vocalizations.
Due to the distance and darkness, it is unlikely that scent or visual information helped these males tell the difference between male and female bats, the study concludes. “Through passive information transfer and eavesdropping, echolocation calls play a crucial and hitherto underestimated role for social communication in a highly mobile and gregarious nocturnal mammal.”
Eavesdropping doesn’t always generate reliable information, however—especially when the subject knows they’re being watched.
Domestic canary (Serinus canaria).
L.E. MacDonald. Creative Commons.
A study published in PLOS One on socially-monogamous domestic canaries looked at the effect of having an audience on males’ behavior. Researchers from Paris West University Nanterre la Défense paired up 21 male canaries and 21 female canaries in cages, allowing them to mate. Later they placed those same male canaries in cages with different, but familiar, females, with whom they could interact. These interactions either took place in front of another empty cage or a cage containing a familiar female or their own mate. When there was no audience, the male canaries courted the most. They courted the least in front of their own mates.
“These results show that male domestic canaries can adjust their behaviour according to the social bond they share with the audience,” the study states. “Indeed, subjects courted less in the presence of their mate than in the presence of a familiar female. This suggests that males suffer costs while engaging in extra-pair behaviours in the presence of their mate.”
A second eavesdropping experiment found that male canaries competed more aggressively against other males for food when they had an audience. The presence of a mate or a familiar female led to more attacks on the competing male.
The study concluded: “One could assume that males losing a contest would suffer a decrease in their reproductive success in both situations: eavesdropping familiar females would not choose them as sexual partners … while eavesdropping mates would engage more in extra-pair copulations … ”