DOI: 10.5061/dryad.r4xgxd2fh
title: Maximum performance expression is affected by octopamine and antennae removal in Acheta domesticus

description: Animals in nature seldom use their maximum performance abilities, likely
 in part due to context-dependent differences in performance motivation.
 Despite interest in the factors affecting performance expression, the
 physiological mechanisms underlying variation in performance motivation
 are poorly understood. We manipulated levels of the biogenic amine
 octopamine (OA) to test the hypothesis that OA drives motivation to
 express maximum bite force in male house crickets. We also tested the
 effect of antenna removal on bite force given prior evidence of potential
 links among antennaectomy, aggression, and OA. We found that
 administration of an OA antagonist, epinastine, significantly decreases
 realized maximum bite force, as does antenna removal. In addition, the
 performance decrement induced by antennaectomy is abolished by
 administration of excess OA, and that rescue effect is itself nullified by
 the simultaneous administration of epinastine. These data show that OA is
 an important mediator of performance, and thus of performance motivation,
 in insects, and potentially a promising candidate for the short term
 manipulation of performance as well.
description: All crickets used in this study were the second-generation
 offspring of A. domesticus crickets obtained from a commercial supplier
 (Fluker’s). Males were raised separately to adulthood on a diet of dried
 cat food (Purina Cat Complete Chow). All experiments were conducted using
 crickets of similar ages (i.e. between 12 and 15 days post-eclosion).
 Following eclosion, we measured maximum bite force of all crickets using
 standard methods (Losos et al. 2002). Briefly, a Tekscan FlexiForce
 wireless ETF flexible force circuit was placed between the mandibles of a
 cricket which would then invariably bite down vigorously on the circuit.
 We measured bite force 5 times per individual and retained the largest of
 those five individual measures for analysis, consistent with both standard
 maximum performance methodology (Losos et al. 2002), and with comparable
 studies in crickets, including A. domesticus (Condon and Lailvaux 2016;
 Hall et al. 2010; Lailvaux et al. 2011). Following initial bite force
 measurement, we allocated crickets randomly to one of five groups:
 control; epinastine; antennectomized; antennectomized + OA; and
 antennectomized + OA + epinastine. Crickets then spent two hours inside
 individual 5x5x5 cm plastic containers with either plain or supplemented
 excess pureed sweetcorn (as in Bubak et al. 2014; Bubak et al. 2015; Bubak
 et al. 2013) under one of five conditions depending on their group
 membership. Control crickets (n = 14) were simply placed in a container
 with untreated pureed sweetcorn. The sweetcorn of epinastine crickets (n =
 15) was supplemented with epinastine at a concentration of 15mg/ml.
 Antennectomized crickets (n = 18) had both antennae removed with scissors
 following initial bite force measurement before being placed in a
 container with untreated pureed sweetcorn. Antennectomized + OA crickets
 (n = 15) had their antennae removed and were placed in a container
 containing pureed sweetcorn supplemented with OA at a concentration of
 15mg/ml. Antennectomized + OA + epinastine crickets (n = 15) had their
 antennae removed and were placed in a container with pureed sweetcorn
 supplemented with both OA and epinastine at the same concentrations as
 above. Following the two-hour treatments, we measured all crickets for
 maximum bite force again using exactly the same procedure as
 before. All experimental methods complied with the
 national and institutional ethical guidelines where this work was
 conducted. <em>Statistical
 analysis</em> We used the
 <em>nlme</em> R package (Pinheiro et al. 2019) to fit a
 mixed-model with bite force as a dependent variable; treatment with the
 levels described above; measurement with the levels pre- and
 post-treatment as an independent variable; thorax size as a covariate to
 account for size effects; and cricket identity as a random factor because
 pre- and post-treatment bite force were measured on the same animals. We
 also included an interaction between treatment and measure to test our
 hypotheses which both pertain to significant differences between pre- and
 post-treatment bite force in certain treatment levels, but not others, and
 an interaction between size and measure to test for the possibility of
 size-dependent plasticity in bite force (Lailvaux et al. 2019). P-values
 associated with individual predictor variables are approximate due to
 penalty factors applied to random effects during calculation of the
 likelihood function. Consequently, we assessed the significance of
 predictors using log-likelihood ratio deletion tests to determine the
 minimum adequate model, which we then re-fit using restricted estimate
 maximum likelihood (REML) (Silk et al. 2020). To test for specific
 differences, we used the <em>emmeans</em> package to extract
 contrasts comparing pre- and post-treatment bite force within each
 treatment level (Lenth 2019). We used R v 3.6.0 for all analyses (R Core
 Team 2019).