The age and body size of Amolops mantzorum between two populations distributed in western China were estimated using skeletochronology. The age at sexual mature of individuals was be estimated 2 yrs old in males for both populations while females reached sexual maturity at 2 yrs old at the low-altitude site and 3 yrs old at the high-altitude site. The oldest males and females from the high-altitude site were 7 yrs and 10 yrs old while longevity of males reached 6 yrs and 7 yrs in females at the low-altitude site. Average age between males and females differed significantly at the high-altitude site, but it did no differ significantly at the low-altitude site. For both sexes, average age did not differ significantly between the populations. On average, adult females had significantly larger body sizes than adult males for both populations. Sexual size dimorphism from the low-altitude site and the high-altitude site was 0.280 and 0.282, respectively. Body size between the populations differed significantly within each sex. Positive correlations were found between age and body size for both sexes within each site. The growth coefficient did not differ significantly within a population both sexes or in a sex between the populations. Our findings suggest that inter-population difference in body size of the frogs seems to be related to longevity of individuals, ambient temperature and construction of dams for electricity.
Elevation that results in changes in climate, duration of breeding season and food resource has long been considered a major influence on the evolution of life-history traits in amphibians. The present study examined differences in reproductive output (clutch size and egg size) of the Omei Treefrog (Rhacophorus omeimontis) at two elevations (1000 m and 1700 m above sea level) in Baoxing County, western China. Within each population, female attributes (size and age) were responsible for much of the reproductive output variation in that larger or older females produced larger clutches of smaller eggs. Clutch size and egg size showed a significantly negative correlation, which was indicative of a trade-off between the two parameters. The high-elevation females were significantly larger than the low-elevation counterparts. After accounting for interpopulational difference in body size, clutch size, egg size and clutch volume differed significantly between the populations. For the high-elevation population relatively smaller clutches tended to be associated with larger eggs. Our findings suggest that females produce smaller clutches relative to body size and larger eggs in the high-elevation population to ensure that each egg is adequately provisioned in the face of cold climate and short duration of development.
In animals, body size is a prominent trait that generally positively affects ecological and reproductive success. Through field observations and experiments, we investigated the effects of two mechanisms of sexual selection on large male mating advantage in the Andrew's toad, Bufo andrewsi, a species widely distributed in western China. We observed a large male mating advantage in the field. Field data were corroborated by experiments in which large males mated more frequently than smaller males. However, in female preference tests, in which females could choose freely between two males differing in body size, choosing females showed no size preference while many females did not exert mate choice at all. We suggest that the large male mating advantage observed in the field and laboratory is caused by competition among males rather than by female choice.
Selection pressure is an important force in shaping the evolution of vertebrate brain size among populations within species as well as between species. The evolution of brain size is tightly linked to natural and sexual selection, and life-history traits. In particular, increased environmental stress, intensity of sexual selection, and slower life history usually result in enlarged brains. However, although previous studies have addressed the causes of brain size evolution, no systematic reviews have been conducted to explain brain size in anurans. Here, we review whether brain size evolution supports the cognitive buffer hypothesis (CBH), the expensive tissue hypothesis (ETH), or the developmental cost hypothesis (DCH) by analyzing the intraspecific and/or interspecific patterns in brain size and brain regions (i.e., olfactory nerves, olfactory bulbs, telencephalon, optic tectum, and cerebellum) associated with ecological factors (habitat, diet and predator risk), sexual selection intensity, life-history traits (age at sexual maturity, mean age, longevity, clutch size and egg size, testis size and sperm length), and other energetic organs. Our findings suggest that brain size evolution in anurans supports the CBH, ETH or DCH. We also suggest future directions for studying the relationships between brain size evolution and crypsis (i.e., ordinary mucous glands in the skin), and food alteration in different developmental stages.
Competition for fertilization in multi-male group spawning can drive variation in male reproductive investment (i.e., testis mass and sperm size). Inter-population comparisons of variation in energetic availability and allocation along geographical gradients allow insights into the mechanisms shaping the reproductive investments of animals. Although inter-population differences in female reproductive investment (i.e., clutch size and egg size) have been studied extensively across a wide range of taxa, little information on variation in reproductive investment in males is available. Here, we studied altitudinal variation in testis mass and sperm length among three populations in the Jingdong tree frog (Hyla gongshanensis jingdongensis), a polyandrous species, in Yunnan Province in China. The results showed that individuals exhibited smaller testes at higher altitudes while testes mass was positively correlated with body size, body condition and age. Longer sperm length was observed in the middle-altitude population. Moreover, we found that sperm number was positively correlated with testes mass and sperm length. Our correlational findings suggest that environmental constraints at high altitude select for less investment in testes and offspring number.
In many taxa, the left and right testes often differ in size. The compensation hypothesis states that an increase in size of one testis can compensate for a reduced function in the other testis. Moreover, the expensive-tissue hypothesis predicts that an increase in investment of a metabolically costly tissue is offset by decreasing investment in the other metabolically costly tissues. Here we tested these two hypotheses in Carassius auratus, by analysing difference between left and right testes mass, and between brain mass and both gut length and gonad mass (testes mass in males and clutch mass in females). We found no difference between left and right testis mass and no correlations between relative testis size and body measurements. These findings suggest that the left testis cannot serve a compensatory role. Nonetheless, contrary to the predictions of the expensive-tissue hypothesis, brain mass was positively correlated with both gut length and gonad mass within each sex. This positive correlation between brain mass and other organs (gut, gonad and clutch tissues) suggests that organisms may compensate for substantial variation in investment in tissues without sacrificing other expensive tissues.
Sexual differences in morphological traits are widespread among animals. Theory predicts that dimorphism in secondary sexual characters evolves as a consequence of sexual selection. We investigated the intersexual difference in mass of forelimb muscles of the Andrew's toad, Bufo andrewsi, an elongated species inhabiting montane regions in western China. Our results showed that average body size of females was significantly larger than males. However, when the influence of body size was removed, the forelimb muscle mass of males significantly exceeded that of females, and total mass of forelimb muscles of amplectant males was significantly larger than that of non-amplectant males. These results suggest that the robustness of the forelimbs can allow amplectant males to retain a firm grip on the female in amplexus, which may aid resistance to inference by conspecific males. Our findings are consistent with the prediction that sexual selection favors large forelimb muscle mass in males.
The digestive tract provides a functional relationship between energy intake and allocation. An understanding of effects of environmental factors on the evolution of digestive tract morphology is especially important. To investigate this, we studied the variation in digestive tract length across 10 populations of the Andrew’s toad (Bufo andrewsi) between 2012 and 2015 in Sichuan province, western China. These populations were collected in different habitats varying in temperature and precipitation. The results reveal an increase in the length of the digestive tract and gut with increasing temperature and decreasing precipitation, when controlling for the effect of body size. Our findings suggest that individuals of populations living in high-temperature and low-precipitation environments have longer digestive tracts, possibly because they consume less animal-based foods and more high-fiber foods.
Social group is associated with life-history traits and can predict brain size variation in cooperative primates and some other mammal groups, but such explicit relationships remain enigmatic in cooperatively breeding birds. Indeed, some compositions of social group in cooperative species (e.g., helper number and group size) would affect the fitness of breeders by providing alloparental care. Here, we conducted comparative tests of the relationship between the social group and both life-history traits and brain size across 197 species of cooperatively breeding birds using phylogenetically controlled comparative analyses. We did not find any correlations between helper numbers and both life-history traits and brain size. However, we found that maximum group size was positively associated with clutch size. Moreover, average group size has positive associations with body mass and relative brain size. Our findings suggest that helper numbers cannot promote variation in relative brain size, while larger groups may predict bigger brains in cooperatively breeding birds.
Muscles are vital for the process of movement, mating and escape of predators in amphibians. During evolution, the morphological and genetic characteristics as well as the size of muscles in species will change to adapt different environments. Theory predicts that low male-male competition in high-altitude/latitude selects for small limb muscles. Here, we used the Andrew’s toad (Bufo andrewsi) as a model animal to test this prediction by analyzing geographical variation in the mass of limb muscles across nine populations from the Hengduan Mountains in China. Inconsistent with the prediction, we found that latitude and altitude did not affect the relative mass of total combined limb muscles and mass of combined hindlimb muscles among populations. Meanwhile, the relative mass of combined forelimb muscles, the two forelimb muscles (flexor carpi radialis and extensor carpi radialis) and the four hindlimb muscles (e.g. biceps femoris, semimebranous, semitendinosus and peroneus) was lowest in middle latitude and largest in low latitude whereas gracilis minor was largest in high latitudes. However, we did not find any correlations between the two forelimb muscles and the four hindlimb muscles and altitude. Our findings suggest that combined forelimb muscles, flexor carpi radialis, extensor carpi radialis, biceps femoris, semimebranous, semitendinosus and peroneus are largest in low latitudes due to pressures of mate competition.