Three beetles—three concepts. Different defensive strategies of congeneric myrmecophilous beetles
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Myrmecophiles, i.e., organisms associated with ants live in a variety of ecological niches in the vicinity or inside ant colonies and employ different strategies to survive ant encounters. Because different niches are characterized by different encounter rates with host ants, strategies used to avoid ant aggressions should depend on these niches. This hypothesis was studied with three rove beetle species of the genus Pella, which are myrmecophiles of the ant Lasius fuliginosus and the non-myrmecophilous relative Drusilla canaliculata. Behavioral tests in the field revealed that Pella species are better adapted to interactions with ants than D. canaliculata, but that they use species-specific strategies in ant interactions. Pella cognata and Pella funesta avoid encounters with ants by swift movements. Chemical analyses of the defensive tergal gland secretions showed that P. cognata has replaced the aggression inducing undecane by the behaviorally neutral tridecane. P. funesta repels the ants by releasing the panic alarm pheromone sulcatone from its tergal gland resulting in an “ant free space” around the beetles. Finally, Pella laticollis uses a specific and unique appeasing behavior. Behavioral and chemical data did not reveal any indication for the mimicry of the ants' cuticular hydrocarbon profiles by any of the beetle species. It is discussed that the employed strategies correlate with the ecological niches of the beetles. P. cognata and P. funesta are living along ant trails with ample space to escape and the employed strategies are probably sufficient to escape from dangerous conflicts. In contrast, P. laticollis lives in refuse areas of ant nests with frequent ant encounters, and its appeasement strategy allows it to stay at the encounter site.
KeywordsColeoptera Staphylinidae Pella Zyras Drusilla canaliculata Lasius fuliginosus Myrmecophily Chemical mimicry Appeasement behavior
We like to thank Robert Pfeifle and Konrad Schwarz who accurately performed behavioral experiments. Many thanks to H. Burger, J. Gögler, and A. M. Rottler from the Institute of Experimental Ecology at the University of Ulm in Germany who helped in the statistics on cuticular hydrocarbons. Thanks to M. Spelleken and K. Peschke (University of Freiburg, Germany) for specimen as a starter for our Drusilla canaliculata rearing. The manuscript was considerably improved thanks to helpful comments of J. Collatz and very valuable remarks of unknown reviewers. All experiments performed for this study were in compliance with the current German laws.
Conflict of interest
The authors declare that they have no conflict of interest.
Movie of behavioral interactions between Pella laticollis and Lasius fuliginosus. P. laticollis shows the appeasing behavior “duck down”, which is almost never observed in the other two Pella species described here. During “duck down”, the beetle stops and crouches to the ground with the abdomen bent upwards. The ants' reaction most often consists in intense antennation of the beetle's abdomen. After this, the beetle or the ant leaves the place. This is shown several times in the video (MPG 9305 kb)
Movie of behavioral interactions between Pella cognata and Lasius fuliginosus. P. cognata actively avoids encounters with ants. Nearly, each ant contact is prompted by immediate and swift changes of the beetle's direction (MPG 7173 kb)
Movie of behavioral interactions between Pella funesta and Lasius fuliginosus. P. funesta avoids aggressive encounters with ants by releasing tergal gland secretion. In this video, the beetle releases tergal gland secretion when it is molested by a large number of ants. The secretion contains sulcatone, a panic-alarm-inducing pheromone. As a consequence, ants actively avoid P. funesta (MPG 6558 kb)
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