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RESEARCH ARTICLE
Dry versus wet and gross: Comparisons between the dry skull
method and gross dissection in estimations of jaw muscle
cross-sectional area and bite forces in sea otters
Chris J. Law | Rita S. Mehta
Department of Ecology and Evolutionary
Biology, University of California Santa Cruz,
Santa Cruz, California
Correspondence
Chris J. Law, Department of Ecology and
Evolutionary Biology, University of California
Santa Cruz, 130 McAllister, Santa Cruz, CA
95060.
Email: cjlaw@ucsc.edu
Funding information
American Cetacean Society; American
Museum of Natural History; American Society
of Mammalogists; Division of Environmental
Biology; NSF Doctoral Dissertation
Improvement Grant, Grant/Award Number:
1700989; National Science Foundation (NSF)
Graduate Research Fellowship
Abstract
Bite force is a measure of feeding performance used to elucidate links between animal
morphology, ecology, and fitness. Obtaining live individuals for in vivo bite-force mea-
surements or freshly deceased specimens for bite force modeling is challenging for
many species. Thomason's dry skull method for mammals relies solely on osteological
specimens and, therefore, presents an advantageous approach that enables researchers
to estimate and compare bite forces across extant and even extinct species. However,
how accurately the dry skull method estimates physiological cross-sectional area
(PCSA) of the jaw adductor muscles and theoretical bite force has rarely been tested.
Here, we use an ontogenetic series of southern sea otters (Enhydra lutris nereis)totest
the hypothesis that skeletomuscular traits estimated from the dry skull method accu-
rately predicts test traits derived from dissection-based biomechanical modeling.
Although variables from these two methods exhibited strong positive relationships
across ontogeny, we found that the dry skull method overestimates PCSA of the mas-
seter and underestimates PCSA of the temporalis. Jaw adductor in-levers for both jaw
muscles and overall bite force are overestimated. Surprisingly, we reveal that sexual
dimorphism in craniomandibular shape affects temporalis PCSA estimations; the dry
skull method predicted female temporalis PCSA well but underestimates male tempo-
ralis PCSA across ontogeny. These results highlight the importance of accounting for
sexual dimorphism and other intraspecific variation when using the dry skull method.
Together, we found the dry skull method provides an underestimation of bite force
over ontogeny and that the underlying anatomical components driving bite force may
be misrepresented.
KEYWORDS
biomechanical modeling, Carnivora, feeding performance, jaw adductor musculature,
physiological cross-sectional area, sexual dimorphism
1|INTRODUCTION
For many vertebrates, the ability to generate relatively high bite
forces is an important character trait that determines an animal's abil-
ity to exploit challenging resources (Gignac & Erickson, 2016; Santana,
Dumont, Dumont, & Davis, 2010; Verwaijen, Van Damme, & Herrel,
2002) compete for mates and territories (Herrel, Spithoven, Van
Damme, & De Vree, 1999; Huyghe, Vanhooydonck, Scheers, Molina-
Borja, & Van Damme, 2005; Lailvaux, Herrel, Vanhooydonck,
Meyers, & Irschick, 2004; Lappin, Hamilton, & Sullivan, 2006), and
Received: 19 June 2019 Revised: 24 July 2019 Accepted: 13 August 2019
DOI: 10.1002/jmor.21061
Journal of Morphology. 2019;1–8. wileyonlinelibrary.com/journal/jmor © 2019 Wiley Periodicals, Inc. 1
presumably defend against predators. Unsurprisingly, many researchers
have quantified bite force as a measure of whole organismal perfor-
mance (Anderson, McBrayer, & Herrel, 2008) to further elucidate rela-
tionships between cranial morphology, ecology, and fitness (Arnold,
1983; Wainwright, 1994). Bite forces are typically quantified empirically
with live animals or estimated using biomechanical modeling approaches.
in vivo bite forces are measured using modified force transducers with
parallel bite plates that are placed into the specimen's mouth (reviewed
by (Lappin & Jones, 2014). However, studies using this in vivo approach
are often limited because wild animals are not easily accessible. Addition-
ally, some species, despite their interesting biology or critical phyloge-
netic position, are poor or risky experimental subjects due to their
unpredictable or aggressive, defensive behavior (but see Erickson, Lappin,
Parker, & Vliet, 1999; Gignac & Erickson, 2016). As a result, many
researchers have turned to biomechanical models where the feeding
apparatus is modeled as a lever system and quantitative descriptions of
the craniomandibular structure and jaw adductor musculature are used
as parameters to estimate bite forces (e.g., Davis, Santana, Dumont, &
Grosse, 2010; Gignac & Erickson, 2016; Hartstone-Rose, Perry, &
Morrow, 2012; Kolmann, Huber, Motta, & Grubbs, 2015). These models
not only estimate bite force but also allow study of the muscular and
morphological components that contribute to generating bite force. The
more recent development of diffusible iodine-based contrast-enhanced
computed tomography (diceCT) techniques have furthermore made
virtual anatomical reconstructions and digital dissections possible (Gignac
et al., 2016; Jeffery, Stephenson, Gallagher, Jarvis, & Cox, 2011;
Metscher, 2009). These novel approaches have allowed researchers to
visualize complex, soft-tissue morphology in three-dimensional and
obtain muscle architecture such as cross-sectional area (CSA), pennation
angles, and fascicle lengths (Hartstone-Rose & Santana, 2018).
Direct measurements of bite forces are difficult to ascertain and
interpret from mammals. Mammals typically do not offer motivated
bites, presumably due to sensitivity of the periodontal ligament and the
need to protect their singleset of adult dentition; consequently, it is diffi-
cult to determine whether they are biting maximally. Similarly, obtaining
the necessary specimens (i.e., freshly euthanized or recently deceased
specimens with intact jaw musculature for modeling and/or time com-
mitment for diceCT preparation) required to model bite force has proven
to be challenging. As a result, bite forces have been empirically measured
or modeled in only a handful of mammals (e.g., Becerra, Echeverría, Casi-
nos, & Vassallo, 2014; Binder & Van Valkenburgh, 2000; Ellis, Thomason,
Kebreab, Zubair, & France, 2009; Hartstone-Rose et al., 2012; Santana
et al., 2010). Instead, many researchers estimate mammalian bite forces
using Thomason's (1991) dry skull method, an approach that relies on
estimated CSA of the jaw adductor muscles from photographs of osteo-
logical specimens. Because skulls are more readily accessible from
museum collections than muscle tissue, the dry skull method is used to
examine how bite forces relate to diet (Christiansen & Wroe, 2007), cra-
nial stress distribution (Christiansen & Adolfssen, 1999; Thomason,
1991), scaling patterns (Law et al., 2018; Wroe, McHenry, & Thomason,
2005), and phylogenetic signal (Sakamoto, Lloyd, & Benton, 2010). The
dry skull method can also estimate bite forces in extinct mammals (Wroe
et al., 2005).The few studies that have examined the accuracy of the dry
skull method found inconsistent relationships between estimated bite
force and in vivo measurements; the dry skull method underestimates
in vivo measurements in Virginia opossums (Didelphis virginiana)and
domestic dogs (Ellis, Thomason, Kebreab, & France, 2008; Thomason,
1991), but provides an accurate estimate of bite force in New World
leaf-nosed bats (family Phyllostomidae; Davis et al., 2010). Despite the
reasonable accuracy of estimated bite force, Davis et al. (2010) found
that the dry skull method inaccurately estimates CSA of the jaw muscles,
for example, physiological CSA (PCSA) of the temporalis was under-
estimated, but PCSA of the masseter was overestimated. These results
suggest that the correspondence between estimated bite force and
in vivo measurements may be due to the fact the bite force is an emer-
gent property of multiple components and that there are multiple combi-
nations that result in similar performance (i.e., many-to-one mapping of
form to function; Alfaro, Bolnick, & Wainwright, 2005). Unfortunately,
the availability of in vivo measurements needed to validate the dry skull
method is scarce.
In this study, we examine how well Thomason's (1991) dry skull
method corresponds with the dissection-based estimates CSA of the
jaw adductor muscles and theoretical bite force. Specifically, we test
the hypotheses that bite force derived from photographs of osteologi-
cal specimens with estimated PCSA and in-lever lengths of jaw adduc-
tor muscles accurately predicts variables derived from dissection-based
measurements of PCSA and in-lever lengths of jaw adductor muscles
and overall theoretical bite force in the same individuals. We test these
relationships using an ontogenetic series of southern sea otters
(Enhydra lutris nereis) as our model species because of the availability of
a large sample of fresh specimens with intact craniomandibular muscu-
lature for dissections and the known morphological specializations for
consuming hard prey such as short, blunt skulls, taller and wider man-
dibular rami, and bunodont dentition (Constantino et al., 2011; Law,
Baliga, Tinker, & Mehta, 2017; Riley, 1985; Timm-Davis, DeWitt, &
Marshall, 2015). We acknowledge that in vivo bite force measurements
are critical in validating both modeling approaches. Nevertheless, it is
informative to determine the accuracy of the dry skull method when
data are available, and this large sample of specimens will allow us to
investigate the strength of correspondence of both modeling methods
while examining the effects of intraspecific variation on bite force.
2|METHODS
2.1 |Specimens and gross dissections
We obtained 70 naturally deceased southern sea otters (34 females
and 36 males) that were freshly frozen from the California Department
of Fish and Wildlife Marine Wildlife Veterinary Care and Research Cen-
ter (CDFW) from October 2013 to February 2017 (U.S. Department of
the Interior, Letter of Authorization #81440-2007-B0009). Of these
70 specimens, 55 were previously analyzed in Law, Young, and Mehta
(2016) and Law, Venkatram, and Mehta (2016). Our data set consists of
an ontogenetic series from day old pups to 11-year-old adults for both
sexes and contains overlapping body sizes between both sexes. All
specimens were stranded along the central California coast, within and
2LAW AND MEHTA
throughout the current southern sea otter range (Pigeon Point in the
north to Gaviota in the south).
We dissected the major jaw adductor muscles (superficial tempo-
ralis, deep temporalis, superficial masseter, deep masseter, and zygo-
maticomandibularis) following Scapino (1968). While there was
distinct separation between the superficial and deep temporalis, there
was inadequate separation between the superficial masseter, deep
masseter, and zygomaticomandibularis muscles (Scapino, 1968).
Therefore, we treated the three subdivisions as one muscle—the mas-
seter. Furthermore, the medial pterygoid is greatly reduced, making
up approximately 3.3–4.2% of the jaw adductor muscle volume in
other mustelids (Davis, 2014; Timm, 2013; Turnbull, 1970) and is posi-
tioned deep along the medial side of the mandible and cannot be
excised intact. Therefore, we were unable to successfully calculate
PCSA from these muscles; instead, we used estimations from (Timm,
2013) in subsequent analyses (see below). Muscles were removed
from the left side of the skull, blotted dry, weighed to the nearest
0.1 g using a digital scale, and digested in a solution of 15% aqueous
nitric acid for 3–7 days depending on muscle size (Biewener & Full,
1992). We then measured muscle fascicle lengths to the nearest
0.01 mm using a digital caliper (Neiko 01407A). Skulls were subse-
quently cleaned by a dermestid beetle colony at the California Acad-
emy of Sciences or with a maceration tank at CDFW.
2.2 |Theoretical bite force model
We measured theoretical bite force following Law, Young, & Mehta
(2016). Briefly, we modeled jaw closing as a static third class lever
where an axis passing through the temporomandibular joints (TMJs)
serves as the fulcrum and muscle forces generated by jaw adductor
muscle contractions create rotation of the lower jaw about this
fulcrum (Davis et al., 2010; Law, Young, & Mehta, 2016). We first
calculated PCSA for each muscle based on muscle mass (m), mean
fascicle length (f), muscle density (ρ= 1.06 g/cm; (Mendez & Keys,
1960), and fiber pennation angle (θ) (Sacks, Sacks, Roy, & Roy, 1982):
PCSA = m
ρ
cos θ½
f
PCSA of the temporalis muscle was totaled as a sum of the PCSA calcu-
lated for the superficial and deep temporalis subdivisions and modeled
as a single muscle. Following recommendations of (Hartstone-Rose,
Deutsch, Leischner, & Pastor, 2018), we excluded pennation angle in
our estimation of PCSA because of the challenges of measuring pen-
nation angle in multiple planes. To estimate muscle forces of the tem-
poralis and masseter, we multiplied PCSA by a muscle stress value of
30 N/cm
2
(Davis et al., 2010; Pfaller, Gignac, & Erickson, 2011; Santana
et al., 2010). We, however, acknowledge that there are limitations to
this muscle stress value because muscle stress can vary between
147 and 500 kPa in various mammals and even vary between different
types of muscles within an individual (Buchanan, 1995). We then
modeled each muscle force as a single force vector (F
T
and F
M
)and
applied them to the insertion points of the temporalis, that is, the top
of the coronoid process of the mandible, and the masseter, that is, the
midpoint between the anteriormost edge of the masseteric fossa and
the angular process of the mandible. Our estimation of muscle forces
assumed that all of the jaw muscles were maximally activated, which is
typical for models estimating maximal performance (Davis et al., 2010;
Pfaller et al., 2011; Santana et al., 2010).
Following Thomason (1991), we calculated maximum theoretical
bite forces (tBF
X
) by adding the moment (product of the force vector
and in-lever length) of each jaw adductor muscle, dividing by the out-
lever, and multiplying by 2 to account for bilateral biting:
tBFX=2 FT*MAT + FM*MAM
OX
where MAT is the perpendicular in-lever length of the temporalis, mea-
sured as the perpendicular distance from the temporalis muscle force
vector to the TMJ; MAM is the perpendicular in-lever length of the mas-
seter, measured as the perpendicular distance from the masseter muscle
force vector to the TMJ; and O
X
is the out-lever length, measured as the
distances between the bite point and the TMJ (Davis et al., 2010). All in-
lever and out-lever measurements were taken from photographs of the
lateral view of the mandible in ImageJ v. 1.48 (Schneider, Rasband, &
Eliceiri, 2012). We calculated theoretical bite forces at two locations, the
lower canine (BF
C
) and in between first and second molar of the lower
jaw (BF
M
). We also incorporated the effects of the medial pterygoid in
our calculations of PCSA and theoretical bite force using Timm's (2013)
estimation of medial pterygoid force generation. Estimation of medial
pterygoid force generation (19.5 N) accounted for just 4.9% of total jaw
muscle force generation in Alaskan sea otters (Timm, 2013). Therefore,
we increased PCSA of the masseter–pterygoid complex and theoretical
BF
C
and BF
M
by 4.9% for each individual.
2.3 |Estimation of bite force with dry skull method
To estimate bite forces using Thomason's (1991) dry skull method, we
photographed each specimen in three views: (a) the cranium in
posteriodorsal view, photographed by orienting the cranium so that the
plane between the orbital processes and posteriormost points of the
zygomatic arches is parallel to the photographic plane; (b) the cranium in
ventral view, photographed by orienting the palate plane parallel to the
photographic plane; and (c) the mandible in lateral view, photographed
by orienting the long axis of the dentary parallel to the photographic
plane. We estimated CSAs of the temporalis and masseter–pterygoid
complex by outlining the left and right infratemporal fossae from the
posteriodorsal cranial and ventral cranial views, respectively (Figure 1a,b).
Following Thomason (1991), the resulting areas were then multiplied by
amusclestressvalue30N/cm
2
to estimate forces of the temporalis and
masseter–pterygoid complex. Each muscle force was modeled as a single
force vector (T
DS
and M
DS
) and assumed to act through their centroids
perpendicular to the plane of the muscle CSA. We then measured the
moment arms (in-levers)ofthetemporalis(MAT
DS
) and masseter–
pterygoid complex (MAM
DS
) as the distance from centroid to the TMJ
LAW AND MEHTA 3
using the lateral and ventral views of the cranium (Figure 1). Bite force
with the dry skull method (dsBF
X
) was then estimated as
dsBFX=2 TDS*MATDS +MDS *MAMDS
OX
All CSAs and linear measurements were taken using ImageJ
v. 1.48 (Schneider et al., 2012).
2.4 |Statistical analyses
If the dry skull method reliably estimates jaw muscle PCSA across
ontogeny, we would expect regressions between variables from the
dry skull method and variables from the biomechanical model to
exhibit strong correlations (R
2
> 0.95) with slopes of 1.0 and inter-
cepts of 0. To test this, we performed a series of standardized major
axis (SMA) regressions of variables derived from the bite force model
(PCSA, MAT, MAM, and tBF) against the variables estimated from the
dry skull method (CSA, MAT
DS
, MAM
DS
, and dsBF). We used ttests
to determine if each slope significantly deviated from 1.0 and if each
intercept significantly deviated from 0. Predicted slopes/intercepts
significantly greater or less than the 95% confidence intervals of the
observed SMA regression slopes/intercepts were interpreted as over-
estimations or underestimations, respectively. Because sea otters
exhibit male-biased sexual dimorphism in craniomandibular size,
shape, and musculature (Law et al., 2017; Law, Venkatram, & Mehta,
2016; Law, Young, & Mehta, 2016), we also used sex as a main factor
in our SMA regressions to determine if slopes differed between
female and male otters. We combined the female and male data sets
FIGURE 1 Photographs of the cranium in (a) posterodorsal, (b) ventral, and (c) lateral views used to estimate cross-sectional areas (CSAs) and
in-lever lengths of the temporalis and masseter–pterygoid complex. Shaded areas are reconstructed muscle CSAs of the (a) temporalis and
(b) masseter–pterygoid group. White dots represent the centroids of the resultant force vectors of the (a) temporalis and (b) masseter–pterygoid.
Photographs of the lateral view of the mandible were used to measure out-levers. MAT
DS
estimated temporalis in-lever; MAM
DS
estimated
masseter in-lever; O
C
out-lever to canine; O
M
out-lever to molar [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1 SMA regressions between parameters from dissection-based modeling (independent variables) and parameters estimated from the
dry skull method (dependent variables)
Slope test Intercept test
R
2
Slope (95% CI) pValue
Different
from 1.0?
Accuracy of dry
skull method? Intercept (95% CI) pValue
Different
from 0?
PCSA
tem
.79 0.78 (0.69–0.87) <.001 Lower Underestimated −0.38 (−1.08–0.33) .293 No
PCSA
mas
.80 2.47 (2.21–2.75) <.001 Greater Overestimated 1.90 (1.29–2.52) <.001 Greater
MAT .76 0.55 (0.49–0.62) <.001 Lower Underestimated 0.69 (0.52–0.88) <.001 Lower
MAM .69 0.86 (0.75–0.90) <.001 Lower Underestimated 0.02 (−0.15–0.18) .832 No
BF
canine
.89 0.77 (0.71–0.83) <.001 Lower Underestimated 22.61 (9.04–36.19) .001 Greater
BF
molar
.88 0.76 (0.72–0.81) <.001 Lower Underestimated 43.89 (18.29–69.50) .001 Greater
Abbreviations: BF, bite force; MAM, masseter in-lever; mas, masseter; MAT, temporalis in-lever; PCSA, physiological cross-sectional area; SMA,
standardized major axis; tem, temporalis.
Note. All pvalues are reported as Benjamini–Hochberg-corrected values. Bold pvalues indicate significance (α= .05).
4LAW AND MEHTA
and reran the regressions in cases where the slope did not signifi-
cantly differ between the sexes. We adjusted all p-values using a
Benjamini–Hochberg correction to reduce the Type I error probability
across multiple comparisons (Benjamini & Hochberg, 1995). All statis-
tical analyses were performed in the smatr (Warton, Duursma,
Falster, & Taskinen, 2011) package in R 3.2.3 (R Core Team, 2017).
3|RESULTS
We found strong positive relationships between measured PCSA and
estimated CSA of the temporalis (R
2
= .79, p< .001) and of the masse-
ter (R
2
= .80, p< .001) across ontogeny (Table 1; Figure 2). However,
the slope (β) between measured PCSA and estimated CSA of the tem-
poralis was significantly lower than 1.0 (β= .778, p< .001) with an
intercept not significantly different from 0 whereas the slope between
measured PCSA and estimated CSA of the masseter was significantly
higher than 1.0 (β= 2.465, p< .001) with an intercept significantly
greater than 0 (Table 1; Figure 2). These results indicated that the dry
skull method underestimated temporalis PCSA, but overestimated
masseter PCSA across ontogeny. These results do not change when
incorporating the medial pterygoid muscle into estimation of masseter
PCSA (β= 2.347, p< .001). Incorporating the effects of sex in the
regression analyses revealed that there were significant differences in
slopes between the sexes for temporalis PCSA (p= .002) but not for
masseter PCSA (p= .485): the dry skull method predicted the tempo-
ralis PCSA well in females across ontogeny (β= 1.038; CI: 0.866–1.244;
p= .679) but underestimated the temporalis PCSA in males across
ontogeny (β= .730; CI: 0.640–0.832; p< .001; Figure 2).
Comparisons between traditional in-lever measurements and the
dry skull method's in-lever measurements revealed strong positive rela-
tionships between measured MAT and estimated MAT
DS
(R
2
=.76,
p< .001) as well as between measured MAM and estimated MAM
DS
(R
2
=.70,p< .001) across ontogeny. However, MAT and MAM were
also both underestimated by the dry skull method, as the slopes were
significantly less than 1.0 (β
MAT
=.549,p<.001;β
MAM
=.858,p=.025;
Table 1; Figure 3).
Theoretical bite forces derived from the dissection-based model
exhibited strong positive relationships with bite forces estimated from
the dry skull method at both the canine and molar across ontogeny
(R
2
= .89, p< .001; Table 1; Figure 4). The slopes between theoretical
bite forces and estimated bite forces were significantly lower than 1.0
(β
canine
= .767, p< .001; β
molar
= .763, p< .001; Table 1; Figure 4), indicat-
ing that the dry skull method underestimated bite forces that were
derived from the dissection-based model.Theseresultsdonotchange
when incorporating the medial pterygoid muscle (β
canine
= .730, p< .001;
(a) (b)
FIGURE 2 Standardized major
axis regressions of estimated cross-
sectional area (CSA) on measured
physiological CSA (PCSA) of
(a) temporalis (y=0.778x−0.375) and
(b) masseter (y=2.465x+1.904).Red
circles indicate female otters, and blue
triangles indicate male otters. Red and
blue solid lines represent regressions
for females and males, respectively.
Black lines represent common
regression for combined female and
males. Dotted lines indicate slope of
1.0. An asterisk indicates that the
slope is significantly different from
slope of 1.0 [Color figure can be
viewed at wileyonlinelibrary.com]
0.5 1.5 2.5 3.5
0.5 1.0 1.5 2.0
1.5
2.0
2.5
3.0
3.5
(a)
(b)
0.25
0.75
1.25
1.75
MAT (cm)
MAMDS
(cm) DS
(cm)
common slope = 0.549*
1:1 slope = 1
common slope = 0.858*
1:1 slope = 1
MAT
MAT (cm)
FIGURE 3 Standardized major axis regressions of estimated in-
lever lengths on measured in-lever lengths of (a) temporalis
(y= 0.549x+ 0.697) and (b) masseter (y= 0.858x+ 0.018). Black lines
represent common regression for combined female and males. Dotted
lines indicate slope of 1.0. An asterisk indicates that the slope is
significantly different from slope of 1.0
LAW AND MEHTA 5
β
molar
= .727, p< .001). Consistent with the dissection-based model
reported in Law, Young, and Mehta (2016), we did not find significant
sexual dimorphism in comparisons of estimated bite force slopes when
using the dry skull method (p> .112).
4|DISCUSSION
Although we found strong positive relationship between all variables
estimated from the dry skull method and measured using the dissection-
based model, evaluation of the slopes and intercepts revealed that the
dry skull model inaccurately represents the anatomical traits of the jaw
adductor muscles. Our results corroborated previous findings in New
World leaf-nosed bats that the dry skull method underestimated tempo-
ralis PCSA, but overestimated masseter PCSA (Davis et al., 2010). We
also found that the dry skull method underestimated in-lever measure-
ments when compared to in-lever lengths measured directly from the
mandibles. Underestimation of temporalis PCSA was not too surprising
considering that the dry skull method estimated temporalis CSA from
only the infratemporal fossae even though the temporalis muscle
extends posterior of the infratemporal fossae and across the entire dorsal
and lateral parts of the cranium (Davis, 2014; Scapino, 1968; Turnbull,
1970). A possible reason for why the dry skull method overestimated
masseter PCSA was that our dissection-based model does not account
for the force generated by the medial pterygoid, which can contribute to
biting ability. However, the medial pterygoid is greatly reduced, making
up approximately 3.3–4.2% of the jaw adductor muscle volume in other
mustelids (Davis, 2014; Turnbull, 1970). We found that incorporating the
medial pterygoid (Timm, 2013) does not affect our results, indicating that
eliminating the PCSA of the medial pterygoid in our analyses is not the
reason why CSA of the masseter–pterygoid complex was overestimated
by the dry skull method.
Together, the underestimations of in-lever measurements and CSA
of the jaw adductor muscles contributed to the dry skull method's under-
estimation of theoretical bite forces obtained through biomechanical
modeling. These underestimations were consistent with findings that the
dry skull method underestimated in vivo measurements in Virginia opos-
sums (D. virginiana) and domestic dogs (Ellis et al., 2008; Thomason,
1991). Thomason corrected for this underestimation in opossums by
using regression equations between estimated bite force (from the dry
skull method) and theoretical bite forces (from dissection-based model),
which closely matched his in vivo measurements (Thomason, 1991).
Although some researchers have adopted this correction method
(e.g., Sakamoto et al., 2010), other researchers simply used higher muscle
stress values (i.e., 37 N/cm
2
instead of the typical 25–30 N/cm
2
range
(e.g., Christiansen & Adolfssen, 1999; Christiansen & Wroe, 2007; Wroe
et al., 2005) rather than extrapolating beyond the range of observed and
validated opossum bite forces. An important caveat to our interpretation
is that the biomechanical model of theoretical bite force has yet to be
validated with empirical bite force measurements from wild southern sea
otters. These empirical data are essential to determine the accuracy of
our biomechanical model. Unfortunately, direct measurements of bite
forces are difficult to obtain from mammals due to lack of motivated
bites or pathogenic risks (but see examples with rodents and bats; Herrel,
De Smet, Aguirre, & Aerts, 2008; Santana et al., 2010; Becerra et al.,
2014). Sea otters, furthermore, are protected under the US Marine
Mammal Protection Act as well as the U.S. Endangered Species Act,
making in vivo studies challenging. Although the current study is unable
to evaluate which method of bite force estimation is the best approach
to predict actual bite forces, we provide evidence that the dry skull
method relies on too many assumptions to accurately capture the
mechanics of biting in mammals.
Surprisingly, we found that intraspecific morphological variation
from sexual dimorphism can affect PCSA estimations using the dry
skull method. We found significant differences in slopes between the
sexes for temporalis PCSA: temporalis PCSA of female otters was well
predicted by the dry skull whereas temporalis PCSA of male otters
was underestimated. Sexual dimorphism in cranial size and shape may
explain the difference in slopes between females and males. Unlike
other mustelid species (Abramov & Tumanov, 2003; Lynch & O'Sulli-
van, 1993; Thom, Harrington, & Macdonald, 2004; Wiig, 1986), the
postorbital constriction in adult female southern sea otters is signifi-
cantly narrower (Law, Venkatram, & Mehta, 2016). Although narrower
postorbital constriction suggests relatively larger temporalis muscula-
ture (Radinsky, 1981a, 1981b), previous work found that there was no
significant difference in temporalis mass per unit size; that is, a female
and male otter of similar sizes exhibited similarly sized temporalis
muscles (Law, Young, & Mehta, 2016). Therefore, sexual dimorphism
in craniomandibular morphology does not directly translate to sexual
dimorphism in functional traits. Because the dry skull method does
not incorporate musculature in its estimation of muscle CSA and relies
only on the osteological specimen, any sexual dimorphic signal in
theoretical bite force from dissection-based model (N)
estimated bite force from dry skull method (N)
0 200 400 600 800
0 200 400 600 800
0
200
400
600
0
200
400
600
(a)
(b) BF at molar
common slope = 0.763*
1:1 slope = 1
BF at canine
common slope = 0.767*
1:1 slope = 1
FIGURE 4 Standardized major axis regressions of bite forces
estimated from the dry skull method on theoretical bite force derived
from biomechanical modeling at the (a) canine (y= 0.767x+ 22.614) and
(b) molar (y=0.763x+ 1.299). Black lines represent common regression
for combined female and males. Dotted lines indicate slope of 1.0. An
asterisk indicates that the slope is significantly different from slope of 1.0
6LAW AND MEHTA
cranial morphology will potentially be captured by the dry skull
method. In the case of female otters, the narrower postorbital con-
striction increased the area of the infratemporal fossae measured
from the posteriodorsal cranial view and therefore led to the increase
in temporalis CSA. These results suggest that sexual dimorphism on
the postorbital constriction and other craniomandibular traits can also
affect estimations of bite force when using the dry skull method. The
extent of these intraspecific effects on other species, however,
remains to be investigated further.
5|CONCLUSION
The dry skull method is widely used to estimate bite forces in mammals
(Thomason, 1991). Because this method relies solely on osteological
specimens rather than live or freshly deceased specimens, researchers
areabletoestimateandcomparebiteforcesacrossmammalsandeven
extinct species. However, few studies have tested how accurately the
dry skull method predicts underlying skeletomuscular components and
theoretical bite force. Although we found strong positive relationships
between variables estimated from the dry skull method and variables
derived from dissection-based modeling, the dry skull method overesti-
mates masseter PCSA and underestimates temporalis PCSA, jaw adduc-
tor in-levers, and overall theoretical bite force. Our results, furthermore,
revealed that sexual dimorphism in craniomandibular morphology affects
estimations of jaw adductor muscle PCSA, highlighting that accounting
for intraspecific variation is essential when drawing relationships
between skeletal traits and musculature. Together, these results indicate
that the dry skull method provides inaccurate estimations of PCSA and
theoretical bite force across ontogeny and may also lead to error in stud-
ies of intraspecific variation of bite forces. Therefore, caution must be
taken when interpreting bite forces derived from the dry skull method in
studies of maximal bite force performances in individuals as well as over
ontogeny. Unfortunately, obtaining empirical bite forces or muscular
samples for theoretical bite forces is not always possible, especially for
large-scale macroevolutionary analyses of feeding performance. Gather-
ing these data will allow for more accurate assessment of the dry skull
method to use in future ecomorphological studies.
ACKNOWLEDGMENTS
Both authors are grateful to Colleen Young, Erin Dodd, Francesca
Batac, and Sue Pemberton for helping clean out the skulls and Lilian
Carswell for permitting logistics. This study was funded partly by a
Grant-in-Aid of Research from the American Society of Mammalo-
gists, a Lerner-Gray Fund through the American Museum of Natural
History, an American Cetacean Society-Monterey Bay Chapter Grant,
a National Science Foundation (NSF) Graduate Research Fellowship,
and an NSF Doctoral Dissertation Improvement Grant 1700989.
CONFLICT OF INTEREST
Both authors declare no conflict of interest.
AUTHOR CONTRIBUTIONS
C.J.L. designed the study, developed dissection-based bite force
model, measured bite force data with the dry skull method, performed
data analyses, and drafted the manuscript. R.S.M. helped develop the
approach and dissection-based bite force model, provided crucial
insights, and revised the manuscript. Both authors read and approved
the final manuscript.
ORCID
Chris J. Law https://orcid.org/0000-0003-1575-7746
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How to cite this article: Law CJ, Mehta RS. Dry versus wet
and gross: Comparisons between the dry skull method and
gross dissection in estimations of jaw muscle cross-sectional
area and bite forces in sea otters. Journal of Morphology. 2019;
1–8. https://doi.org/10.1002/jmor.21061
8LAW AND MEHTA
