Abstract: Intensive male-infant caretaking (IMC) in the Barbary macaque (Macaca sylvanus),which is promiscuous and highly uncertain in the paternity of infants,has theoretically embarrassed primatologists since 1980s because it is beyond the sphere of action of kin selection and sexual selection,two powerful tools for interpreting social behavior at this time.Fortunately the same kind of behavior has recently been observed in the Tibetan macaque (M.thibetana),in which male mating opportunity is rank- and age dependent,and the paternity is higher than that in Barbary macaques.But both species have similar habitats at high latitude and altitude,and in short of fruits.The comparison of etho-ecological data suggests a logical connection of the IMC with the habitat.For wild Tibetan macaques,except supplied with fruits and bamboo shoots,which are suitable for macaques,in autumn,they have to feed mainly on leaves and grass in other seasons.Interestingly,the onset of weaning (bottleneck for infant survival) and the mating are concentrated on the best season,autumn,but remaining half of births in harsh winter.The contradiction between very low nutrient-intake and increased energy demand on maintaining body temperature for both sexes and on pregnancy and lactation for females in winter is solved by way of storing energy as fat before winter,and consuming intrinsic body material in late winter.This is suggested by the fluctuation of body weight in a year period:peaked in late autumn and dropped to valley point in late winter.Taking the mean of the two extremes,18.3 kg for males and 14.1 for females,as the baseline,the margin of the fluctuation is ±19% for females and ±7% for males.Of the margin,the weight loss of females is over that of males for 2.7 times,giving an approximate measurement of the difference of parental investment to offspring.The difference demonstrates the necessity and possibility of IMC,males timely help to exhaust females,which would benefit the reproductive success of the group.Thus so far,the poor habitat shaping the IMC is proven with both logical and physiological evidences.In fact,the rule seems held right for the Old World primates regardless of their mating system because the only monogynous form,Hylobates syndactylus showing the IMC also lives in habitat lacking of fruit.In addition,the male-infant-male interaction in Barbary and Tibetan macaques is also recognized as a byproduct of IMC.Accordingly,our work has offered a firm foothold for a more general hypothesis of energy-related habitat factors shaping social behavior.Obviously,the IMC possesses the nature of"cooperation for existence".The cooperation then is integrated with Darwins "struggle for existence"—predator defense and competition for food,forming a concept of "ecological selection".Furthermore,the ecological selection is integrated with sexual selection and kin selection in a way of parallel and not contrary to each other,constructing a unifying framework of"general natural selection".Within the framework,a series of conceptual confusions,which are produced by the absence of ecological selection in the evolutionary theory of social behaviors,can be effectively eliminated.That is,most of the behaviors can be related to ones own shaping force(s).For this,what we need to do is to find the problem,or the need for survival and/or reproduction,behaviorally addressed to by the animal,and then to match the problem with specific one or more selective pressure.At the end,the operatedness of"ecological selection"and its potential value of applying to the management of human societies are also discussed.