During mating rituals, animals usually communicate through touch, sight, smell, or hearing. Some species communicate in unconventional ways, relying on vibrational and electric signals rather than the more common sensory approaches. These unique signals are often invisible to other animals that do not possess the organs needed to perceive them. For instance, electric fish require specialized cells to send and receive messages, the absence of which renders other fish oblivious to their communication. However, sometimes predators may intercept these secret courting signals, and use it to their advantage.
The ghost knifefish, native to freshwaters of South America, is rumoured to be named after an ancient tribal legend proclaiming that ghosts of the dead inhabited these fish. Folklore aside, knifefish are in fact quite unusual. They have developed a unique way of finding mates in dark, turbid waters where their vision may be compromised: they communicate with each other using electricity.
Brown ghost knifefish (Apteronotus leptorhynchus) are weakly electric fish; they have a specialized electric organ in their tail that generates a signal called Electric Organ Discharge (EOD). One fish produces an electric field to send a message, and the receiver uses electroreceptors distributed over its skin to interpret the signal. The frequency and waveform of the message give it a unique code that conveys recognition, aggression, location of food, warnings of danger, and, in some cases, even courtship.
Male and female knifefish recognize each other from their unique EOD frequencies (EODfs). Males produce higher frequency signals in the range of 800-1100 Hz, whereas females emit signals in the range of 600-800 Hz. EODfs do more than just help these fish identify the opposite sex; they may actually be indicators of “male quality”. Female knifefish generally prefer males with high EODfs, perhaps because they tend to have larger bodies and gain precedence over others while competing for limited resources.
EODfs are regulated by hormones, which play an important role in courting behaviour. When levels of the male sex hormone 11-ketotestosterone (11-KT) are high, there is an increase in the emitted EODf, as well an accelerated development of male secondary sexual characteristics. This phenomenon has inspired the hypothesis that 11-KT regulates masculinization of the EODf in order to attract females. Similarly, the female sex hormone 17-beta-estradiol lowers EODfs.
Associate Professor Rüdiger Krahe in McGill University is an expert on communication in weakly electric fish and has recently demonstrated how the environment can manipulate mating behaviour in ghost knifefish.
In his experiment, male and female fish were housed in the same aquarium for a month, during which researchers simulated breeding conditions within the aquarium by imitating a rainy environment – knifefish prefer to breed in the rainy season. They recorded the electric signals produced by the fish using electrodes placed on the fish and in the water. They found that in the rainy environment males increased their EODfs, likely to attract females.
The same team also performed a second experiment in which they housed one group of males knifefish in isolation and a second group surrounded by other male and female knifefish. The males housed in a social environment had higher EODfs and 11-KT levels than those kept alone, suggesting that 11-KT plays a role in influencing male behaviour in a scenario where breeding is more likely to occur. Thus, ghost knifefish not only use electric signals to find their partners but also are capable of continuously adjusting the nature of these signals according to changing environmental conditions.
Spiders exploit sexual vibrational signals to kill insects
Some species of insects recognize their own by using vibrational signals that convey specific information. The male leafhopper, for instance, transmits low-frequency vibrational signals through plants to recognize, locate, and ultimately approach females. The insects possess specialized receptor cells in their legs to detect acoustic mating signals. However, unlike electric signals, these vibrational signals sometimes stand out from background noise and can therefore be intercepted by predators.
Tangle-web spiders prey on leafhoppers, and scientists at Cardiff University have shown that they locate their prey by exploiting the leafhopper’s sexual vibrational signals. In the study, spiders were placed on a plant and exposed to recordings of the leafhopper’s mating calls, generated using a vibrational device attached to the plant. These signals were adjusted to match the amplitude and frequency of the leafhopper’s natural calls.
The researchers found that the spiders changed their behaviour in response to different signals. In the presence of the vibrational calls, the spiders spent more time on the plant and began to display typical foraging behaviour. They started to make their way towards the vibrating leaf, probably because they thought it was a source of food. This directed movement was only observed in the presence of the male calling signal. The authors postulate that since male vibrational calls are more conspicuous and have higher amplitudes than female calls, spider foraging behaviour may be influenced by signal amplitude.
Tangle-web spiders preferentially prey on adult leafhoppers and not on nymphs. This is further evidence that the spiders locate their prey using vibrational cues, since only adults produce these sexual communication signals. Additionally, wolf spiders that do not prey on leafhoppers at all remained unaffected by the insect’s vibrational signals in this experiment.
Vibrational signaling in animals has been well documented, but this is the first evidence for its use in predator-prey relationships. It is likely that there are other predators that intercept sexual vibrational cues to locate and capture their prey, and that this tactic is an unrecognised driver of evolution in many invertebrates.
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