Highlights d Acacia ants discriminate browser-induced vibrations from wind-induced vibrations d Browser-induced vibrations serve as a long-distance alarm cue for ants d Acacia ants make use of tropotactic directional vibration sensing Correspondence felix.hager@rub.de In BriefHager and Krausa show that the acacia ant Crematogaster mimosae defends its host tree by exploiting plant-borne vibrations caused by browsers feeding on the tree. Browser-induced vibrations serve as a long-distance alarm cue, and the ants make use of tropotactic directional vibration sensing to orient to the attacked part of the tree. SUMMARYLiving in the African savanna is dangerous, especially for plants. Many plants therefore engage in mutualism with ants, in which plants provide food and shelter in exchange for protection against herbivores. Ants become alarmed when the plant takes on some sort of damage. They immediately emerge from their plant shelter and aggressively defend the plant. Mammalian herbivores can have devastating effects on trees by browsing, breaking tree branches, stripping bark, and pushing over entire trees. However, mutualistic ants substantially reduce the amount of damage. To efficiently protect the tree, ants need to rapidly react together when the tree is under attack. Here, we show that the acacia ant Crematogaster mimosae defends its host tree by exploiting plant-borne vibrations caused by browsers feeding on the tree. Experiments with controlled vibrations show that ants discriminate browser-induced vibrations from those induced by wind, become alarmed, and patrol on the branches. Browser-induced vibrations serve as a long-distance alarm cue. The vibrations propagate through the whole acacia tree and trigger ants' defensive behavior, even on the other side of the tree. Furthermore, the ants make use of tropotactic directional vibration sensing to orient to the attacked part of the tree and fight back the attacker.

Worldwide only a few of the more than 500 stingless bee species has been studied in any detail. Most studies on stingless bees have been conducted in the Neotropics, whereas a very few have been undertaken in Africa. Foraging success is dependent to a greater or lesser extent on olfactory cues or signals. A prerequisite to effective foraging via odors is that the bees are able to associate odors with a nectar reward and memorize this information. In the context of olfactory learning, only five species of stingless bees have been studied using classical conditioning of the proboscis extension reflex (PER). Foragers are capable of using previously experienced olfactory information when choosing food sources, but they rarely respond in differential PER assays. This study examines for the first time the olfactory learning abilities of African stingless bees. A differential conditioning assay was used to study learning and memory of Meliponula ferruginea and Meliponula bocandei. Our results clearly show that both stingless bee species associate odors with a reward. Learning performance of both stingless bee species was poor compared to Apis m. scutellata. This might reflect that the experimental procedure has been optimized for Apis mellifera. However, the PER paradigm seems to be suitable to study learning, memory, and olfactory perception in Meliponula. As in honeybees, this paradigm will open the way to answer ecological, psychological, and neurobiological questions in these species.

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