Geographical variation in allometry in the guppy (Poecilia reticulata)

Abstract

Variation in static allometry, the power relationship between character size and body size among individuals at similar developmental stages, remains poorly understood. We tested whether predation or other ecological factors could affect static allometry by comparing the allometry between the caudal fin length and the body length in adult male guppies (Poecilia reticulata) among populations from different geographical areas, exposed to different predation pressures. Neither the allometric slopes nor the allometric elevations (intercept at constant slope) changed with predation pressure. However, populations from the Northern Range in Trinidad showed allometry with similar slopes but lower intercepts than populations from the Caroni and the Oropouche drainages. Because most of these populations are exposed to predation by the prawn Macrobrachium crenulatum, we speculated that the specific selection pressures exerted by this predator generated this change in relative caudal fin size, although effects of other environmental factors could not be ruled out. This study further suggests that the allometric elevation is more variable than the allometric slope.

On the Relationship between Ontogenetic and Static Allometry

“…In the guppy, a comparison of static allometries of the caudal fin length in males among 21 natural populations from different drainages and exposed to different predation intensities revealed no effect of these ecological variables on the allometric slope, while the allometric intercept differed between geographic areas (Egset et al 2011). In addition, the static allometric slopes observed in these populations were similar to the one observed in females in our study ( ), despite the fact that caudal fin size in b p 0.79 ‫ע‬ 0.05 males is under sexual selection (Bischoff et al 1985;Endler and Houde 1995).…”

“…[3]). For the guppy data, the various estimates of the repeatability (ratio between among-individual and total variance) for the body size measurement showed that this repeatability is high (between 0.98 and 0.99; Egset et al 2011Egset et al , 2012. Therefore, the downward bias of the static allometric slope due to measurement error should be on the order of 1%-2% (Hansen and Bartoszek 2012) and should not strongly affect differences between ontogenetic and static allometry as well as the changes in static allometry during ontogeny.…”

“…Therefore, we believe that the results presented here are robust and relatively independent from the size measurement used. For the guppy, we decided to present the data on body length and caudal fin length because these traits are less subject to measurement errors and allow comparison with the allometry estimated for the same traits in male guppies from natural populations (Egset et al 2011). Furthermore, in fish, swimming performances seem to depend more strongly on the length of the caudal fin than on the area, because of its effect on the rigidity of the fin (Langerhans 2008;see Egset et al 2011 for further discussion).…”

“…However, because of the lack of biological significance of the static allometric intercept (which strongly depends on the unit of the Xaxis), it is more relevant to investigate the effect of the ontogenetic allometry on the elevation of the static allometry, that is, on the average value of the trait, , at thē z t population mean body size (see Egset et al 2011). At m t the developmental stage t, the elevation will bē¯¯z…”

“…[7]) can be tested statistically with hierarchical random regression models (contextual models). Contextual models are multiple regressions that include predictor variables at several levels (e.g., individuals, populations, and species ;Blalock 1984;Raudenbush and Bryk 2002;and see Heisler and Damuth 1987;van de Pol and Wright 2009;Bolstad et al 2010Bolstad et al , 2012Egset et al 2011 for some biological applications). relationship between ontogenetic and static allometry when ontogenetic allometry is quadratic (curved dash line) at the population level.…”