Size-mediated response to public cues of predation risk in a tropical stream fish

In order to investigate any size-dependent differences between behavioural patterns, wild-caught Hart's rivulus Rivulus hartii of varying sizes were exposed to chemical alarm cues extracted from the skin of conspecifics or heterospecific Poecilia reticulata, or a tank water control, in a series of laboratory trials. In response to conspecific alarm cues, R. hartii subjects of the range of body sizes tested exhibited consistent, size-independent antipredator behaviours that were characterized by decreased locomotory activity and foraging levels and increased refuging behaviour. Conversely, focal R. hartii demonstrated significant size-dependent trends in response to heterospecific alarm cues, with smaller individuals exhibiting antipredator responses and larger individuals shifting their behaviour to increased levels of activity consistent with a foraging, or predatory, response. These results show that the behavioural responses of individual R. hartii to publicly available chemical alarm cues from heterospecifics are mediated by the size of the receiver

Male mate choice in the Trinidadian guppy is influenced by the phenotype of audience sexual rivals

In populations with male mate-choice copying, males may mitigate their risk of sexual competition by reducing their preference for a particular female in the presence of sexual rivals (audience). Because of the cost of missed mating opportunities from such an audience effect, males should reduce their mating preference to a greater extent in the presence of more sexually competitive rivals compared with less competitive ones. We tested this hypothesis using the Trinidadian guppy (Poecilia reticulata). We compared a focal male's baseline mating preference for either of 2 stimulus females, which differed in overall body size, in the absence of any sexual rival to his preference for the same females in the presence of a sexual rival using dichotomous-choice tests. Focal and audience males differed in body length and proportion of their body covered in orange and black pigmentation. In the presence of a larger rival, focal males exhibited a greater reduction in preference for their initially preferred female compared to focal males in the presence of a smaller rival, irrespective of whether the latter male was more or less ornamented than the focal. The strength of the initial mating preference of focal males and the magnitude of the audience effect were significantly positively correlated when the audience male was larger than the focal male. Male guppies are thus sensitive to the phenotype of nearby males and alter their preference for a particular mate to a greater extent in the presence of relatively larger eavesdropping males compared to smaller ones

Size-assortative shoaling in the guppy (Poecilia reticulata): The role of active choice

Many fish species exhibit size-assortative shoaling, which is often thought to be driven by predation risk. Recent fieldwork has revealed that guppies (Poecilia reticulata) are more size assorted in high-predation populations than in low-predation ones. However, size assortment does nonetheless occur in some low-predation populations, suggesting that predation is unlikely the sole driving force behind size-assortment. Here, we investigated in the laboratory the potential role of active choice in size-assortative shoaling in wild-caught female guppies originating from two populations of the same river system in Trinidad. Small or large focal females from each population were offered a binary choice of shoaling with either four small female conspecifics or four large ones. Observed shoaling preferences depended on the body size of the focal fish, suggesting phenotype-mediated conflict over group composition. Large focal fish preferred to shoal with the size-matched stimulus shoal of large fish. In contrast, small focal fish did not shoal assortatively but also preferred to shoal with larger females. Our results suggest that size-assortative shoaling in female guppies is likely to be due to factors other than active choice, such as habitat segregation and sexual harassment

Sperm competition risk and mate choice in male Trinidadian guppies, Poecilia reticulata

Sperm competition theory predicts that males should be sensitive to socio-sexual cues that provide information about the risk of sperm competition at any mating and correspondingly adjust their mating tactics in a manner that maximizes their reproductive success. Here, we investigated male mating preferences in response to socio-sexual cues as predictors of sperm competition risk (SCR) in the Trinidadian guppy, Poecilia reticulata, as a model study species. In a natural Trinidadian population, we observed that free-ranging females were most commonly pursued sexually by only one male at a time, which presumably represents a SCR associated with that female from the vantage of a nearby male observer. We tested whether wild-caught male guppies would modify their initial mating preference for either of two stimulus females presented by experimentally increasing the male's apparent SCR. This was done by allowing focal males to observe a rival male either placed near (but not physically interacting nor copulating with) or sexually interacting with their initially preferred female. In the absence of any apparent increase in SCR, the preference of focal males for either of the two stimulus females presented remained consistent. However, males significantly reduced their preference for their initially preferred female after having observed her either sexually interact with or merely in the vicinity of a rival male. Our results indicate that male guppies are sensitive to perceived SCR and adjust their mate choice behaviour in an apparently adaptive manner, as predicted by sperm competition theory