Meeting Abstract
Offspring size is a key life history trait that varies widely and affects the function of neonatal forms within their immediate environment. Using the Trinidadian guppy (Poecilia reticulata) as a model we ask, why do the smallest offspring reach a lower limit of size (5.5 mm SL) in high predation (HP) environments and why do the largest offspring reach an upper size limit (7.5 mm standard length, SL) in low predation (LP), high competition environments? We predict that the lower limit of size runs up against hydrodynamic limitations, whereas the upper limit is set by diminishing returns on improved feeding performance at larger sizes. We employ viscosity manipulations to examine the effects of hydrodynamics during suction feeding, and measured jaw kinematics during scrape feeding on an encrusting substrate across a size range of guppy offspring. We find that hydrodynamics do not constrain feeding rate and suction generation at small size, but that across treatments performance varied as a function of both size and maturity. Over the size range examined, guppy neonates varied four-fold in their degree of morphological maturation (20% ossification in HP; 80% ossification in LP). During scrape feeding, rotation at the intramandibular joint doubled among guppy offspring, ranging from 11.7° in HP to 22.9° in LP neonates. The finding that neonatal guppy offspring covary in both size and maturity at birth means that being smaller also means being less mature, which amplifies the negative performance effects of being born small. These findings also indicate that degree of maturity begins to plateau in larger offspring, which could reduce selection for even larger offspring in LP environments, if maturity is the main driver of scrape feeding performance.
