Effects of neonatal size on maturity and escape performance in the Trinidadian guppy

1. The livebearing Trinidadian guppy (Poecilia reticulata) produces bigger offspring in populations exposed to low predation and produces smaller, more numerous offspring in populations subject to high predation (HP). 2. Like most fishes, guppies respond to predator attacks with a fast-start escape response. From the scaling of teleost fast-start performance, we predict that larger guppy neonates should exhibit faster, more effective escape responses than smaller neonates. Increasing performance with increasing size could be due simply to size, or to both size and the acquisition of more mature body forms, as is seen in larval-stage fishes. 3. We find no difference in external body proportions among guppy offspring varying in size from 5.1 mm to 7.1 mm at birth, suggesting offspring are born morphologically mature. However, based on the degree of skeletal ossification, as a proxy for internal maturity, we find that guppies are still maturing rapidly around the time of birth. The smallest neonates from the very HP Caroni confluence lack ossification of key skeletal elements that are present in their larger low predation counterparts. 4. In guppies from the Aripo/Caroni drainage, we show that neonatal escape performance covaries with responsiveness and increases with size along a gradient of HP offspring, but is lower in the largest low predation offspring. The scaling of escape performance in the HP populations performing maximally (scaling with length as L-2.76) exceeds predictions from size alone (scaling exponent of L-1.0 indicates performance increases linearly with body size). This suggests that guppy neonates, like rapidly developing larval fishes, vary substantially in morphological maturity among populations. 5. 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 escape performance effects of being born small. Despite the negative consequences of being born both small and immature, HP environments select heavily for high fecundity, and thus small offspring size. We find selection favours female life-history traits over offspring escape performance.