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

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).
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.
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 neona...