Study On The Evolutionary Ecology Of Small Herbivorous Mammals: Life History Strategy Of Plateau Pika (Ochotona Curzoniae)

Life history research lies at the heart of evolutionary ecology. It studies the complex relationships in the living process, which can help us understand the Evolution Theory and analyze and predict population dynamics. After studying life history of plateau pika, an endemic herbivorous small mammal habituating in the Qinghai-Tibet Plateau, the following results are reported:1. General life history features of plateau pika.The results support our hypothesis: (1) the fitness of plateau pika population is at relatively low level compared with those of other mammal species; (2) plateau pika adoptes a quick grow strategy; (3) the life history features of plateau pika are concordant with the classification standard of Charnov (2002), i.e. E/a 1.35, C·E~-1.7, I/m~- 0.3, in which E stands for average adult life span; a stands for age-at-first-reproduction; C is reproductive effort; I is the size of an offspring at independence from the parent; m is average adult body mass.2. The similarities and differences of life history features between plateau pika and boreal pika.The life history features of plateau pika are different from those of boreal pika (Ochotona princeps). Boreal pikas occupy territories in talus surrounded by suitable vegetation, while plateau pikas live in habitat in Kobresia humilis meadow in the alpine meadow ecosystem; both pikas do not hibernate, boreal pika constnict a haypile of loose vegetation during the summer to serve as food during the winter, while plateau pika do not store hay for winter; both of them have 2 litters per year, boreal pika breed as yearlings and with a postpartum estrus, while plateau pikas breed at the second year after birth; the litter size of plateau pika is two size of that of boreal pika; the mortality of boreal pika is lower than that of plateau pika; fecundity of boreal pika does not vary with age, however, fecundity of plateau pika varies with age; the sex ratio of adult boreal pikas is near unity, the sex ratio of adult plateau pika is near 1.31:1; the maximum longevity of boreal pikas is 7 years, which is more than two times that of plateau pika, i.e. 3 years; average adult weight of plateau pika is 150g, which is a little higher than that of boreal pika, which is 140g.3.Survival features of plateau pika in life historySurvival rate of plateau pika variate with age. The general mortality pattern of plateau pika is similar to those of other mammal species. Average mortality variate with season. Age-specific mortality pattern is similar to boreal pika(Ochotona princeps)habituating in Alberta, though juvenile mortality and adult mortality are higher than those of boreal pikas. There are 3 mortality peak in plateau pika population. The first time is in neonate period, whenonly one half of juveniles can pass through this period, which implies that juvenile period is influenced strongly by natural selection; the second time is in fecundity peak period, which reflects the cost of reproduction; the third time is in the old-age of plateau pika, a significant loss occur during this period, which implies the senescence of plateau pika and is the result of natural selection.The average longevity of females is longer than that of males. A female reaches the maximum life span recorded, i.e. 931 days. The average longevity of all individuals is 16.33 Mon." 490 days.The survival rate of females is significantly higher than that of males, which reflects the cost of reproduction and society cost of males. It is the outcome of natural selection. The sex ratio of neonates is 1:1, however, the sex ratio of adults is female: male=1.31:l, which is caused by higher mortality of males over females in life history.4. Reproductive features of plateau pikaPlateau pikas have 2 litters every year. The average gestation period of females is 18~20 day. The average litter size is 4.57 individual. The average weight of neonates is 9.28 g. The average litter size of adult female plateau pika does not variate with age. Breeding season is between April ~ June. The reproductive value and fertility of 15~18 Mon.- age females are highest The reproductive value and fertility increased wth age before reproductive value and fertility peak-age, however, the reproductive value and fertility decreased with age after peak-age.5. The body growth rule of plateau pika and its growth modelThe juveniles grow quickly before 30-day age, exhibiting a J-form growth curve, and then grow slower between 30-65 day age, and reach weight equilibrium at about 65-day old, which implies that 65-day is the maturity age of plateau pika.There does not exist dimorphism in plateau pikas. The configurations of females and males iire similar; the average weight of males is a little heavier than that of females, but the difference is not significant (F =1.0854, df=154, P>0.3058).The growth model of juveniles is: dm/dt =6.5266*ma75 - 12.1787nno6. Trade-off between life history featuresThe relationships between fertility and survival, residual reproductive value and fertility, future fertility and present fertility, fertility and mortality, reproductive value and mortality were analyzed. The results indicated that only reproductive value and mortality showed significantly negative relationship, while other features showed insignificant correlation. The correlation coefficients between future fertility and present fertility, fertility and survival were positive, but insignificant. The significant negative correlation between reproductive value andmortality perhaps open out trade-off between these two life history features, which deserves further research.7. The fitness level of plateau pika in MammaliaA life history feature table and a life history strategy analysis table of 65 mammal species belonging to 9 Order have been compiled according to their life table data, among which the fitness indexes of 46 species have been calculated. The fitness index of plateau pika is ranked at the 43rd location, only higher than those of bank vole (Clethrionmys glareolus), chimpanzee (Pan troglodytes) and African elephant (Loxodonta africana). The fitness index of plateau pika (r=0.1125) was lower than that of boreal pika (0. princeps) (r=2.172). The survival rate is the main factor influencing fitness.8. The dynamics of plateau pika populationThe density of plateau pika population in three years was calculated with a computer program compiled in Turbo C language, according to Jolly-Seber method. The dynamics of plateau pika population was coincident with the low intrinsic population growth rate (r=0.1125), i.e. plateau pika population was stable.Reproductive effort is an important factor influencing population fitness. We argue that average weaning weight of juveniles (Wo) / average weight of adults (Wa) as reproductive effort of mammals.