Larger testes are associated with a higher level of polyandry, but smaller
ejaculate volume, across bushcricket species (Tettigoniidae).
K. Vahed, D. J. Parker & J. D. J. Gilbert
Electronic Supplementary Material 2:
Data Analysis
For each species, the values obtained for body mass, testis mass, ampulla mass, sperm
number, spermatophylax mass and the degree of polyandry were ln-transformed to
meet the assumptions of parametric linear regression. Data on testis mass were
analysed by fitting phylogenetic generalized least squares models (PGLS; Grafen
1989, Martins & Hansen 1997) using the ape package (Paradis et al. 2009) in R 2.10.0
(R Core Development Team 2008). We used the function corPagel in ape, which
assumes a Brownian motion model of trait evolution whilst simultaneously estimating
Pagel's , a measure of how closely covariance in the model residuals matches the
structure of the phylogeny (Pagel 1999, Freckleton et al. 2002). Phylogenetic
relationships between the different genera in the sub-family Tettigoniinae were taken
from Rentz & Coless (1990), while phylogenetic relationships between the four taxa
of Anonconotus in this study were provided by R. Szabo, G. Carron & M. Ritchie
(unpublished molecular phylogeny based on Mitochondrial DNA). The relationships
between the sub-families Tettigoniinae, Phaneropterinae and Bradyporinae were taken
from Naskrecki (2000). Branch lengths were not available and so were arbitrarily set
to 1. As candidate predictor variables to explain testis mass, we included male body
mass, ampulla mass, sperm number, spermatophylax mass and polyandry, plus all
possible two-way interactions between these variables. We fitted multiple models
incorporating all combinations of these predictors under an Information Theoretic
framework, using AICc as a criterion for model selection. Models were ranked
according to their AICc, and models within 2 points of the lowest-ranked model were
treated as equally likely (Burnham & Anderson 2002).
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