Body size is a fundamental feature of animals,which is linked to many physiological and life-history traits of organisms,but its relationship with population density and mechanisms remain largely unknown.Based on 27-years of survey data,we investigated the potential influencing factors affecting body mass of the Chinese striped hamster (Cricetulus barabensis,Pallas,1773) and greater long-tailed hamster (Tscherskia triton,de Winton,1899) in Northern China Plain.Focusing on three criteria of body mass of a population (average,maximum and minimum body mass) ,we proposed three hypotheses to explain the body mass shift in the two hamsters-the growth,survival and reproduction hypotheses.Our results showed that the average and maximum body mass of both hamster species were positively correlated with population density,and that this density-dependent change of body mass is more likely to be caused by a change of mortality rates,not by a change of growth rates or reproduction rates.In high-density years,reproduction of hamsters was reduced but survival increased,which resulted in large average and maximum body mass in high-density years.High irrigation probably contributed to low average and maximum body mass by increasing the mortality rate of hamsters.Our results suggest that,in the absence of survival and growth data,maximum body mass (which represents survival rate) ,minimum body mass (which represents growth rate) and average body mass are very useful criteria in revealing intrinsic and extrinsic effects on population dynamics and the underlying mechanisms.