Geographic variation in maternal investment and trade-offs between egg size and clutch size in an endemic toad of the Qinghai-Tibet Plateau

Abstract

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...

Negative carry‐over effects on larval thermal tolerances across a natural thermal gradient

“…Interestingly, egg diameters were smaller in the spring than fall. This pattern could reflect variability in unmeasured factors (such as food availability) or life-history trade-offs between egg size and egg number or future reproductive potential (Roff 1992, Yu andDeng 2020). Importantly, both reproductive traits (GSI and egg diameters) indicate higher maternal investment in lower stress habitats (at lower tide heights), a pattern that occurred seasonally for GSI but not egg diameters.…”

The evolution of reproductive strategies in turtles

“…Macroevolutionary patterns in amniote reproduction ( Battistella et al, 2019 ; Murray, Crother & Doody, 2020 ; Starck, Stewart & Blackburn, 2021 ) can be investigated based on the diversity of traits in egg and clutch ( e.g ., Kaplan & Salthe, 1979 ; Deeming & Birchard, 2007 ; Jetz, Sekercioglu & Böhning-Gaese, 2008 ; Deeming & Ruta, 2014 ). The idea of an “optimal” correlation between egg and clutch size, based on trade-offs associated to K/r strategies, has led to several discussions without a consensus about the distribution or reasons of such correlations ( Smith & Fretwell, 1974 ; Congdon & Gibbons, 1987 ; Wilbur & Morin, 1988 ; Elgar & Heaphy, 1989 ; Godfray, Partridge & Harvey, 1991 ; Kuchling, 1999 ; Zhao, Chen & Liao, 2017 ; Yu & Deng, 2020 ). Optimal egg/clutch size theory assumes that changes in the egg and clutch are driven by selection, resulting in adjustments for the largest possible production of offspring with the highest fitness, at the lowest cost to their progenitors ( Brockelman, 1975 ; Congdon & Gibbons, 1987 ; Janzen & Warner, 2009 ).…”