Comparison of SSD of marine to non-marine aquatic turtles
Data for NMAT were derived from published summaries in two recent
analyses of sexual dimorphism in turtles (Agha et al., 2018; Regis &
Meik, 2017). Only sex-specific mean body size data were available in
these studies. We aggregated data from the two most comprehensive
studies and updated the taxonomic assignments of the studied
populations. Additional details of this process and the resultant
dataset (Supplementary Table S3) are provided in the Electronic
Supplementary Material.
We used two approaches to test the hypothesis that marine and NMAT
should be considered as a single group (aquatic turtles) in analyses of
SSD as has been assumed in previous analyses of turtle SSD (Agha et al.,
2018; Berry & Shine, 1980; Ceballos et al., 2013; Gosnell et al.,
2009). First, we plotted male versus female mean body size for each
species of both non-marine and marine turtles and fit separate
regressions for the two groups. We then computed an ANOVA in which we
included habitat (marine vs non-marine) as a classification variable.
Because there is a correlation between habitat and overall body size
(average sizes of marine turtles are far greater than those of NMAT), we
might wrongly ascribe to ‘habitat’ a difference driven simply by average
size. Therefore, we also included an interaction term between female CL
and habitat to account for this association.
Second, we computed the Lovich-Gibbons Sexual Dimorphism Index
(SDILG) (Lovich & Gibbons, 1992) as modified by D J
Fairbairn (1997), hereafter SDILGF, for all species:
SDILGF =\(\frac{\text{mean}\ \text{size}\ \text{of}\ \text{largest}\ \text{sex}}{\text{mean}\ \text{size}\ \text{of}\ \text{smallest}\ \text{sex}}-1\)
with positive values (arbitrarily) indicating female-biased SSD, 0
indicating no SSD, and negative values (arbitrarily) indicating
male-biased SSD. We plotted these for visual comparison, but we also
computed the distributional properties of the SDILGF for
each group. We then computed an ANOVA in which we included habitat
(marine vs non-marine) as a classification variable. All model effects
are interpreted using the Type III Sums of Squares.
RESULTS
We obtained reliable body size data for 36 populations representing all
seven species of marine turtles, with a total sample size of 38,569
individuals (36,761 females; 1,808 males), the most comprehensive
dataset of marine turtle body size to date (Supplementary Table S2). By
comparison, prior analyses of SSD in turtles (Agha et al., 2018; Berry
& Shine, 1980; Ceballos et al., 2013; Gibbons & Lovich, 1990; Gosnell
et al., 2009; Halámková et al., 2013; Regis & Meik, 2017) included data
for only two to five marine turtle species, and often each species was
represented by a single population (See Supplementary Table S1 for
details).