| The life history strategies of animals consist of a series of balanced life history characteristics such as growth,age and body size at sexual maturity,reproductive investment,and survival.For the limitation of available energy resources,animals must make trade-offs and reasonably allocate energy among self-maintenance-reproduction-growth and development.Especially under environmental stress conditions,it is particularly important for animals to maximize their own fitness to maintain the development of populations by optimizing energy allocation between reproduction and self-maintenance and optimizing reproductive strategies.Our previous studies have shown that environmental heavy metal stress has disturbed the growth,survival and population stability of Bufo raddei,resulting in changes in its trade-offs of growth-reproduction strategy and clutch size-egg size.These changes must be accompanied by the trade-off optimization of energy distribution during the life cycle,while the nonbreeding season is a critical period for amphibians to accumulate energy for selfmaintenance or even damage repair.Under the background of environmental heavy metal stress,it is still unknown how amphibians balance the energy allocation in the non-breeding season,and what is the difference in energy trade-off strategies for amphibians under different environmental pollution stresses.In this study,in order to explore the response mechanism of self-maintenance and reproductive strategy of B.raddei to environmental heavy metal stress,we continued our research in the same plots—Baiyin(BY),the heavy metal polluted area,and Liujiaxia(LJX),a relatively unpolluted area.Heavy metal enrichment levels in organ,self-maintenance indicators and gonad growth and development levels of B.raddei were monitored in non-breeding seasons,and the response mechanism of self-maintenance and reproductive strategy to environmental heavy metal stress was explored by comparing the differences in the above indicators between individuals of different genders,different ages and different plots.Meanwhile,the energy allocation strategy between self-maintenance and reproduction was clarified in order to explore the effects and mechanism of long-term heavy metal stress on the survival and reproduction of amphibians.The main results are as follows:1.Heavy metal concentration in tissues of B.raddei from the two plotsThe concentrations of heavy metals(Cu,Zn,Cd and Pb)in various organs and tissues of B.raddei were generally higher in BY than those individuals in LJX in the same age and gender;the concentration of Zn in organs and tissues in individuals in LJX increased with age,while the concentrations of Cu and Cd(male)in some organs and tissues decreased.Meanwhile,the concentration of Cu in some organs and tissues of BY individuals showed a decreasing trend.Except an increasing trend with age in the concentration of Pb in the gonad of individuals from BY,overall,the concentrations of Zn,Pb and Cd in the tissues of individuals from BY were not significantly correlated with age.2.Differences in energy distribution between self-maintenance and reproduction investment of B.raddei from the two plots(1)Differences in growth and development indicators: Overall,there was no significant difference in the body condition of B.raddei from the two plots,and only the body condition of 3-year-old females from BY was significantly higher than the individuals of the same age from LJX;the body length,body weight,and body condition of B.raddei from the two plots increased extremely significant with age.The organ coefficients of liver,kidney,fatbody,and stomach of 1 to 3-year-old individuals from BY were higher than individuals of the same age from LJX,but the 4-year-old individuals showed the opposite result;the organ coefficients of male individuals in both plots were overall higher than those of female individuals.(2)Differences in nutrition and health maintenance indicators: The nutrition level(fat and protein content in tissues,organ coefficient of fatbody)of individuals aged 1-3 years in BY was overall higher than individuals of the same age in LJX,but the 4-year-old individuals showed the opposite result;overall,the nutrition level of male was higher than that of female.The antioxidant enzyme activity and oxidative stress level in the liver of adult B.raddei were higher in LJX,but the female subadults showed the opposite result,while the antioxidant enzyme activity and oxidative stress level in the kidney of BY individuals were higher;with the increase of age,the antioxidant enzyme activity and oxidative stress level in the liver of LJX individuals increased,but the opposite result was shown in kidney;simultaneously,the antioxidant enzyme activity in liver(only in female)and in kidney of BY individuals decreased.The immune level,especially the plasma immunoglobulin Y(Ig Y)level,of LJX individuals was significantly higher than that of BY individuals.In addition,the Ig Y level of LJX male individuals decreased significantly with age,while the Ig Y level of BY individuals remained at a relatively stable and significantly lower level.(3)Differences in gonad development indicators: No significant differences were shown in organ coefficient of oviduct,organ coefficient of spermary,or spermary development level in B.raddei from the two plots at the same age;individual organ coefficient of gonad increased significantly with age in female and male in LJX and female in BY,the spermary development level of males in LJX increased with age.3.Differences in energy trade-off strategies between self-maintenance and reproductive investment of B.raddei from the two plots(1)Differences in energy investment of self-maintenance and reproduction:Correlation analysis showed that there were negatively correlations between the organ coefficient,nutrient level,antioxidant index and immune level with gonad organ coefficient or gonad development level of female B.raddei from the two plots and male B.raddei from LJX,indicating that the increasing energy input for reproduction reduced the energy input for growth and maintenance of individuals,and this was particularly significant in BY female individuals.(2)Differences in energy investment between growth and survival: The organ coefficients of stomach,liver and kidney of B.raddei in BY plot were all higher than individuals of the same age in LJX,indicating that BY individuals accumulated more nutrients through the increase of food intake during the non-breeding season.However,B.raddei in BY plot were under a much higher oxidative stress,and the immune level of organism was generally lower.Moreover,the nutrition level of elderly individuals in this plot was lower.(3)Differences in trade-off strategies between genders: The growth and development,nutrient storage,antioxidant defense levels and immune levels of female B.raddei were lower than male individuals of the same age,indicating that the higher energy input for reproduction limited the energy input for self-maintenance of females.Compared with individuals in LJX with the same age and the younger individuals in the same plot,the nutrition level and organ coefficient of older female individuals in BY were significantly lower,indicating that under environmental heavy metal stress,the female's energy input for reproduction and growth in the early stage of life greatly limited the energy input for self-maintenance in the late stage of life.Overall,under long-term environmental heavy metal pollution,the energy input of B.raddei for growth and energy saving was higher,and the energy input of B.raddei for self-maintenance especially for immune and antioxidant functions was lower.Moreover,the energy input for self-maintenance in the early stages of life was significantly higher than that in the late stages.The results of the present study would provide a reference for further exploring the mechanism of amphibian population stability changes caused by environmental pollution. |
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