Background
Sexual size dimorphism (SSD) is a phenomenon that has received a great
deal of theoretical and empirical attention, as it is thought to reflect
variation in sex-specific selection due to ecological performance, and
fitness effects arising from fecundity selection and mating systems (D J
Fairbairn, 1997; Daphne J Fairbairn, Blanckenhorn, & Székely, 2007;
Shine, 1989, 1990). Turtles (Order Testudines) have been prominent model
systems for comparative analyses aimed at understanding the causes of
SSD, owing to the diversity of their mating systems, habitats
(freshwater, terrestrial, marine) they occupy, and the wide availability
of data on body size of many species (Agha et al., 2018; Berry & Shine,
1980; Ceballos, Adams, Iverson, & Valenzuela, 2013; Gibbons & Lovich,
1990; Gosnell, Rivera, & Blob, 2009; Halámková, Schulte, & Langen,
2013; Regis & Meik, 2017). All of these analyses assign turtle species
to different habitat types (aquatic, semi-aquatic, terrestrial, etc.),
but with varying degrees of detail. In all cases, marine turtles, the
subject of this paper, are lumped with non-marine aquatic turtles
(NMAT).
The seven species of marine turtles comprise a monophyletic lineage
(superfamily Chelonioidea) containing two families (Cheloniidae,
Dermochelyidae) (reviewed in Figgener, Bernardo, and Plotkin (2019)).
Both extant and extinct marine turtles are well-known to exhibit
striking adaptations to the marine environment including forelimbs
highly modified into flippers with concomitant neuromuscular
repatterning, and streamlining of body form as is seen in other highly
vagile marine vertebrates (Frank E Fish, 1993; Kelley & Pyenson, 2015;
Pyenson, Kelley, & Parham, 2014). Three issues pertaining to the marine
turtle data that have been used in prior analyses of testudine SSD
prompted this study. The first issue is that most reviews do not include
data for all seven species, (two to five species have been included)
although data exist for all seven species in the literature. Second,
most studies include species’ mean values that are often based on a
single population ignoring a large amount of literature data on body
sizes in different populations. Further, some studies report values
whose origin in the primary literature is unclear (Supplementary Table
1). Because most marine turtles occupy far more expansive geographic
ranges (Figgener et al., 2019) than any other turtle species including
both temperate and tropical regions, intraspecific diversity in body
size may influence overall conclusions about SSD in marine turtles. The
third and possibly most concerning issue is that all the prior analyses
cited above are consistent in grouping marine turtles with other aquatic
turtles despite their well-known distinct morphology and ecology, which
includes long-distance, often trans-oceanic migrations (Godley et al.,
2008; Graeme C. Hays & Hawkes, 2018; Plotkin, 2003, 2010).
In this paper, we critically examine these issues. First, we address the
lack of SSD data for marine turtles in previous studies by assembling
the most comprehensive dataset to date on body size of all seven marine
turtle species, including estimates from multiple populations. We then
analysed these data to describe quantitatively intraspecific and
interspecific patterns in marine turtle SSD. Finally, we compared these
new estimates of marine turtle SSD to data from other fully aquatic
turtles to test the hypothesis that the previous grouping of marine
turtles with other aquatic turtles in comparative analyses of sexual
size dimorphism is justified.
Methods
We carefully reviewed all data on marine turtle body size reported and
used in prior analyses of testudine SSD to validate their accuracy and
to gauge any omissions. As part of this process, we re-examined all the
primary sources reported in these studies. This exercise revealed many
omissions and errors (summarised in Supplementary Table 1). Therefore,
we generated a novel, comprehensive dataset (Supplementary Table 2) of
sex-specific body sizes (carapace length, CL) of adult marine turtles in
which data for both sexes were reported from the same population using
data from primary sources. The detailed methodology and the resultant
dataset for this synthesis are detailed in the Electronic Supplementary
Material.
We analysed this new dataset to test the hypothesis that marine turtles
exhibit significant sexual size dimorphism. First, to gain an overview
of species differences as well as intraspecific variation, we computed
sex-specific mean values for each population and species. Then we
plotted male vs female size for each population and computed a
regression of males versus females (Ranta, Laurila, & Elmberg, 1994).
The null hypothesis, in this case, is that males and females for a given
species do not differ in size, which implies a slope of one and an
intercept of zero. This null hypothesis thus differs from the standard
null in regressions that both the slope and intercept are zero. Because
the data were unbalanced with respect to the number of populations per
species (Fig. 1A), we repeated the analysis using only mean values for
each species. In both models, each component of the null hypothesis
(slope, intercept) was evaluated using a one sample, two-tailed t-test.