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To better understand the importance of body size in hermaphroditic mating, we studied the mating behaviour of the pond snail Radix auricularia. This simultaneous hermaphrodite exhibits a positive correlation between body size and egg production and provides weak evidence for size-assortative mating. In addition to this mating pattern, the partner performing the male role was significantly smaller than the mounted partner, performing the female role. In this study, two laboratory experiments were done to test whether body size has (1) an effect on mate choice of the sperm donor, and (2) poses mechanical constraints. These snails did not display physical limitation in mating with a much larger or smaller partner. In the mate choice experiment, we found no evidence for significant preferences for large partners. Therefore, mate choice of sperm donor and mechanical constraints are not likely to contribute to the formation of size-assortatively mating pairs in this species, which contributes to understanding about how these processes work in hermaphrodites.

Life history theory predicts that animals often produce fewer offspring of larger size and indicate a stronger trade-off between the number and size of offspring to cope with increasing environmental stress. In order to evaluate this prediction, we tested the life history characteristics of Bufo minshanicus at eight different altitudes on the eastern Tibetan Plateau, China. Our results revealed a positive correlation between female SVL and clutch size or egg size, revealing that larger females produce more and larger eggs. However, high-altitude toads seem to favor more offspring and smaller egg sizes when removing the effect of female SVL, which is counter to theoretical predictions. In addition, there was an overall significantly negative relationship between egg size and clutch size, indicative of a trade-off between egg size and fecundity. Therefore, we suggest that higher fecundity, rather than larger egg size, is a more effective reproductive strategy for this species of anuran living at high-altitude environments.Life-history theory suggests that females should be able to optimize their allocation of resources between current breeding efforts and future survival 1 . Higher altitudes can be stressful environments because temperatures are colder, seasonality is greater, and food availability can fluctuate greatly. For example, there are negative correlations between temperature and altitude 2 , with rainfall and altitude found to be positively correlated 3 . High-altitude females are expected to select for producing larger eggs 2-6 because larger eggs can lead to larger initial sizes, faster rates of growth, faster developmental rates, or both 7-12 . Yet, a significant increase in egg size may prolong the duration of the embryonic period, which results in increasing total mortality 13 . In addition, individuals that metamorphose at larger size may be more easily discovered by predators, thus Rollinson and Hutchings (2010) 14 suggest that bigger is not always better. However, egg or hatching size may not affect mass at metamorphosis and survival because other ecological factors, such as temperature, density and competition, food level and quality, predation, or a combination of these, can alter the growth and developmental rate of each life stage of amphibians 9,15-18 .The number of offspring produced, and their size, are fundamental life history traits because they are intimately linked with fitness and therefore population viability 19 . The trade-off between offspring number and size suggests that selection will produce more small eggs or abundant larvae under scarce resources and stressful environments, which is known as r-selection 9 . The advantage associated with producing smaller eggs is due to the corresponding increase in fecundity 9 . Conversely, selection will prefer to produce larger eggs or more robust larvae under abundant essential resources and good environmental conditions, which is regarded as K-selection 5,20 . However, amphibians respond to varying environments by changing egg size or a t...

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