Growth, metabolism, anatomy, behaviour, invertebrate drift

Adaptive trade-offs are fundamental mechanisms underlying phenotypic
diversity, but the presence of generalizable patterns in multivariate
adaptation and their mapping onto environmental gradients remain unclear.
To understand how life-history affects multivariate trait associations, we
examined relationships among growth, metabolism, anatomy and behaviour in
rainbow trout juveniles from piscivore vs. insectivore ecotypes along an
experimental gradient of food availability. We hypothesized that i)
selection for larger size in piscivorous adults would select for higher
juvenile growth at the cost of lower active metabolism; ii) elevated
growth of piscivores would be supported by a greater productivity of their
natal stream and more proactive foraging behaviours; and iii) general
patterns of multivariate trait associations would match the predictions of
the Pace-Of-Life Syndrome. Relative to insectivores, piscivorous fry
showed a pattern of higher growth (+63%), maximum food intake (+33%),
growth efficiency (+41%), and standard metabolic rate (SMR; +47%), but
lower active metabolic capacity (maximum metabolic rate (MMR; -17%),
aerobic scope (AS; -48%)), suggesting that faster piscivore growth is
supported by greater food intake and digestive capacity but is traded-off
against lower scope for active metabolism. A similar trade-off appeared
among organ systems, with piscivorous fry exhibiting an 83% greater
investment in average mass of organs associated with food consumption and
processing (i.e. stomach and intestine), but an apparently smaller
relative investment in organs involved in cardio-vascular or cognitive
activities (heart and brain, respectively). Higher invertebrate drift in
their natal rearing habitat, quicker behavioural transition to a novel
food source and lower anxiety after a frightening event in piscivorous fry
suggest that faster growth requires both proactive foraging behaviours and
higher prey availability in the environment. Finally, the sampling of
replicate insectivore populations confirmed their lower juvenile growth
(-73% on average) and reduced environmental productivity of their natal
streams (-45% lower drift abundance) relative to the piscivore ecotype.
Our results suggest that selection for large adult body size influences
selection on high juvenile growth, high basal metabolism and proactive
behaviours, and that the intense phenotypic divergence between piscivorous
and insectivorous rainbow trout may be constrained by environmental
productivity.