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Crayfish are excellent model organisms to study the proximate mechanisms underlying the maintenance of dominance hierarchies in invertebrates. Our aim here was to investigate whether Procambarus clarkii males use social eavesdropping to discriminate dominant from subordinate crayfish. To this end, we conducted an experiment composed of a "passive" and an "active" phase. In the passive phase, "focal" individuals were allowed (treatment 1) or not (treatment 2) to see and smell two size-matched crayfish fighting while, in the subsequent active phase, they were allowed to freely interact with the fighting dyad. None of the recorded variables showed any significant difference between the two treatments, but, invariably, focal individuals were able to promptly discriminate dominant from subordinate crayfish. This study provides evidence that male crayfish recognize the social status of a conspecific without the need of direct or indirect experience with it and avoid dominants-and thus dangerous opponents-by means of a badge of status. A form of "winner and loser effects" could also contribute to the structuring of dominance/subordinate relationships. The implication of these results in understanding the maintenance of dominance hierarchies in invertebrates are discussed and compared with findings previously achieved in the context of mate choice by P. clarkii females, who do appear to use eavesdropping to identify dominants and subordinates.

Predation can be an important force of selection, resulting in the evolution or learning of antipredator defences in amphibian larvae. In the laboratory, we compared the behavioural responses of the tadpoles of Alytes cisternasii subjected to the chemical stimulus of an exotic predator, Procambarus clarkii, with the responses to the chemical stimuli of two of its native predators, the snake Natrix maura and the fish Squalius pyrenaicus, which employ different predation strategies. Tadpoles reacted more intensely to N. maura and then to P. clarkii, with no significant responses to S. pyrenaicus. The alteration in the use of the vertical axis of the aquaria was the antipredator behaviour more frequently used towards both native and exotic predators, and the adopted behaviour was adequate to the activity period and predation strategy of each predator. Alytes cisternasii tadpoles reacted to P. clarkii, a predator introduced about 20 years ago in the study area. These reactions may result from a micro-evolutionary process, but may also be a fortuitous response to a non-familiar cue. We can also not discard the possibility of learning by tadpoles since, due to the reproductive characteristics of this species, it was not possible to collect egg masses before their release in the aquatic environment.Key words: alien predators; Alytes cisternasii; antipredator behaviour; chemoreception; invasions; Procambarus clarkii.Respuestas conductuales de las larvas de sapo partero ibérico (Alytes cisternasii) a los estímulos químicos de depredadores nativos (Natrix maura y Squalius pyrenaicus) y exóticos (Procambarus clarkii). La depredación es una importante presión selectiva que resulta en la evolución o el aprendizaje de conductas antipredatorias en larvas de anfibios. Comparamos en el laboratorio las respuestas conductuales de larvas de Alytes cisternasii expuestas a los estímulos quí-micos procedentes de un depredador exótico, Procambarus clarkii, con las respuestas a los estímulos químicos de dos de sus depredadores naturales, la culebra viperina (Natrix maura) y el cacho (Squalius pyrenaicus), los cuales utilizan estrategias de depredación diferentes. Las larvas reaccionaron de manera más intensa a N. maura, seguida de P. clarkii, sin que existieran respuestas significativas a S. pyrenaicus. La alteración en el uso de la columna de agua fue la conducta antipredatoria más utilizada ante los depredadores, ya fueran nativos o exóticos, adecuando las larvas su comportamiento al periodo de actividad y a la estrategia de cada depredador. Las larvas de A. cisternasii reaccionaron ante los estímulos procedentes de P. clarkii, un depredador introducido tan solo hace 20 años en el área de estudio. Estas reacciones podrían ser resultado de un rápido proceso micro-evolutivo, pero también ser una respuesta fortuita mostrada ante un estímulo novel para las larvas. Tampoco podemos descartar la posibilidad de aprendizaje por parte de las larvas ya que, dadas las características reproductoras de la especie, no fue posible colectar masas de h...

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