Daily rhythms occur in numerous physiological and behavioral processes across an immense diversity of taxa, but there remain few cases in which mechanistic links between rhythms of trait expression and organismal fitness have been established. We construct a dynamic optimization model to determine whether risk allocation provides an adaptive explanation for the daily foraging rhythm observed in many species using the orb-weaving spider Cyclosa turbinata as a case study. Our model predicts that female C. turbinata should generally start foraging at lower levels of energy reserves (i.e., should be less bold) during midday when predators are most abundant. We also find that individuals' foraging efficacy determines whether daily rates of encounters with predators or with prey more strongly influences boldness under high risk. The qualitative model predictions are robust to variation in our parameter estimates and likely apply to a wide range of taxa. The predictions are also consistent with observed patterns of foraging behavior under both laboratory and field conditions. We discuss the implications of our study for understanding the evolution of daily rhythms and the importance of model predictions for interpreting empirical studies and generating additional hypotheses regarding behavioral evolution.
Observations of C. Turbinata foraging behavior in the field
These data describe 10 days of continuous observations of the behavioral responses ("Event"-- attack or rest) of 9 individual female Cyclosa turbinata ("ID") to prey items hitting the web at different times of day ("TimeofDay.h").
Watts_etal_FieldObservations2013.csv
Metabolic Calculations for C. turbinata
Metabolic calculations used to estimate the proportion of prey energy consumed per time step in the dynamic model. Includes estimates of metabolic rate for C. turbinata in microliters of oxygen per hour (Greenstone & Bennett 1980) and conversion factors from Gnaiger 1983 and Riechert 1991.
Watts_etal_MetabolicCalculations2017.csv
Collated Predator and Prey Abundances for C. turbinata
These data describe daily patterns in the abundances of potential predators (wasps) and prey (arthropods <1.5 cm in length) of C. turbinata in the field. The data include pooled estimates of encounter rates per 3 hour interval collated from Moore et al. 2016 ("PreyTrapped", "PredatorsTrapped"), and subsequent calculations to yield encounter rates per day ("Prey.per.Day", "Pred.per.Day") and per time step ("Prey.per.t", "Pred.per.t"), as well as encounter probabilities per time step ("ProbEncPrey.per.t", "ProbEncPred.per.t").
Watts_etal_CollatedPredatorPreyAbund.csv
R Code for Dynamic Model of Sit-and-Wait Foragers
This file contains R code for the dynamic model used to determine whether temporal variation in predation risk favors daily patterns of foraging behavior in a sit-and-wait predator. This file was created using R v3.4.1. Additional versions of the model code (i.e., those used to plot effects of various model parameters) are available by request.
Basic_model_simplepredictions.R
R Code for Dynamic Model of Searching Foragers
This file contains R code for an extension of our dynamic model that considers scenarios in which foragers must actively search for prey to attack. The R code was produced in R v.3.4.1.
Basic_model_searchingforprey.R