Data from: Heterochrony in the evolution of Trinidadian guppy offspring size: maturation along a uniform ontogenetic trajectory

The size and maturity of Trinidadian guppy (Poecilia reticulata) offspring
vary among populations adapted to environments of differential predation.
Guppy offspring born to low-predation, high-competition environments are
larger and more mature than their high-predation ancestors. Here we ask:
what specific changes in developmental or birth timing occur to produce
the larger, more mature neonates? We collected specimens across the
perinatal window of development from five populations and quantified
musculoskeletal maturation. We found that all populations undergo similar
ontogenetic trajectories in skeletal and muscle acquisition; the only
difference among populations is when neonates emerge along the trajectory.
The smallest neonates are born with 20% of their skeleton ossified,
whereas the largest neonates are born with over 70% of their skeleton
ossified. The area of the major jaw-closing muscle is relatively larger in
larger offspring, scaling with length as L2.5. The size range over which
offspring are birthed among populations sits along the steepest part of
the size–maturity relationship, which provides a large marginal increase
in fitness for the high-competition female. Because of the functional
effects of producing more mature offspring at birth, offspring size may be
the first and most critical life-history trait selected upon in highly
competitive environments.