Predation risk and the metabolic stoichiometry of Trinidadian guppies (Poecilia reticulata): Consequences for foraging strategies and life history traits

Background/Question/Methods

By addressing the need for prey to balance demand for multiple chemical substrates, multi-currency ecological frameworks (e.g., ecological stoichiometry, nutritional geometry) are providing novel insights into predator prey dynamics. Predators often induce stress in prey that increases their demand for carbon (C; lipids and carbohydrates) relative to nitrogen (N; amino acids). This shift in metabolic demand may change the C:N of preferred foods, tissues, and feces of prey  with implications for organism, community, and ecosystem dynamics. We explored the effects of predation risk on the metabolic stoichiometry of Trindadian guppies (Poecilia reticulata) and the corresponding implications for their trophic ecology. We exposed guppies to simulated predation risk by introducing kairomones from the guppy predator Crenicichla sp. We assessed the influence of predators on the C:N of guppy metabolism by measuring resting oxygen (O) and N metabolism. We used atomic O:N ratios to describe the proportion of guppy metabolism comprised of nitrogen compounds. We also measured the influence of predators on guppy trophic efficiency over a seven-week period by feeding individual guppies ad libitum amounts of either high C:N (high lipid) or low C:N (high protein) foods and measuring both food consumption and fish growth.  

Results/Conclusions

Cue-exposed guppies had higher metabolic O:N than guppies not exposed to predator cues, indicating predation risk reduced N use for metabolism. In the seven week growth study, predation risk altered the nutrient-specific foraging of guppies. Without predation risk, guppies fed to C satiation. No-cue guppies on high C:N diets ingested comparable amounts of C but less N than those reared on low C:N diets. Cue-exposed guppies, however, fed to N satiation. Cue-exposed guppies fed high C:N food consumed more C and not less N than guppies fed low C:N food. Though predation risk did not strongly affect carbon trophic efficiency, it was the primary driver of nitrogen trophic efficiency. Guppies exposed to predator cues were more efficient at turning ingested N into tissue than those reared without predator cues. These results suggest the currency driving guppy trophic behavior changed with predation risk. Non-risk exposed guppies fed to meet C demand, but, with predator cues, guppies fed to meet N demand and retained ingested N at a higher rate. It is unclear whether this shift is adaptive conservation of N for growth under predator-restricted foraging or a general physiological response of guppies to food restriction.