Morales, M., Barone, J. and Henry, C. (2008) Acoustic alarm signalling facilitates predator protection of treehoppers by mutualist ant bodyguards. Proceedings of the Royal Society B, 275: 1935-1941.

This study examined the role of the low amplitude vibrational signaling of the treehopper Publilia concava to alert its mutualistic ants of their shared predator, the Ladybeetle, when encountered. Ladybeetles typically feed on treehopper juveniles, or nymphs, unless defended by an adult treehopper female. P. concava is an insect that provides honeydew, a carbohydrate-rich excretion, to ants in return for protection from predators. The purpose of this study was to provide evidence for interspecific alarm signaling in the insect protection mutualism of the ants and P. concava. The researchers were able to demonstrate a functional link between the acoustic alarm signaling and efficacy of protection.

In the encounter laboratory trials described, a single starved ladybeetle was placed on a potted golden rod plant with an untended adult female treehopper and her offspring. Here, the number of signals produced during a 10 second interval following the initial encounter of the treehopper and ladybeetle and the 10 second interval starting at the time of contact were determined. The same procedure was followed but ants were used as replacements for the ladybeetles to see if the same signals were produced by the treehopper when it encountered ants. All vibrational signals were digitally recorded at a bit rate of 16 and a sampling rate of 48 kHz using either a digital audio tape deck or a DVCAM recorder. The researchers used a mixed-effects model to analyze 84 signals from the 10 s interval immediately following predator contact, which allowed them to separate any variance components between individual treehoppers (8 treehoppers were used). Four properties were examined from the vibrational alarm signals: duration of each signal, peak frequency, bandwidth, and pulse rate of signals. Field trials consisted of a playback of a recorded treehopper alarm signal to test the effect of signal production on the probability of the ants being able to discover a predator. Control trials used absence of the alarm signal and a courtship signal recorded from a male P. concava treehopper to test the hypothesis that alarm signal production rather than signaling increases the probability of ladybeetle discovery by ants.

The results demonstrated that P. concava treehoppers produce alarm signals in response to predator threat and this signaling works effectively as predator protection to ants. Laboratory trials resulted in a significant increase of putative alarm signal production following initial contact with the ladybeetle predators, but not after initial contact with ants (alarm signal production increased by a factor of four following contact with predators but remained unchanged following contact with ants). Field trials resulted in a significant increase in both ant activity and the probability of ladybeetle discovery by ants relative to both silence and treehopper courtship signal controls (the odds of beetle discovery increased by a factor of 2.7 and 2.9). Because signaling was not observed in response to contact with ants, researchers concluded that signaling is fairly specific to instances of predator attack in this system. Another conclusion the researchers were able to make was that the response of the ants to the alarm signal was not a general response to any vibrational signal, since they did not observe an increase in ant activity or predator discovery following playback of a male treehopper courtship signal.

This study relates to the environmental signaling lecture’s topic of predator alarm signals that function to alert others. While the material covered in lecture dealt mostly with conspecifics, this study presented an interesting look at interspecific alarm signaling but continued to support the same premises covered in lecture. Both lecture and this study showed a direct benefit of alarm calls from sender to receiver to deter future attack and signal the predator’s presence. I chose this article because I wanted to see if there really was a relationship and benefit for the alarm signals produced from the treehopper to the mutualistic ants. It was interesting to see the evolutionary payoffs of the alarm signal for both the treehopper and ant species. The researchers inferred that differences in the frequency of signaling may follow differences in the relative cost of reward production and that the energetic cost of signal production is lower than the cost of producing ant rewards. Therefore, signal production could represent a ‘lower-cost strategy’ for attracting ants, since there is such a high efficiency of energy transfer for vibrational signals.