Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
Bite force, cranial morphometrics and size
in the fishing bat Myotis vivesi (Chiroptera: Vespertilionidae)
Sandra M. Ospina-Garcés
, Efraín De Luna
L. Gerardo Herrera M.
(https://orcid.org/0000-0001-9348-5040), Joaquín Arroyo-Cabrales
& José Juan Flores-Martínez
1. Posgrado en Ciencias Biológicas, Instituto de Biología, Universidad Nacional Autónoma de México, Apartado Postal
70-153, 04510 México, Distrito Federal; ospinagarcess@gmail.com https://orcid.org/0000-0002-0950-4390
2. Instituto de Ecología, A.C. Carretera Antigua a Coatepec 351, El Haya, Xalapa, 91070, México,
ospinagarcess@gmail.com; https://orcid.org/0000-0002-0950-4390
3. Instituto de Ecología A.C. Biodiversidad y Sistemática, Xalapa, Veracruz, 91070; efrain.deluna@inecol.mx, https://
4. Estación de Biología de Chamela, Instituto de Biología, Universidad Nacional Autónoma de México, Apartado Postal
21, San Patricio, Jalisco, 48980, México; gherrera@ib.unam.mx
5. Laboratorio de Arqueozoología, ‘M. en C. Ticul Álvarez Solórzano’ INAH, Moneda # 16 Col. Centro, 06060 México,
Distrito Federal; arromatu5@yahoo.com.mx
6. Laboratorio de Sistemas de Información Geográfica, Departamento de Zoología. Instituto de Biología, Universidad
Nacional Autónoma de México, Circuito Exterior, Edificio Nuevo, Módulo C, Apartado Postal 70-153, 04510
México, Distrito Federal; jj@ib.unam.mx
* Correspondence
Received 06-IV-2018. Corrected 31-VII-2018. Accepted 18-IX-2018.
Abstract: Fish-eating in bats evolved independently in Myotis vivesi (Vespertillionidae) and Noctilio leporinus
(Noctilionidae). We compared cranial morphological characters and bite force between these species to test
the existence of evolutionary parallelism in piscivory. We collected cranial distances of M. vivesi, two related
insectivorous bats (M. velifer and M. keaysi), two facultatively piscivorous bats (M. daubentonii and M. capac-
cinii), and N. leporinus. We analyzed morphometric data applying multivariate methods to test for differences
among the six species. We also measured bite force in M. vivesi and evaluated if this value was well predicted
by its cranial size. Both piscivorous species were morphologically different from the facultatively piscivorous
and insectivorous species, and skull size had a significant contribution to this difference. However, we did not
find morphological and functional similarities that could be interpreted as parallelisms between M. vivesi and N.
leporinus. These two piscivorous species differed significantly in cranial measurements and in bite force. Bite
force measured for M. vivesi was well predicted by skull size. Piscivory in M. vivesi might be associated to the
existence of a vertically displaced temporal muscle and an increase in gape angle that allows a moderate bite
force to process food.
Key words: bite force; cranial morphology; Myotis; Noctilio; piscivory; size; gape angle.
The relationship between the structure of
the masticatory apparatus and diet has long
been investigated in bats (Freeman, 1981;
1984; Van Cakenberghe, Herrel, & Aguirre,
2002, Swartz, Freeman, & Stockwell, 2003;
Nogueira, Monteiro, Peracchi, & De Araújo,
2005; Dumont, Herrel, Medellin, Vargas-Con-
treras, & Santana, 2009; Nogueira, Peracchi,
Ospina-Garcés, S.M., De Luna, E.,
Herrera M., L.G., Arroyo-Cabrales, J., & Flores-Martínez,
J.J. (2018). Bite force, cranial morphometrics and size in the fishing bat Myotis vivesi
(Chiroptera: Vespertilionidae). Revista de Biología Tropical, 66(4), 1614-1628.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
& Monteiro, 2009). Morphological differences
in cranial size are related to differences in diet
and have consequences in the performance of
the masticatory muscles. For example, volume,
length and position of the temporal and mas-
seter muscles vary among bat species from
different dietary groups (Dumont, 1999; Free-
man, 1981; 1984, Nogueira et al., 2005, Swartz
et al., 2003; Van Cakenberghe et al., 2002;
Dumont et al., 2009). Such morphological
variation could be related to size and hardness
of food items because these two features affect
the performance of the masticatory muscles
(Dumont, 1999; Nogueira et al., 2005; Ghazali
& Dzeverin, 2013).
Masticatory performance in bats has been
estimated analyzing bite force in relation to
body size and masticatory muscle morphology.
Body size and bite force are highly correlated,
as well as bite force and mass and length of
fibers of the masticatory muscle (Aguirre,
Herrel, Van Damme, & Matthysen, 2002;
2003; Herrel, De Smet, Aguirre, & Aerts,
2008). Cranial size, muscle masses and the
fiber length of the temporal muscle are the
best predictors of bite force: species with a
larger cranium, a larger temporal muscle mass
and shorter temporal fiber lengths bite harder
(Dumont & Herrel, 2003). In general terms,
the cranial muscular system is a good predic-
tor of bite force (Dumont & Herrel, 2003;
Dumont et al., 2009).
Cranial traits related the performance
of masticatory muscles suggest parallelisms
among some bat species with the same diet. For
example, insectivorous bats have narrow and
elongated faces, and their zygomatic breadth
is about 55-70 % of the condylocanine length.
In contrast, bat species that feed on small ver-
tebrates (e.g., Noctilio leporinus, Cheiromeles
torquatus, Scotophilus gigas and Saccolaimus
peli) have wide and short faces, with a zygo-
matic breadth about 70-80 % of the condyloca-
nine length (Freeman, 1981). Cranial size and
the change to vertical orientation of the tempo-
ral muscle might allow wider gapes facilitat-
ing prey processing of vertebrate-eating bats
(Emerson & Radinsky, 1980; Van Cakenberghe
et al., 2002). Other cranial traits have been
associated with carnivory, not only in bats but
also in saber-tooth cats (Emerson & Radinsky,
1980). A laterally-flaring angular processes,
a low coronoid process, a vertically oriented
temporal fossa, and a high origin/insertion ratio
of the temporal muscle suggest wide gapes and
strong bites in carnivorous mammals (Emer-
son & Radinsky, 1980; Freeman, 1988). The
piscivorous bat N. leporinus (Linnaeus, 1758)
presents these features (Freeman, 1981; Her-
ring & Herring, 1974).
Piscivory in bats appeared in parallel in N.
leporinus (Noctilionidae) and some species in
the genus Myotis (Vespertilionidae). M. vivesi
(Menegaux, 1901) is the only truly piscivorous
species (Blood & Clark, 1998), whereas seven
other species are insectivores that occasionally
feed on fish (M. ricketti, M. daubentonii, M.
capaccinii, M. adversus, M. macrotarsus, M.
albescens and M. stalkeri; Aihartza, Almenar,
Goiti, Salsamendi, & Garin, 2008; Flan-
nery, 1995; Law & Urquhart, 2000; Siemers,
Dietz, Nill, & Schnitzler, 2001; Whitaker &
Findley, 1980). These eight species were ini-
tially grouped in the subgenus Leuconoe with
other entomophagus Myotis, due to similar
external characteristics (Findley, 1972). This
subgenus has not been supported by molecular
techniques and is treated at present not as taxon
but as an ecomorph (Ruedi & Mayer, 2001).
Typically, Leuconoe bats prey insects near the
water surface and this can possibly make easier
a transition to piscivory in large-sized Leuco-
noe. Additionally, external morphological traits
are associated with the presence of fish in their
diets including laterally compressed and large
claws to catch prey on the water surface (Free-
man, 1981; Norberg & Rayner, 1987; Lewis-
Oritt, Van Den Bussche, & Baker, 2001).
However, a phylogenetic analysis showed that
the facultatively piscivorous and piscivorous
Myotis species are not a monophyletic group
(Ruedi & Mayer, 2001; Stadelmann, Herre-
ra, Arroyo-Cabrales, Flores-Martinez, May,
Ruedi, 2004). Members of the genus Myotis
represent an excellent model to examine chang-
es in the masticatory apparatus morphology
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
associated to the evolution of piscivory from
insectivorous ancestors (Lewis-Oritt et al.,
2001). Geometric morphometric analyses per-
formed recently in piscivorous Myotis species
describe morphological changes in mandibular
masticatory process position related to gape
capacity and diet, and a significant effect of
size in skull shape variation (Ospina-Garcés,
De Luna, Gerardo-Herrera, & Flores-Mar-
nez, 2016). In the present study we explored
if there are morphological differences in the
masticatory apparatus of M. vivesi with respect
to their facultatively piscivorous (M. capaccinii
and M. daubentonii) and insectivorous rela-
tives (M. keaysi and M.velifer), and the effect
of skull size in this variation. We included
N. leporinus in the comparison to evaluate
if there are parallel trends of variation in the
masticatory apparatus of M. vivesi. In addition,
we compared masticatory performance of M.
vivesi measured as bite force in relation to skull
size with data previously reported for insec-
tivorous and facultatively piscivorous Myotis,
N. leporinus and unrelated insectivorous spe-
cies (Aguirre et al., 2002; Herrel et al., 2008).
We addressed the following questions: 1)
does cranial morphology and performance of
M. vivesi differ compared to their facultative-
ly piscivorous or insectivorous relatives and
are cranial morphotypes in M. vivesi similar
enough to be interpreted as parallelism in the
non-related piscivorous N. leporinus?, and 2)
are the morphological and functional differ-
ences explained by the increase of skull size
in both piscivorous species? We hypothesized
that cranial morphology and performance of
M. vivesi would differ with respect to insec-
tivorous and facultatively piscivorous Myotis,
but present similarities with N. leporinus. We
expected that the position and development of
the masticatory muscles would be more differ-
ent between M. vivesi and insectivorous Myotis
than with respect to the masticatory morpholo-
gy in facultatively piscivorous Myotis. We also
anticipated that bite force in M. vivesi would
be similar to that expected for its size, and that
size-corrected bite force would be similar to
that of N. leporinus.
Study system: We examined skulls and
jaws from 228 adult bat specimens of two
piscivores (Myotis vivesi: 20 females, 27 males;
Noctilio leporinus: 27 females, 24 males), two
insectivores (M. velifer: 26 females, 27 males;
M. keaysi: 31females, 19 males) and two
facultatively piscivores (M. daubentonii: 12
females, 4 males; M. capaccinii: 5 females, 6
males). Age of specimens was assessed follow-
ing the criteria proposed by Pacheco & Pat-
terson (1992). Specimens (Appendix) belong
to the National Mammal Collection (México),
the Osteological Collection of the National
Institute of Anthropology and History (Méxi-
co), the Doñana Biological Station (Spain),
and the Humboldt University Natural History
Museum (Germany).
Measurements: Inter-landmark distances
were collected using a digital caliper (Mitu-
toyo CD-6´´ Mitutoyo U.S.A.) to the nearest
0.01 mm on the right side of skull and jaw.
We measured nine dimensions on the skull and
five on the jaw (Fig. 1) from each individual:
1. Greatest skull length (GSL), 2. Maximum
zygomatic breadth (MZB), 3. Condylocanine
lenght (CCL), 4. Toothrow lenght (MTR), 5.
Maxillary breadth across M3 (M3B), 6. Tem-
poral origin (Ori T), 7. Temporal insertion (ins
T), 8. Postorbital constriction width (POC), 9.
Masseter origin (Ori M), 10. Masseter insertion
(Ins M), 11. Dentary lenght (DL), 12. Dentary
depthness under the protoconid of m2 (DD),
13. Coronoid process height (CPH), 14. Mas-
seteric fosa length (MFL), 15. Elevation of
condyle angle (ECA), 16. Coronoid process
angle (CPA).
Specific average and standard deviation
were presented in Table 1. Some of these mea-
surements are related to the development of
the masticatory muscle (Emerson & Radinsky
1980; Freeman 1984). Other measurements are
related to the size of the skull, the zygomatic
breadth, and the condyle canine length. Maxil-
lary breadth indicates the length of the skull,
and the dentary depth under the protoconid of
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
m2 (DD) reflects the thickening of the dentary.
Because there are differences in skull size
among species considered, measurements were
size-adjusted with a geometric mean. Dis-
tances for each individual were divided by the
geometric mean of all 14 measurements from
the same individual (Dumont, 2004).
Two angles were estimated on digital
photographs of the right side of the mandible
of each individual with the program tpsDIG2
Cranial morphological distances and angles measured in five Myotis and one Noctilio species
M. cappacinii M. daubentonii M. keaysi M. velifer M. vivesi N. leporinus
(1) GSL 15.33 (0.21) 14.80 (0.32) 13.01 (0.31) 16.33 (0.34) 21.44 (0.41) 26.72 (1.44)
(2) MZB 9.33 (0.19) 8.98 (0.31) 7.96 (0.25) 10.69 (0.26) 14.06 (0.3) 19.27 (0.73)
(3) CCL 13.45 (0.18) 12.89 (0.33) 11.51 (0.41) 14.83 (0.25) 19.57 (0.56) 23.56 (0.65)
(4) MTR 5.66 (0.08) 5.36 (0.12) 4.91 (0.22) 6.59 (0.29) 9.18 (0.24) 10.61 (0.3)
(5) M3B 6.07 (0.12) 5.79 (0.16) 5.16 (0.2) 6.88 (0.15) 8.88 (0.22) 13.02 (0.38)
(6) Ori T 8.78 (0.22) 8.51 (0.22) 7.42 (0.31) 9.16 (0.23) 11.91 (0.36) 20.5 (1.35)
(7) Ins T 5.79 (0.14) 5.42 (0.31) 4.94 (0.22) 6.09 (0.29) 6.99 (0.22) 13.91 (1.28)
(8) POC 3.79 (0.09) 4.13 (0.13) 3.31 (0.13) 3.99 (0.12) 5.56 (0.15) 7.3 (0.27)
(9) Ori M 3.83 (0.14) 3.74 (0.09) 3.4 (0.18) 4.76 (0.17) 5.91 (0.36) 7.86 (0.57)
(10) Ins M 2.23 (0.75) 2.08 (0.08) 1.81 (0.09) 2.50 (0.12) 3.03 (0.14) 4.9 (0.5)
(11) DL 11.22 (0.319) 10.63 (0.27) 9.61 (0.3) 12.92 (0.25) 17.08 (0.4) 19.42 (0.71)
(12) DD 1.13 (0.10) 1.08 (0.05) 1.04 (0.07) 1.46 (0.1) 1.74 (0.08) 3.14 (0.36)
(13) CPH 3.00 (0.13) 3.00 (0.13) 2.69 (0.16) 3.94 (0.14) 5.03 (0.17) 6.69 (0.27)
(14) MFL 2.83 (0.15) 2.73 (0.15) 2.47 (0.15) 3.51 (0.2) 4.32 (0.2) 7.04 (0.6)
(15) ECA 0.91 (0.02) 0.86 (0.05) 0.86 (0.05) 0.86 (0.05) 0.97 (0.06) 1.20 (0.05)
(16) CPA 0.99 (0.06) 1.02 (0.06) 1.06 (0.04) 1.09 (0.04) 1.15 (0.06) 1.26 (0.04)
* Values are described as mean (± SD). Abbreviations of 14 cranial measurements and two angles are ordered as in Fig. 1.
Fig. 1. Cranial measurements and angles recorded in this study. Numbers designate
14 inter-landmark distances and two angles.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
(Rohlf, 2005). We used the elevation of the
coronoid process and the elevation of the con-
dyle angles to infer the direction of the masse-
ter and temporal muscles. The elevation of the
coronoid process angle (CPA) was measured
between the lines corresponding to the height
of the coronoid process and the dentary line
(Fig. 1). The elevation of the condyle angle
(ECA) was measured between the lines cor-
responding to the coronoid process height and
a line from the condyle to the coronoid process
(Fig. 1). The angles estimate the height of the
coronoid and condylar processes respect to the
dentary and therefore the orientation of the
masticatory muscles (Emerson & Radinsky,
1980). Both angle values were transformed to
radians for further statistical analyses (vari-
ables 15 and 16, Table 1).
We measured bite force using a piezo
resistive force sensor (Flexiforce A 201-100,
Tekscan, Inc., Boston), with a sensitivity range
of 0-100 pounds. This device consists of a
plate that works like a load cell in an electrical
circuit; it has a resistance that changes with the
force applied in a sensible area at the end of the
plate where the lowest electrical resistance cor-
responds to the maximum force. We obtained
the resistance of the cell with an industrial
multi-meter (Techmaster D-8500, Techmaster
de México S.A., Tijuana) with a sensibility of
400 MΩ. This instrument requires a pressure
of at least three seconds to show any recording.
The force applied is calculated by a regression
using resistance values from the application of
known weights.
Bite force was estimated from 15 adults of
M. vivesi (3 males and 12 females; body mass:
30.85 ± 4.8 g, mean ± SD). Specimens were
caught in Partida Norte Island, México (28°
53´ 30” N - 113° 02´ 25” W) and weighted
with a PESOLA precision scale (100 g maxi-
mum capacity, MICROLINE, Switzerland).
Bite force measurements were made in situ
between 09:00 am and 1:00 pm. The specimens
were placed in front of the sensor so that the
mouth occupied the first third of the sensitive
area because the plate is larger than the mouth.
This procedure was performed three times
for each individual and low-pressure readings
were discarded. We used the lowest resis-
tance measurements recorded that correspond
to the highest bite force (Freeman & Lemen,
2008). After the measurements, individuals
were released at the capture site. Procedures
followed guidelines approved by the American
Society of Mammalogists (Sikes & Gannon,
2011) and were collected under permission by
the Secretaría de Medio Ambiente y Recursos
Naturales (SEMARNAT).
Statistical analysis: Data for analyses
consisted in fourteen variables measured on
the skull and two angles (CPA: Coronoid
Process angle and ECA: Elevation of Condyle
angle) expressed as radians (Table 1). Statisti-
cal analyses were structured to compare six a
pRiori groups (six species). Data without trans-
formation and size-adjusted variables were
tested for normality within each species with
the Shapiro-Wilk test. Since differences in size
between sexes could hide differences between
species, we evaluated sexual dimorphism. We
performed a two-way analysis of variance
(ANOVA) to test differences between species
and sexes in the greatest skull length (GSL).
Post-hoc comparisons between sexes within
species were performed to identify species with
significant dimorphism.
Multivariate differences in cranial mor-
phology among six species were first examined
with a Canonical Variate Analysis (CVA) based
on variances of 14 measurements on skulls and
jaws and the two angles. These sixteen vari-
ables were transformed to base 10 logarithms.
A second CVA analysis with fifteen size-
adjusted variables, with GSL removed from
the data matrix, was performed to determine
the contribution of variables to discriminate
groups when the size effect is reduced. Size
contribution to the contrast between species
with different body mass (Freeman, 1981) was
examined by the comparison between resulting
CVA analyses and calculating the correlation
between the GLS and the scores of the first CV
from the size-adjusted CVA.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
Bite force in M. vivesi was inferred from
a linear regression between known weights of
steel nuts (N = 8), transformed in force (N), and
resistance (MΩ). We measured the resistance
of the different steel pieces added one at time
to increase the force measured in Newtons and
therefore the resistance of the sensor, and we
performed the bivariate regression between
force of each group of steel pieces and its resis-
tance (Fig. 2). Resistances of maximum bites
measured in the 15 individuals of M. vivesi
were replaced in equation of this regression
to obtain the bite force in Newtons. Bite force
mean values previously reported for 10 insec-
tivorous bat species, two facultatively piscivo-
rous Myotis and the piscivorous N. leporinus
(Aguirre et al., 2003, Freeman & Lemen, 2010;
Krüger, Clare, Greif, Siemers, Symondson, &
Sommer, 2014) were used to conduct a linear
regression analysis with the estimator of skull
size (GLS). We performed the Blomberg’s K
phylogenetic signal test on the residuals of this
linear model following Revell (2010), using the
super tree of bats (Jones, Purvis, Maclarnon,
Bininda-Emonds, & Simmons, 2002) with all
branches scaled to 1. We conducted this test
with the library Phytools (Revell, 2012) for
the statistical program R 3.3.3 (R Core Team,
2017). We examined if bite force estimated for
M. vivesi fitted the 95 % confidence interval of
the regression to determine if it corresponded
to the value predicted by its GLS. All multi-
variate analyses were performed in the program
STATISTICA 10 (StatSoft Inc, 2011) using a
significance level of α = 0.05.
Morphological variation: Among sixteen
variables, only CCL and the CPA were not nor-
mally distributed within the six species of bats
(W < 0.941, P < 0.05). The sixteen variables
transformed to logarithms were normally dis-
tributed (W > 0.861, P > 0.05). Size-adjusted
variables were normally distributed within the
six species of bats (W > 0.899, P > 0.05). There
were significant differences in GLS among
species (F
5, 216
= 3 111, P < 0.001), but not sig-
nificant differences between sexes (F
1, 216
= 0,
P = 0.803) or the interaction between sexes and
species (F
5, 216
= 0.5, P = 0.576). The results
of post-hoc comparisons showed significant
Fig. 2. Estimations of the bite force from the resistance measurements. Points correspond to known control weights of steel
nuts (N= 8) applied on the plate of the piezo resistive force sensor. The resistances recorded (MΩ) for Myotis vivesi were
transformed to bite force values (N) using this linear regression.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
differences between most pairs of species (P <
0.001), except for the comparison between M.
capaccinii and M. daubentonii, and with M.
velifer (P > 0.05).
The first CVA from sixteen variables
(unadjusted for size) estimated five significant
canonical vectors (P < 0.0001) for 6 a pRiori
groups. The first canonical function accounted
for 86 % of variance (Wilks’ λ < 0.0001; χ
= 2 508.21, P < 0.0001). The second canonical
variable accounted for an additional 12 % of
the variance (Wilks’ λ = 0.002; χ
= 1 340.53,
p <0.0001). The first two canonical axes
indicate differences between species groups
Fig. 3. Morphological variation of the masticatory apparatus in the fishing bat Myotis vivesi compared to insectivorous (I),
facultatively piscivorous (FP), and piscivorous (P) bat species. A. Plot of first two significant axes of the CVA analysis from
14 measurements and two angles on skulls and jaws for six species (N= 228). B. Plot of first two significant axes of the CVA
analysis from 13 size corrected measurements (GLS excluded) and two angles on skulls and jaws for six species (N= 228).
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
(Fig. 3A). The contributions of variables to
the first two canonical axes are presented in
Fig. 4A. The species with insectivorous diets
(M. keaysi and M. velifer) together with the
facultatively piscivorous species (M. capac-
cinii and M daubentonii) appeared on the right-
hand side on axis 1, and the piscivorous species
N. leporinus is in the extreme left-hand side.
Along the first canonical variate, N. leporinus
had the most contrasting morphology of the six
species examined. Therefore, the comparison
of this species against all others forms the basis
of the first discriminant function. The first two
canonical variates revealed that the cranial
Fig. 4. Comparison of variable loadings from two CVA analyses of A. original and B. size corrected measurements.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 66(4): 1614-1628, December 2018
morphology of M. vivesi was very different
from the morphology of the two insectivorous
and the two facultatively psicivorous species.
Both canonical variables also reveal that the
masticatory apparatus of M. vivesi was very
different from N. leporinus. Variables that
mostly contribute to the separation of both
species were the cranial distances GLS, M3B,
POC and Ori T, on the first canonical axis. The
GLS, Ins T and DL had the highest contribution
on the second canonical axis, discriminating M.
vivesi from other Myotis species.
In the second CVA analysis (size corrected
variables and GSL excluded), the ordination of
species was different to the first CVA analysis,
with Myotis species at the same position on
CV1 and non-closely related Noctilio placed
apart (Fig. 3B). This analysis determined five
significant canonical vectors (P < 0.0001) for
6 a pRiori groups. The first canonical func-
tion accounted for 79 % of variance (Wilks’
λ = 0.00013; χ
= 1 939.26, P < 0.0001).
The second canonical variable accounted for
an additional 15 % of the variance (Wilks’
λ= 0.0081; χ
=1 041.31, P < 0.0001). The
contributions of variables to the first canonical
axes are presented in Fig. 4B. Abbreviations of
14 cranial measurements and two angles are
ordered as in Fig. 1. The values on the scale
are the correlation coefficients between each
measurement and the first two Canonical axes
from both analyses. As in the first CVA, the
first two canonical axes adequately described
differences between species groups but showed
more overlapping among Myotis species along
the first axis (Fig. 3B). This size adjusted CVA
ordination also reveals that cranial morphology
of M. vivesi differs from N. leporinus. The con-
tribution of variables was more uniformly dis-
tributed, however the DD and the MFL had the
highest contribution on the first canonical axis
(Fig. 4B). On the second canonical axis, we
observe contrasting shape differences among
M. daubentonii and M. capaccini with M. vivesi
also explained by differences in DD, MFL and
the Ori M. The scores of the size-adjusted CV1
remain highly correlated to GLS (R = 0.81, F
= 446.7, P < 0.001).
Bite force and diet: Values from the
Blomberg’s K test (K = 0.324, P = 0.938) reveal
there is no phylogenetic signal in the residuals
of the linear regression between bite force and
skull size (GSL). The original values of bite
force and GLS were highly correlated among
species with different diets (R = 0.839, F
1, 11
26.29, P < 0.001, Fig. 5). Bite force measured
in M. vivesi (10.37 ± 5.39 N) was positioned
within the 95 % confidence interval of the
regression. Size-corrected bite force of M.
vivesi (0.48 ± 0.25 N/mm GSL) was lower than
in N. leporinus (0.68 ± 0.26 N/mm GSL).
Our comparison of Myotis species sepa-
rated insectivorous, facultatively piscivorous,
and piscivorous bats based in original and size-
adjusted variables of their skull morphology.
As expected, cranial morphology in M. vivesi
diverged from its insectivorous (M. keaysi
and M. velifer) and facultatively piscivorous
relatives (M. capaccinii and M. daubentonii).
Differences in the two CVA analyses indicate
significant contribution of skull size to the
discrimination among species in the first CVA
and more uniformly distributed variable loads
in the second CVA, where similarities among
Myotis species are most evident. There were
also differences in the two CVA analyses in
the pattern of variable correlations. The con-
tributions to discrimination axes were more
dispersed among variables in the second CVA
than in the first CVA. In the analysis of vari-
ance from variables not adjusted for size, the
measurements that contributed most to the
discrimination of M. vivesi on the first two
canonical axes were the greater length of the
skull, the postorbital constriction, the maxil-
lary breadth across M3, the origin of the mas-
ticatory muscles (Ori T and Ori M), and the
elevation of the condyle angle. Most of these
traits describe the masticatory muscle position
suggesting differences among bats with differ-
ent diets in the performance of the masticatory
muscle (Nogueira et al., 2005), as in the case
of facultatively piscivorous and insectivorous