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

Sexual Dimorphism in the Limb Bones of Asiatic Toad (Bufo gargarizans) in Relation to Sexual Selection

Sexual dimorphism is often considered to be the result of differences in the intensity of sexual selection between sexes. From this point of view, the sexual dimorphism of the limb bones of the Bufo gargarizans in southwest China was studied. Results showed that the fore- and hindlimb skeletons of this species were sexually dimorphic in anatomy. The humerus, radioulna, and total lengths of the forelimb skeleton of males were substantially longer than those of females, but the hand length of males was smaller than that of females. Several other features of males, such as deltoid and medial crest areas and humerus and radioulnar weights, were also significantly larger than those of females. The femoris, tibiofibula, talus-calcaneus, and foot lengths; total hindlimb skeleton length; and femoral upper crest areas of males were significantly greater than those of females. However, no significant intersexual difference in femoris and tibiofibular weights was observed. These findings suggested that robust forelimb bones and long hindlimb bones could contribute to the mating success of males; if so, sexual selection promotes the evolution of sexual size and shape dimorphism in the limb bones of the B. gargarizans.

Fatigue resistant jaw muscles facilitate long-lasting courtship behaviour in the southern alligator lizard (Elgaria multicarinata)

The southern alligator lizard (Elgaria multicarinata) exhibits a courtship behaviour during which the male firmly grips the female's head in his jaws for many hours at a time. This extreme behaviour counters the conventional wisdom that reptilian muscle is incapable of powering high-endurance behaviours. We conducted in situ experiments in which the jaw-adductor muscles of lizards were stimulated directly while bite force was measured simultaneously. Fatigue tests were performed by stimulating the muscles with a series of tetanic trains. Our results show that a substantial sustained force gradually develops during the fatigue test. This sustained force persists after peak tetanic forces have declined to a fraction of their initial magnitude. The observed sustained force during in situ fatigue tests is consistent with the courtship behaviour of these lizards and probably reflects physiological specialization. The results of molecular analysis reveal that the jaw muscles contain masticatory and tonic myosin fibres. We propose that the presence of tonic fibres may explain the unusual sustained force properties during mate-holding behaviour. The characterization of muscle properties that facilitate extreme performance during specialized behaviours may reveal general mechanisms of muscle function, especially when done in light of convergently evolved systems exhibiting similar performance characteristics.

Do muscle contractile properties drive differences in locomotor performance in invasive populations of Xenopus laevis in France?

Jumping and swimming are key locomotor traits in frogs intimately linked to survival and dispersal. French populations of the frog Xenopus laevis from the invasion front are known to possess greater terrestrial locomotor endurance. Here, we tested whether individuals from the invasion front show differences in their muscle physiology that may underlie the observed whole-organism performance differences. We measured muscle contractile properties of the isolated gastrocnemius muscle in vitro, including isometric stress, activation and relaxation time, and work loop power output, both before and during a period of fatiguing contractions. We found that frogs from the centre of the range can produce tetanus force in their gastrocnemius muscle faster than animals from the periphery of the range, which could contribute to higher performance in one-off jumps. Yet, populations did not differ in muscle endurance. These results, coupled with previous work on this invasive population of Xenopus laevis, suggest that the greater stamina observed in individuals from the periphery may be more due to anatomical differences such as longer hind limbs and larger hearts along with potentially other as of yet untested physiological differences rather than differences in the mechanical properties of skeletal muscle.

Flesh and bone: An integrative approach towards sexual size dimorphism of a terrestrial salamander (genus Salamandrina)

Males and females face different selection pressures due to a sexually biased investment into reproduction. This often results in different morphologies. Sexual size dimorphisms (SSD) can give us important hints on the evolution and biology of a species. Salamanders are a perfectly suited system for investigating SSD, including a diversity of reproductive modes and behaviors, and patterns of SSD combined with life history traits in a phylogenetic context help us to understand the evolution of these processes. Because spectacled salamanders (genus Salamandrina) are the phylogenetically most basal taxon of the Salamandridae, they play a key role in reconstructing the evolutionary pattern of SSD. Combining extensive external and skeletal measurements of the cranium, limbs, and the pelvic girdle using high-resolution micro Computer Tomography (μCT) yielded an integrative analysis of expressed SSD of morphology and osteology of Salamandrina perspicillata. Multivariate analysis of external characters showed that males generally had larger cloacae, heads, and limbs relative to body size, while females had larger trunks. Analysis of osteology confirmed this pattern but also revealed new dimorphic characters in the cranium and the pelvic girdle. Dimorphic characters in external morphology and osteology are likely linked to the different reproductive roles of the sexes and support sexual rather than ecological selection as the primary force acting on the phenotype of the phylogenetically basal salamandrids.

Sex and weaponry: The distribution of toxin-storage glands on the bodies of male and female cane toads (Rhinella marina)

The parotoid macroglands of bufonid anurans store (and can expel) large volumes of toxic secretions and have attracted detailed research. However, toxins also are stored in smaller glands that are distributed on the limbs and dorsal surface of the body. Female and male cane toads (Rhinella marina) differ in the location of toxin‐storage glands and the extent of glandular structures. Female toads store a larger proportion of their toxins in the parotoids than males as well as (to a lesser extent) in smaller glands on the forelimbs. Males have smaller and more elongate parotoids than females, but glands cover more of the skin surface on their limbs (especially hindlimbs) and dorsal surface. The delay to toxin exudation in response to electrostimulation varied among glands in various parts of the body, and did so differently in males than in females. The spatial distribution of toxin glands differs between the sexes even in toads that have been raised under standardized conditions in captivity; hence, the sexual dimorphism is due to heritable factors rather than developmentally plastic responses to ecological (e.g., habitat, predation risk) differences between the sexes. The selective advantages of this sexual dimorphism remain unclear. A priori, we might expect to see toxin widely dispersed across any part of the body likely to be contacted by a predator; and a wide distribution also would be expected if the gland secretions have other (e.g., male–male rivalry) functions. Why, then, is toxin concentrated in the parotoids, especially in female toads? That concentration may enhance the effectiveness of frontal displays to deter predation and also may facilitate the transfer of stored toxins to eggs.

Greater reproductive investment, but shorter lifespan, in agrosystem than in natural-habitat toads

Global amphibian decline is due to several factors: habitat loss, anthropization, pollution, emerging diseases, and global warming. Amphibians, with complex life cycles, are particularly susceptible to habitat alterations, and their survival may be impaired in anthropized habitats. Increased mortality is a well-known consequence of anthropization. Life-history theory predicts higher reproductive investment when mortality is increased. In this work, we compared age, body size, and different indicators of reproductive investment, as well as prey availability, in natterjack toads (Epidalea calamita) from agrosystems and adjacent natural pine groves in Southwestern Spain. Mean age was lower in agrosystems than in pine groves, possibly as a consequence of increased mortality due to agrosystem environmental stressors. Remarkably, agrosystem toads were larger despite being younger, suggesting accelerated growth rate. Although we detected no differences in prey availability between habitats, artificial irrigation could shorten aestivation in agrosystems, thus increasing energy trade. Moreover, agrosystem toads exhibited increased indicators of reproductive investment. In the light of life-history theory, agrosystem toads might compensate for lesser reproductive events-due to shorter lives-with a higher reproductive investment in each attempt. Our results show that agrosystems may alter demography, which may have complex consequences on both individual fitness and population stability.

Population density and structure drive differential investment in pre- and postmating sexual traits in frogs

Sexual selection theory predicts a trade-off between premating (ornaments and armaments) and postmating (testes and ejaculates) sexual traits, assuming that growing and maintaining these traits is costly and that total reproductive investments are limited. The number of males in competition, the reproductive gains from investing in premating sexual traits, and the level of sperm competition are all predicted to influence how males allocate their finite resources to these traits. Yet, empirical examination of these predictions is currently scarce. Here, we studied relative expenditure on pre- and postmating sexual traits among frog species varying in their population density, operational sex ratio, and the number of competing males for each clutch of eggs. We found that the intensifying struggle to monopolize fertilizations as more and more males clasp the same female to fertilize her eggs shifts male reproductive investment toward sperm production and away from male weaponry. This shift, which is mediated by population density and the associated level of male-male competition, likely also explains the trade-off between pre- and postmating sexual traits in our much broader sample of anuran species. Our results highlight the power of such a multilevel approach in resolving the evolution of traits and allocation trade-offs.

Sperm competition and the evolution of precopulatory weapons: Testis size and amplexus position, but not arm strength, affect fertilization success in a chorusing frog

Trade-offs between pre- and postcopulatory traits influence their evolution, and male expenditure on such traits is predicted to depend on the number of competitors, the benefits from investing in weapons, and the risk and intensity of sperm competition. Males of the chorusing frog Crinia georgiana use their arms as weapons in contest competition. Previously, we showed that increased numbers of rivals elevated the risk and intensity of sperm competition due to multimale amplexus, and caused a reversal in the direction of precopulatory selection on arm girth. Here, we focused on the factors affecting postcopulatory fertilization success during group spawning, using paternity data from natural choruses. Competitive fertilization success depended on the time spent amplexed and amplexus position. Relative testes size but not arm girth, contributed to fertilization success, but the effect of testes size depended on amplexus position. Our findings offer within species empirical support for recent sperm competition models that incorporate precopulatory male-male competition, and show why an understanding of the evolution of animal weapons requires a consideration of both pre- and postcopulatory episodes of sexual selection.

Sexual dimorphism and age of Mediterranean salamanders

We analysed sexual size dimorphism (SSD) for two Mediterranean species of the "true" salamander clade possessing distinct life histories (Salamandra algira and Mertensiella caucasica) and equilibrated the morphometric approach to individual age by using skeletochronology. For species that have a short breeding season and live at high altitudes, such as Mediterranean amphibians, the fecundity advantage hypothesis predicts female-biased SSD to maximise reproductive success. Our results showed no SSD in either species; however, morphometric data indicated a male-biased dimorphism in limb (arm and leg) dimensions in both species when compared to body size. Limb dimorphisms are likely related to the particular mating system, which involves an amplexus during spermatophore transfer. Arm length appeared sexually dimorphic during ontogeny both in viviparous S. algira and oviparous M. caucasica. A review on SSD indicated monomorphy of body size as a common lineage-specific pattern among the "true" salamander clade, but also the common presence of other traits such as sexually dimorphic limb proportions.

Jumping performance in the highly aquatic frog, Xenopus tropicalis: sex-specific relationships between morphology and performance

Frogs are characterized by a morphology that has been suggested to be related to their unique jumping specialization. Yet, the functional demands associated with jumping and swimming may not be that different as suggested by studies with semi-aquatic frogs. Here, we explore whether features previously identified as indicative of good burst swimming performance also predict jumping performance in a highly aquatic frog, Xenopus tropicalis. Moreover, we test whether the morphological determinants of jumping performance are similar in the two sexes and whether jumping performance differs in the two sexes. Finally we test whether jumping capacity is positively associated with burst swimming and terrestrial endurance capacity in both sexes. Our results show sex-specific differences in jumping performance when correcting for differences in body size. Moreover, the features determining jumping performance are different in the two sexes. Finally, the relationships between different performance traits are sex-dependent as well with females, but not males, showing a trade-off between peak jumping force and the time jumped to exhaustion. This suggests that different selective pressures operate on the two sexes, with females being subjected to constraints on locomotion due to their greater body mass and investment in reproductive capacity. In contrast, males appear to invest more in locomotor capacity giving them higher performance for a given body size compared to females.

Effects of testosterone on contractile properties of sexually dimorphic forelimb muscles in male bullfrogs (Rana catesbeiana, Shaw 1802)

This study examined the effects of testosterone (T) on the contractile properties of two sexually dimorphic forelimb muscles and one non-dimorphic muscle in male bullfrogs (Rana catesbeiana, Shaw 1802). The dimorphic muscles in castrated males with testosterone replacement (T₊) achieved higher forces and lower fatigability than did castrated males without replaced testosterone (T₀ males), but the magnitude of the differences was low and many of the pair-wise comparisons of each muscle property were not statistically significant. However, when taken as a whole, the means of seven contractile properties varied in the directions expected of masculine values in T₊ animals in the sexually dimorphic muscles. Moreover, these data, compared with previous data on male and female bullfrogs, show that values for T₊ males are similar to normal males and are significantly different from females. The T₀ males tended to be intermediate in character between T₊ males and females, generally retaining masculine values. This suggests that the exposure of young males to T in their first breeding season produces a masculinizing effect on the sexually dimorphic muscles that is not reversed between breeding seasons when T levels are low. The relatively minor differences in contractile properties between T₊ and T₀ males may indicate that as circulating T levels rise during breeding season in normal males, contractile properties can be enhanced rapidly to maximal functional levels for breeding success.

No sexual dimorphism in limb muscles of a frog not engaging in amplexus

Sexual dimorphism in limb muscles is widespread among anurans, with males having stronger limbs than females. This phenomenon has been interpreted in the context of intrasexual selection: 1) the robust forelimb muscles in males are associated with amplexus, in which the male tries to grasp the female tightly, and also with rejection of rivals’ attempts at taking over, and 2) massive hindlimb muscles favor the ability to kick away rivals during scramble competition. However, in a few species, fertilization occurs without any form of amplexus and in these species the limb muscle dimorphism is expected to be absent. We tested this prediction inFeirana taihangnicus: a species without amplexus. As expected, we detected non-significant sexual differences in the mass of both forelimb and hindlimb muscles after accounting for body size and age. Our findings represent an interesting example of coevolution of form and function.

Prolonged relaxation after stimulation of the clasping muscle of male frog, Rana japonica, during the breeding season

We investigated the mechanical properties of the flexor carpi radialis muscle (FCR), a forelimb muscle used mainly for amplexus in the breeding season (February to March), of the male Japanese brown frog, Rana japonica. In the present experiment, the changes in force and stiffness of the FCR before, during, and after contraction were measured at 4 degrees C. The total time from the end of stimulation to the end of relaxation was about 30 min. The time course of this prolonged relaxation was fitted by two exponential decay processes. Stiffness decreased during prolonged relaxation, but stayed higher than force, when normalized to peak values. These mechanical properties of the FCR were different from those of the glutaeus magnus muscle (GM) in the hindlimb, used for jumping. When a quick release was applied to the FCR during relaxation, the force recovered gradually after a sudden decrease. The time course of this force recovery was fitted by a single exponential term, and the rate constant decreased as the prolonged relaxation proceeded. The possible involvement of active process(es) in the prolonged relaxation is discussed.

Thermal relationships and exercise physiology in anuran amphibians: Integration and evolutionary implications

Thermal and water balance are coupled in anurans, and species with particularly permeable skin avoid overheating more effectively than minimizing variance of body temperature. In turn, temperature affects muscle performance in several ways, so documenting the mean and variance of body temperature of active frogs can help explain variation in behavioral performance. The two types of activities studied in most detail, jumping and calling, differ markedly in duration and intensity, and there are distinct differences in the metabolic profile and fiber type of the supporting muscles. Characteristics of jumping and calling also vary significantly among species, and these differences have a number of implications that we discuss in some detail throughout this paper. One question that emerges from this topic is whether anuran species exhibit activity temperatures that match the temperature range over which they perform best. Although this seems the case, thermal preferences are variable and may not necessarily reflect typical activity temperatures. The performance versus temperature curves and the thermal limits for anuran activity reflect the thermal ecology of species more than their systematic position. Anuran thermal physiology, therefore, seems to be phenotypically plastic and susceptible to adaptive evolution. Although generalizations regarding the mechanistic basis of such adjustments are not yet possible, recent attempts have been made to reveal the mechanistic basis of acclimation and acclimatization.

What factors contribute to an ownership advantage?

In most taxa, owners win fights when defending a territory against intruders. We calculated effect sizes for four factors that potentially contribute to an 'owner advantage'. We studied male fiddler crabs Uca mjoebergi, where owners won 92% of natural fights. Owners were not more successful because they were inherently better fighters (r=0.02). There was a small effect (r=0.18) of the owner's knowledge of territory quality (food availability) and a medium effect (r=0.29) of his having established relations with neighbours (duration of active tenure), but neither was statistically significant. There was, however, a significant effect due to the mechanical advantage the owner gained through access to the burrow during fights (r=0.48, p<0.005).

Plasticity of muscle function in a thermoregulating ectotherm (Crocodylus porosus): biomechanics and metabolism

Thermoregulation and thermal sensitivity of performance are thought to have coevolved so that performance is optimized within the selected body temperature range. However, locomotor performance in thermoregulating crocodiles (Crocodylus porosus) is plastic and maxima shift to different selected body temperatures in different thermal environments. Here we test the hypothesis that muscle metabolic and biomechanical parameters are optimized at the body temperatures selected in different thermal environments. Hence, we related indices of anaerobic (lactate dehydrogenase) and aerobic (cytochrome c oxidase) metabolic capacities and myofibrillar ATPase activity to the biomechanics of isometric and work loop caudofemoralis muscle function. Maximal isometric stress (force per muscle cross-sectional area) did not change with thermal acclimation, but muscle work loop power output increased with cold acclimation as a result of shorter activation and relaxation times. The thermal sensitivity of myofibrillar ATPase activity decreased with cold acclimation in caudofemoralis muscle. Neither aerobic nor anaerobic metabolic capacities were directly linked to changes in muscle performance during thermal acclimation, although there was a negative relationship between anaerobic capacity and isometric twitch stress in cold-acclimated animals. We conclude that by combining thermoregulation with plasticity in biomechanical function, crocodiles maximize performance in environments with highly variable thermal properties.