Size-based differences determine the contextual value of risky information in heterospecific information use

Behavioural responses to damage-released chemical cues in aquatic species are often examined with emphases on the potential survival benefits accrued by conspecific receivers in accordance with the hypothesis of taxonomic cue conservation. Nevertheless, chemical cues are publically available to any capable receivers in aquatic systems, and therefore convey information on ambient risk levels to unrelated prey species and on the availability of foraging opportunities to potential predators. Many documented alarm response patterns by heterospecific receivers closely match those of conspecifics, suggesting that the contextual value of public information on risk conveyed by alarm cues may be at least as important as their conserved value. To further explore this relationship, we compared the antipredator responses of shoals of three large or three small female Trinidadian guppies, Poecilia reticulata, to conspecific cues or cues from larger or smaller Hart's rivulus, Rivulus hartii, donors to quantify the relative values of publicly available information. Shoals of large female guppies demonstrated functionally similar antipredator responses to damage-released cues from conspecifics and larger heterospecifics, and no response to the damage-released cues from smaller heterospecifics or the kairomones of larger heterospecifics. Shoals of small guppies demonstrated significant antipredator responses to the damage-released cues of size-matched rivulus, but not to those of larger rivulus or to the odours of either size class. Collectively, these results indicate that potential prey assign at least equal importance to conspecific and public chemical cues from unrelated members of the same prey guild indicating elevated levels of predation risk in natural systems. •We examined risk assessment in guppies via size-based heterospecific (rivulus) alarm and odour cues.•Small guppies responded to alarm cues of size-matched, but not larger, rivulus.•Large guppies responded to alarm cues from larger, but not smaller, rivulus.•Neither size of guppy responded to rivulus odours.•In certain contexts, the value of public information outweighs the value of intraspecific cues.