Maintenance of phenotypic plasticity is linked to oxidative stress in spadefoot toad larvae

Phenotypic plasticity allows organisms to improve the match between their
phenotype and heterogeneous environments. Theoretical models have argued
that costs of maintaining the sensory and response machinery necessary for
adaptive phenotypic plasticity are important determinants to the evolution
of plasticity. Despite recurrent arguments invoking putative metabolic
costs associated with maintenance of cellular machinery, no studies have
yet attempted to quantify it from a molecular standpoint. Here we
experimentally examine physiological differences across genotypes
(sibships) of spadefoot toad larvae with different degrees of plasticity
in response to predator cues. We observed marked differences across
sibships in developmental, growth and morphological responses to
predators, and tested whether increased plasticity was associated with
oxidative stress or immune suppression. We observed that more plastic
sibships experienced higher antioxidant enzymatic activity when reared in
the absence of predator cues, i.e. not expressing their plastic responses.
The degree of plasticity was also associated with higher lipid
peroxidation and slightly greater granulocyte-to-lymphocyte ratio. Higher
antioxidant activity in highly plastic sibships suggests that maintenance
of phenotypic plasticity may be linked to energy demanding metabolic
processes. Our findings suggest that having the potential to produce
plastic responses may incur oxidative and immunological costs. In the long
term, such maintenance costs may erode individual fitness and even
constrain the evolution of plasticity. To our knowledge, this is the first
empirical evidence indicating the existence of a physiological cost to the
maintenance of phenotypic plasticity.