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.

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Journal Ethology
Jones, K.A. (Katherine A.), Croft, D.P. (Darren P.), Ramnarine, I.W. (Indar W.), & Godin, J.-G.J. (2010). Size-assortative shoaling in the guppy (Poecilia reticulata): The role of active choice. Ethology, 116(2), 147–154. doi:10.1111/j.1439-0310.2009.01727.x