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First published online January 31, 2007
Journal of Experimental Biology 210, 715-721 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000646
Sexual dimorphism of extensor carpi radialis muscle size, isometric force, relaxation rate and stamina during the breeding season of the frog Rana temporaria Linnaeus 1758
1 Departamento de Fisiologia, Instituto de Biociências, Universidade de
São Paulo, Rua do Matão–Travessa 14 No 321, CEP 05508-900,
São Paulo, SP, Brasil
2 Department of Biomolecular and Sport Sciences, Faculty of Health and Life
Sciences, Coventry University, CV1 5FB, UK
* Author for correspondence (e-mail: navas{at}usp.br)
Accepted 18 December 2006
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Summary |
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Key words: amplexus, contractile, fatigue, sex, morphology, tetanus
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Introduction |
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; Duellman and Trueb,
1994; Irwin et al.,
2003; Lee and Corrales,
2002; Wells,
1977). The relative importance of each of these behaviours depends
upon the species, breeding male density, male size and sex ratios
(Arak, 1983;
Wells, 1977). In some species
of explosive breeders such as Rana temporaria many more males than
females are present in the breeding pond at any one time, resulting in
competition between males. Many males may attempt to clasp (hold) the same
female to try to ensure they copulate with her, or they will try to dislodge a
male that is already grasping a female
(Arak, 1983;
Elmberg, 1991;
Wells, 1977). In some species
such competition can lead to intrasexual, interspecific or multiple amplexus
(Marco and Lizana, 2002), and
can even result in multiple paternity
(Lode and Lesbarreres, 2004;
Roberts et al., 1999). In
higher male densities of explosive breeders, males of large body length appear
to have an advantage that increases the likelihood of them mating
(Arak, 1983;
Elmberg, 1991;
Lee, 2001); however, it now
appears that the real key to male mating success in such species may be
greater absolute mass of adductor and flexor forelimb muscles, regardless of
body size of the male (Lee,
2001).
Males of many frog species undergo seasonal changes in both size and
contractile properties of the muscles used in amplexus to improve their
performance. Flexor carpi radialis muscle from male Rana temporaria
undergoes seasonal changes in contractile characteristics
(Melichna et al., 1972) such
that during the breeding season half twitch relaxation time and maximal rate
of force development are both slower than during the summer. These alterations
in contractile characteristics are linked to seasonal increases in
gonadotrophin concentrations (e.g. LH, luteinizing hormone and FSH,
follicle-stimulating hormone), which affect both amplexing behavior and muscle
morphology (Itoh and Ishii,
1990). In Rana esculenta, amplexing males exhibit higher
levels of testosterone and lower levels of estradiol than non-amplexing
individuals (Gobbetti and Zerani,
1999), and in Rana pipiens testosterone increases
forelimb muscle dry mass by more than 150% under experimental conditions
(Kim et al., 1998).
Testosterone treatment of castrated male Xenopus laevis frogs leads
to increases in muscle size, force and twitch relaxation times in flexor carpi
radialis, yet this treatment has little effect on the hindlimb locomotor
muscle ileofibularis (Regnier and Herrera,
1993). Testosterone treatment also increases the number of tonic
fibres in coracoradialis muscle (another forelimb muscle used to clasp
females) in Xenopus laevis, which could explain the seasonal
prolongation in `clasp' muscles of time to fatigue and relaxation time
(Rubinstein et al., 1983).
The prevalence of amplexus as an important stage of the reproductive mode
of anurans has resulted in the evolution of sexual dimorphism in the forelimb
muscles of anurans (Clark and Peters,
2006; Oka et al.,
1984; Peters and Aulner,
2000; Sidor and Blackburn,
1998; Yekta and Blackburn,
1992). Some forelimb muscles such as extensor carpi radialis,
flexor carpi radialis, sternoradialis and abductor indicus longus that are
involved in amplexus have been found to be larger in mass and cross-sectional
area in male than in female frogs (Clark
and Peters, 2006; Kirby,
1983; Oka et al.,
1984; Peters and Aulner,
2000; Yekta and Blackburn,
1992) and to have more oxidative fibers and/or higher oxidative
capacity in males than in females (Kirby,
1983; Melichna et al.,
1972; Peters,
2001; Rubinstein et al.,
1983) (but see Oka et al.,
1984). Such differences in muscle morphology have been found to
lead to higher absolute isometric force in male muscles, without a difference
in isometric stress (force per cross-sectional area) between sexes in two
distantly related species, Rana catesbeiana and Bufo marinus
(Clark and Peters, 2006;
Peters and Aulner, 2000).
However, these previous studies did not compare muscle mechanics during the
reproductive season, but did propose that sexual dimorphism probably increases
during the reproductive phase of males. Flexor carpi radialis (FCR) muscles
from both male and female Rana temporaria had a significantly slower
rate of twitch force activation and a slower half twitch relaxation time in
the breeding season when compared to outside of the breeding season
(Melichna et al., 1972).
However, throughout the year sexual dimorphism was evident with higher FCR
ATPase activity and faster twitch activation and relaxation rates in females
than in males. Resting membrane potential of sternoradialis increased during
the breeding season, such that very little stimulation was required to elicit
muscle force (Kirby,
1983).
The aims of our study were to use an explosive breeding frog (Rana
temporaria Linnaeus 1758) during the breeding season to: (1) determine
typical male forelimb joint angles used during amplexus; (2) measure in
situ extensor carpi radialis (ECR) muscle length in male forelimbs held
at the typical forelimb joint angles used during amplexus; (3) compare ECR
muscle mass and length between male and female frogs; and (4) compare ECR
muscle isometric activation times, relaxation times, absolute force, relative
force (stress) and fatigue between male and female frogs. For this study we
have focused on the ECR, a muscle that plays a fundamental role in amplexus by
extending the wrist in a dorsal direction to help prevent the female from
moving (Duellman and Trueb,
1994). We propose that the degree of sexual dimorphism in ECR
muscle contractile properties will be higher during the breeding season than
previously found outside the breeding season
(Peters and Aulner, 2000).
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Materials and methods |
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Amplexus joint angle measurements
The amplexus joint angle measurements were obtained from two couples found
amplexing in the field and from two amplexing couples formed in captivity.
Pieces of flexible wire were bent into the shapes of the shoulder, elbow and
wrist angles and held against the amplexed couple to check the wire shape was
representative before further reshaping of the wire or measurement of the
angle from the wire. More specifically the amplexus joint angles were measured
as follows: the interior angle between the posterior border of the scapula and
the posterior humerus (shoulder angle); the interior angle between the humerus
and the radioulna (elbow angle); the posteromedial angle between the radioulna
and the carpus (wrist angle). Amplexed couples were not disturbed by this
process and did not attempt to move.
Morphometrics and dissection
Snout–vent length and maximal diameter of the forelimb (both
proximally and distally to the elbow) were measured to the nearest 0.1 mm
using Vernier calipers. Frog body mass was measured to the nearest 0.1 g using
an electronic balance.
Individuals were killed by pithing and transection of the spinal cord in accordance with the British Home Office Animals (Scientific Procedures) Act 1986, Schedule 1. Both forelimbs were removed from the frog and pinned out in oxygenated (95% O2, 5% CO2) Ringer solution (composition in mmol l–1: NaCl, 115; KCl, 2.5; Na2HPO4, 2.15; NaH2PO4, 0.85; sodium pyruvate, 5.0; CaCl2, 1.8; pH 7.4 at 20°C). The left forelimb was pinned out in the position corresponding to the mean shoulder, elbow and wrist angles observed in amplexus (see Results). The total in situ length of the ECR muscle was determined by measuring the length of a piece of suture silk placed along the surface of the muscle and cut to the same length as the muscle. The ECR muscle was dissected, for subsequent muscle mechanics, from the left forelimb with a piece of the humerus left at the end of the proximal tendon, and the thumb left at the end of the distal tendon.
Isometric studies on isolated muscle
The ECR muscle preparation was transferred to a bath of oxygenated (95%
O2, 5% CO2) circulating frog Ringer solution maintained
at 10.0±0.5°C. The humerus was attached via a crocodile
clip to a strain gauge (model UF1, Pioden Controls Ltd, Canterbury, UK; range
±2 kg for male frogs, ±55 g for female frogs) and the thumb was
attached via a crocodile clip to a servomotor (V201, Ling Dynamic
Systems, Royston, UK). A linear variable displacement transformer (DFG5.0,
Solartron Metrology, Bognor Regis, UK) was used for position detection. The
ECR muscle from male frogs was initially set at the length that had occurred
in situ when the leg was set at the mean joint angles measured during
amplexus. Muscle length was measured using a microscope fitted with an
eyepiece graticule. The ECR muscle from female frogs was not initially set at
any particular length of relevance. Electrical stimulation was delivered to
the muscle in 1.5 ms pulses via parallel platinum electrodes. A
series of isometric twitches were used to optimize stimulation amplitude to
yield the maximum isometric twitch force (achieved at a stimulation amplitude
between 18 and 20 V). Muscle stimulation was controlled using custom written
software (Testpoint, CEC, Massachusetts, USA) via a D/A board
(KPCI3108, Keithley Instruments, Ohio, USA). Force data were sampled at a rate
of 10 kHz. An isometric tetanic response was elicited by subjecting the muscle
to a 500 ms train of stimulation. Stimulation frequency was optimized (between
100 and 120 Hz) to yield the maximal isometric tetanic force. The stimulation
amplitude and frequency found to generate maximal force for a muscle at in
vivo amplexus length was then used for all subsequent work with that
muscle. A 5 min recovery period was allowed between each tetanic response.
Time to half peak twitch, time from peak twitch to half relaxation, time to
half peak tetanic force and time from the last stimulus to 50% tetanus
relaxation were also measured (the times reported in the Results were measured
at the length at which the muscle preparation produced maximal stress). The
muscle length was then altered (in no set order) to at least six new lengths
to produce a length–force curve via repetition of the twitch
and tetanus at each new length. At every fourth length change the muscle was
set back to the amplexus length (or in the case of female frogs, the first
muscle length) so that the condition of the muscle could be monitored over the
time course of the experiment. Any changes in muscle force production between
these repeated (control) tetanic responses were assumed to have occurred in a
linear manner over time and actual muscle force production was corrected
accordingly. Usually muscles improved slightly during the time course of the
experiment and then decreased to 95% of maximum control force by the end of
the experiments. The muscle length for maximum force production
(L0) was recorded.
Fatigue resistance was assessed at in vivo amplexus length for males and at the length for maximal force production for females, by subjecting the muscle to a 200 ms train of stimulation (delivered at the stimulation frequency that yielded maximum tetanic force), repeated at a frequency of 1 Hz, for 3 min. Muscle force was monitored during these fatigue tests using a thermal arraycorder. Recovery from the fatigue run was monitored in four ECR muscle preparations that were each subjected to a fatigue run lasting 3 min. Each preparation recovered to greater than 66% of the prefatigue force within 5 min of the fatigue run finishing. Additionally a second fatigue test was delivered 15 min after the end of the first test in one individual. 10 min after this second fatigue test the peak tetanic force was 76% of the peak tetanic force achieved before the first fatigue test. Therefore, we concluded that the fatigue protocol used in this study fatigued rather than killed the muscle preparation.
ECR muscle mass was determined to the nearest 0.1 mg for male frogs (0.01
mg for female frogs) at the end of the experiment using an electronic balance,
having first blotted the muscle on absorbent paper to remove excess Ringer
solution. Mean muscle cross-sectional area was calculated from mean muscle
fibre length, muscle mass and an assumed muscle density of 1060 kg
m–3 (Méndez and
Keys, 1960). Muscle stress was calculated as force divided by mean
muscle cross-sectional area. To aid comparisons between sexes and different
sized animals muscle force was also expressed as a percentage of body weight
(N).
Statistical analysis
To meet the requirements of parametric statistical analyses all percentage
data were converted to proportions, then arcsine transformed prior to
statistical analysis. Sex comparisons of equality of variance were performed
using Levene's test. Independent samples t-tests were used to
determine whether significant differences existed between sexes in each
variable tested, assuming equal variance where appropriate. The effect of
fatigue on tetanic force production was analysed using a general linear model
to determine the effect of sex on force produced and the interaction between
sex and time. The truncated product method
(Zaykin et al., 2002) was used
to combine each set of P values to determine whether it was likely
that significant results in the set were truly significant or were due to
chance. The summary P value from the truncated product method was
then stated in each Table legend.
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Results |
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Morphometrics
The female frogs had a significantly (P<0.05) longer (7.8%)
snout–vent length and tended to have a higher body mass
(Table 1). However, the male
frogs had significantly (P<0.05) larger maximum lower forelimb and
upper forelimb diameters than the females, more than 30% higher in each case
(Table 1). Male frogs had
significantly ECR muscle mass than females (P<0.001), comprising
means of 0.22% of body mass in males, yet only 0.019% in females
(Table 1). Male frogs also had
significantly longer (38%) ECR muscles than females (P<0.01;
Table 1).
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Muscle mechanics
The ECR muscle in males produced a significantly higher peak tetanic
absolute force and a significantly higher maximal isometric stress (relative
force) than in females (Table
1; P<0.001 in each case), with values 1500% and 115%
higher than female values, respectively. There was no significant difference
in time to half peak twitch force between male and female ECR muscles
(P>0.05; Table 2).
However, time from peak twitch force to half relaxation, time to half peak
tetanic force and time from last stimulus to half tetanic relaxation were all
significantly slower in male ECR muscles being 58%, 74% and 189% longer,
respectively, than in female ECR (P<0.05 in each case;
Table 2). Muscle length had a
large effect on force production with maximum force in males produced at
94.4±9.4% of in vivo amplexus length.
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Male ECR muscle was significantly more fatigue resistant than female ECR (Fig. 1; P<0.001 for effect of sex and interaction between time and sex). Peak tetanic force increased in the first few tetani produced by male ECR muscles, then subsequently decreased whereas the peak tetanic force produced by female ECR muscles gradually decreased (Fig. 1). After the first 2–3 s of a fatigue run, male ECR muscles did not relax inbetween trains of stimuli and approached a force plateau. In contrast, female muscle relaxed almost to baseline force inbetween each train of stimuli (Fig. 2).
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). Previous studies have also demonstrated sexual dimorphism
in mass of forelimb muscles used by males during amplexus
(Lee, 2001;
Oka et al., 1984;
Peters and Aulner, 2000;
Yekta and Blackburn, 1992).
Oka et al. found an approximately sevenfold greater ECR wet muscle mass in
male than in female Bufo japonicus in breeding state
(Oka et al., 1984). In
contrast only a 3.4-fold sex difference in ECR muscle mass was found in
Rana pipiens experimentally induced into breeding state
(Yekta and Blackburn, 1992),
compared with only a 1.4-fold difference in ECR muscle in Rana
catesbeiana outside of the breeding season
(Peters and Aulner, 2000).
Some of these differences in degree of sexual dimorphism between experimental
studies may reflect differences between ECR muscles from the natural breeding
state compared with experimentally induced breeding state or the non-breeding
state. However, differences between species would also be expected because
amplexus is a highly diverse aspect of anuran reproductive biology,
particularly regarding duration and force requirements. A couple might remain
in amplexus for several months, as with some species in the genus
Atelopus, or for less than an hour, as in Colostethus, Hyla
andersonii and Pseudacris nigrata
(Wells, 1977). Interspecific
differences in sexually dimorphic muscles do appear to exist as four of the
muscles that exhibit sex differences in muscle mass in Rana pipiens
(Yekta and Blackburn, 1992)
show no such dimorphism in Bufo japonicus
(Oka et al., 1984).
Our proposed ECR length for amplexed males was taken as an estimate of the
operating conditions of this muscle in nature, although this value would
clearly differ according to the size of female the male was copulating with.
ECR produced maximal force at lengths compatible with those we estimated for
amplexed males, suggesting that in vivo operating conditions during
the breeding season might generally be close to the functional optima of the
muscle. In contrast, ECR out of breeding season produced maximal force at 120%
of their estimated amplexus length (Peters
and Aulner, 2000). The significance of this difference is
difficult to evaluate at this point because we lack information on the main
use of this muscle during the non-breeding season, force–length
relationships might differ between Ranid species, and male Rana
temporaria appear not to choose females according to body length, body
mass or fecundity (Elmberg,
1991), so that amplexed Rana temporaria males in nature
might experience a relatively wide range of in vivo muscle
lengths.
Muscle mechanics
Male Rana temporaria ECR muscle produced a 25.8-fold significantly
higher isometric absolute tetanic force and a 2.2-fold significantly higher
relative tetanic force (stress) than in females. It appears that most of the
difference in absolute force between sexes is due to the large increase in
cross-sectional area with a relatively small contribution from the increase in
stress. Peters and Aulner also found higher (1.09-fold higher) ECR muscle
absolute forces in males than females
(Peters and Aulner, 2000);
however, they found no significant differences between sexes in muscle stress
(relative force), indeed muscle stress tended to be higher in female ECR. They
reported values of 237 kN m–2 for maximal isometric stress
and an overall force production equal to 150% of body weight in ECR from male
non-breeding state Rana catesbeiana
(Peters and Aulner, 2000). In
contrast, our corresponding values for male Rana temporaria at peak
reproductive season were 381 kN m–2 and 560%, respectively.
Undoubtedly, intraspecific comparisons would be more appropriate to enable
conclusive statements; however, given the magnitude of the differences found
and previous morphological studies, we suggest that the force production of
male forelimb muscles responsible for amplexus is probably much higher when
frogs are in reproductive state, and that such changes might be hormonally
induced causing increased muscle mass, although we have no evidence to exclude
some of the change being due to increased muscle stress. However, protein
concentration in seven different forelimb muscles has not previously been
found to differ between male and female Rana pipiens that have been
experimentally induced into breeding state
(Yekta and Blackburn,
1992).
We found that ECR relaxation times were significantly longer in males than
in females. This slower relaxation in males led to very limited relaxation
(sustained force) between tetani during the fatigue run, such that the male
ECR muscle could maintain almost constant isometric force when stimulated for
only one fifth of the time (for 200 ms in every 1 s). Previous studies on
anuran dimorphic forearm muscles outside of the reproductive season have also
reported a higher sustained force in males than in females
(Clark and Peters, 2006;
Peters and Aulner, 2000), but
nothing like as high as we have shown in frogs of reproductive state. The
twitch half relaxation time of flexor carpi radialis muscle in Rana
temporaria was 94% longer (slower) in males than in females during
the breeding season and such large differences persisted throughout the year,
with muscle in males possessing more fibres with low myosin ATPase activity
(Melichna et al., 1972).
Peters and Aulner found ECR twitch relaxation times were 52% slower in male
than in female Rana catesbeiana outside of the reproductive season,
although these values did not reach the level of significance
(Peters and Aulner, 2000).
However, twitch relaxation times were significantly slower in male than in
female abductor indicus longus (AIL) and flexor carpi radialis (FCR) muscle
(Peters and Aulner, 2000).
Similar (although non-significant) trends in twitch relaxation times, but
significantly slower tetanus relaxation times, in male than in female AIL and
FCR in Bufo marinus outside of the reproductive season were also
found (Clark and Peters, 2006).
Peters and Aulner (Peters and Aulner,
2000) found much faster ECR relaxation times in male Rana
catesbeiana than we found in male Rana temporaria but these
differences may again reflect differences in reproductive state between the
two studies; indeed the contractile properties of flexor carpi radialis muscle
tend to become faster outside of the breeding season
(Melichna et al., 1972). In
the present study the rate of fatigue of male ECR muscle was relatively slow
compared to female ECR muscle. In contrast there was no significant difference
in fatigue index between sex in ECR muscle of Rana catesbeiana,
studied during the non-reproductive phase
(Peters and Aulner, 2000).
Seasonal changes in contractile properties are likely initiated by hormonal
changes. Testosterone treatment of castrated Xenopus laevis increased
tetanus half relaxation time by 38% and increased both tetanic force and
muscle cross-sectional area by more than 80% in flexor carpi radialis muscle
when compared with castrated control males
(Regnier and Herrera, 1993).
Changes in muscle fiber type proportion can also be induced by testosterone,
as has been reported for Xenopus laevis
(Rubinstein et al., 1983) and
suggested for Rana pipiens
(Renaud, 1991).
Application to reproductive behaviour
Our results suggest that the extensor carpi radialis (ECR) muscles from
male Rana temporaria in breeding state are suited to maintaining
low-to-moderate forces for long periods of time, but are also able to produce
very high peak forces if required. These physiological properties match the
two different clasping behaviours previously described for Xenopus
laevis: "(i) a prolonged but loose maintenance grip and (ii)
brief rapid tightening"
(Regnier and Herrera, 1993).
The results from the present study, combined with evidence from previous
studies, suggest that extensor carpi radialis in Ranid species undergoes large
increases in mass, relaxation time and fatigue resistance as males enter their
reproductive state, enabling the forelimbs to produce prolonged relatively
high force to clasp (hold) females during amplexus. These characteristics
agree with what is known about the reproductive biology of Rana
temporaria. At low male densities, the reproductive mode of Rana
temporaria is characterized by random mating, with no clear size ratio
between males and females and an absence of takeover events
(Arak, 1983;
Elmberg, 1987). Non-random
mating, however, occurs at high male densities and possibly also when the
prespawning period is relatively long. Under experimental conditions involving
high male densities and high male to female ratios, takeover attempts become
common and are successful in 20% of couples
(Elmberg, 1991). Therefore,
takeovers, even if rare, might have modulated the evolution of forelimb muscle
traits in this species. High strength of the forelimbs might offer selective
advantages enabling males to better prevent both interference from
non-amplexed males and, to a lesser extent, female rejection. Our
understanding of anuran sexual selection and reproductive success would
benefit from additional comparative studies of the physiology and ecological
correlates of forelimb muscles.
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Acknowledgments |
|---|
|
References |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Arak, A. (1983). Male–male competition and mate choice in anuran amphibians. In Mate Choice (ed. P. Bateson), pp. 181-210. Cambridge: Cambridge University Press.
Clark, D. L. and Peters, S. E. (2006).
Isometric contractile properties of sexually dimorphic forelimb muscles in the
marine toad Bufo marinus Linnaeus 1758, functional analysis and
implications for amplexus. J. Exp. Biol.
209,3448
-3456.
Duellman, W. E. and Trueb, L. (1994). Biology of Amphibians. Baltimore: Johns Hopkins University Press.
Elmberg, J. (1987). Random mating in a boreal population of European Common frogs Rana temporaria. Holarctic Ecol. 10,193 -195.
Elmberg, J. (1991). Factors affecting male yearly mating success in the common frog, Rana temporaria. Behav. Ecol. Sociobiol. 28,125 -131.
Gobbetti, A. and Zerani, M. (1999). Hormonal and cellular brain mechanisms regulating the amplexus of male and female water frog (Rana esculenta). J. Neuroendocrinol. 11,589 -596.[CrossRef][Medline]
Irwin, J. T., Costanzo, J. P. and Lee, R. E. (2003). Postfreeze reduction of locomotor endurance in the freeze-tolerant wood frog, Rana sylvatica. Physiol. Biochem. Zool. 76,331 -338.[CrossRef][Medline]
Itoh, M. and Ishii, S. (1990). Changes in plasma levels of gonadotropins and sex steroids in the toad, Bufo japonicus, in association with behavior during the breeding season. Gen. Comp. Endocrinol. 80,451 -464.[CrossRef][Medline]
Kim, J. W., Im, W. B., Choi, H. H., Ishii, S. and Kwon, H. B. (1998). Seasonal fluctuations in pituitary gland and plasma levels of gonadotropic hormones in Rana. Gen. Comp. Endocrinol. 109,13 -23.[CrossRef][Medline]
Kirby, A. C. (1983). Physiology of the sternoradialis muscle: sexual dimorphism and role in amplexus in the Leopard frog (Rana pipiens). Comp. Biochem. Physiol. 74A,705 -709.[Medline]
Lee, J. C. (2001). Evolution of a secondary sexual dimorphism in the toad, Bufo marinus. Copeia 2001,928 -935.[CrossRef]
Lee, J. C. and Corrales, A. D. (2002). Sexual dimorphism in hind-limb muscle mass is associated with male reproductive success in Bufo marinus. J. Herpetol. 36,502 -505.
Lode, T. and Lesbarreres, D. (2004). Multiple paternity in Rana dalmatina, a monogamous territorial breeding anuran. Naturwissenschaften 91, 44-47.[CrossRef][Medline]
Marco, A. and Lizana, M. (2002). The absence of species and sex recognition during mate search by male common toads, Bufo bufo. Ethol. Ecol. Evol. 14,1 -8.
Melichna, J., Gutmann, E., Herbrychova, A. and Stichova, J. (1972). Sexual dimorphism in contraction properties and fibre pattern of the flexor carpi radialis muscle of the frog (Rana temporaria L.). Experientia 48, 89-91.
Méndez, J. and Keys, A. (1960). Density and composition of mammalian muscle. Metabolism 9, 184-188.
Oka, Y., Ohtani, R., Satou, M. and Ueda, K. (1984). Sexually dimorphic muscles in the forelimb of the Japanese toad, Bufo japonicus. J. Morphol. 180,297 -308.[CrossRef]
Peters, S. E. (2001). Muscle fiber types in sexually dimorphic forelimb muscles of the bullfrog, Rana catesbeianaAm. Zool. 41,1553 .
Peters, S. E. and Aulner, D. A. (2000). Sexual dimorphism in forelimb muscles of the bullfrog, Rana catesbeiana: a functional analysis of isometric contractile properties. J. Exp. Biol. 203,3639 -3654.[Abstract]
Regnier, M. and Herrera, A. A. (1993). Differential sensitivity to androgens within a sexually dimorphic muscle of male frogs (Xenopus laevis). J. Neurobiol. 24,1215 -1228.[CrossRef][Medline]
Renaud, J. M. (1991). Seasonal-variation in muscle fatigue in the sartorius muscle of the frog Rana Pipiens.Can. J. Zool. 69,1712 -1715.
Roberts, J. D., Standish, R. J., Byrne, P. G. and Doughty, P. (1999). Synchronous polyandry and multiple paternity in the frog Crinia georgiana (Anura: Myobatrachidae). Anim. Behav. 57,721 -726.[CrossRef][Medline]
Rubinstein, N. A., Erulkar, S. D. and Schneider, G. T. (1983). Sexual dimorphism in the fibers of a `clasp' muscle of Xenopus laevis. Exp. Neurol. 82,424 -431.[CrossRef][Medline]
Sidor, C. A. and Blackburn, D. G. (1998). Effects of testosterone administration and castration on the forelimb musculature of male leopard frogs, Rana pipiens. J. Exp. Zool. 280,28 -37.[CrossRef][Medline]
Wells, K. D. (1977). The social behaviour of anuran amphibians. Anim. Behav. 25,666 -693.[CrossRef]
Yekta, N. and Blackburn, D. G. (1992). Sexual dimorphism in mass and protein content of the forelimb muscles of the Northern Leopard frog, Rana pipiens. Can. J. Zool. 70,670 -674.
Zaykin, D. V., Zhivotovsky, L. A., Westfall, P. H. and Weir, B. S. (2002). Truncated product method for combining P-values. Genet. Epidemiol. 22,170 -185.[CrossRef][Medline]
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