Abstract
Body mass components (dry mass, lean dry mass, water mass, fat mass) in each sex correlate strongly with body mass and pronotum length in Gryllus texensis and Acheta domesticus. Ordinary least squares (OLS) regression underestimates the scaling relationship between body mass and structural size (i.e., pronotum length) in both cricket species compared with standard major axis (SMA) regression. Standardized mass components correlate more strongly with scaled mass index () than with residual body mass (R i). R i represents the residuals from an OLS regression of log body mass against log pronotum length. Neither condition index predicts energy stores (i.e., fat content) in G. texensis. R i is not correlated with energy stores in A. domesticus whereas is negatively correlated. A comparison of condition index methods using published data showed that neither sex nor diet quality affected body condition at adulthood in G. texensis when using the scaled mass index. However, the residual index suggested that sex had a significant effect on body condition. Further, analysis of covariance (ANCOVA) suggested that diet quality significantly affects body mass while statistically controlling for body size (i.e., body condition). We conclude that the statistical assumptions of condition index methods must be met prior to use and urge caution when using methods that are based on least squares in the y -plane (i.e., residual index ANCOVA).
Highlights
Our overall objectives in this paper are to identify a condition index that is, a reliable indicator of energetic condition in two cricket species, Gryllus texensis and Acheta domesticus, that are commonly used in studies of evolutionary ecology, and to critically compare the performance of different indices whose use is advocated in the literature: Residual index (Ri) (Jakob et al 1996; Marshall et al 1999), mass index (M^ i) (Peig and Green 2009, 2010), and analysis of covariance (ANCOVA) (Garcia-Berthou 2001)
The reference data set for G. texensis (n = 86 males; n = 103 females) comprised lab-reared descendants of crickets collected in Austin, TX (U.S.A.)
Validating a condition index by correlating either absolute or relative size of body mass components with a measure of structural size can be misleading in the absence of isometry either because a lack of variation in body size is assumed or because the size of different components can scale differently with increasing total body size
Summary
Body condition refers to an animal’s energetic state and is generally considered to be an indicator of an animal’s health, quality, and vigor (Schulte-Hostedde et al 2001; Tomkins et al 2004; Schulte-Hostedde et al 2005; Peig and Green 2009; Cox et al 2014; see Rowe and Houle 1996). Our overall objectives in this paper are to identify a condition index that is, a reliable indicator of energetic condition (i.e., fat content) in two cricket species, Gryllus texensis and Acheta domesticus, that are commonly used in studies of evolutionary ecology, and to critically compare the performance of different indices whose use is advocated in the literature: Ri (Jakob et al 1996; Marshall et al 1999), M^ i (Peig and Green 2009, 2010), and ANCOVA (Garcia-Berthou 2001) We achieve the latter goal using data previously presented in Kelly and Tawes (2013) on the effect of sex and nutritional conditions during development on body condition at adulthood in G. texensis. It is essential to examine these issues in invertebrates for the simple fact that their unique biology could affect scaling relationships between measures of body size and body mass components, fat content
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