The interplay between socioecologlcal and biological processes manifests Itself at the level of individuals, populations, and species. The biology of Individuals is deeply modified when they are groups; many of the attributes of populations such as size, distribution, composition, etc. are related to social interactions, and at the level of species, patterns of social relations within groups tend to be structured in ways that influence survival, reproduction, and exchange among populations.
In one experimental approach to these problems, the social ecology of freely growing populations of mice In large enclosures was related to behavioral, physiological, and health changes of individuals, to demographic changes and to changes of gene frequencies. Another experiment examined the process and effects of artificial selection for the same trait in different social environments.
The population enclosures were octagonal structures subdivided Into central and peripheral sections with a total surface area of 13.3 square feet. From a founder group of mice of known genetic (progeny of a four-way cross among inbred mouse strains C57L/J, SWR/J, C3HeB/FeJ, 129/J) and environmental background, three equivalent samples of mice were distributed into replicate population enclosures (Pop A and B) and into standard laboratory cages as randomly mated male-female pairs—the control group (Pop C).
During the first year of study, daily observations of the enclosures were made, and several censuses were performed. Identifiable cohorts, animals born during each census interval, were established to provide an additional way of analyzing changes in the populations.
In Pop C, reproduction remained constant and mortality was negligible. Marked changes occurred in Pop A and B. The sizes (1000-A and 800-B mice) and densities (85-A and 60-B mice per square foot) are several times greater than those of any previously reported population of small mammals. However, there would have been 100,000 mice in each enclosure at the end of a year had the populations continued to grow as they did at first. Changes of reproductive physiology constituted prominent aspects of self-regulation in the enclosures. Peak demographic input rates occurred during the third month, but were already associated with decreased productivity per adult female. Analysis of maturation and reproduction pointed to inhibition of reproduction in sexually mature females as the most important factor in the decline of productivity. Pregnancy rates fell steadily and inhibition of full-term gestation occurred. Gonads and reproductive cells of males were adult, but a large proportion of males showed little sexual activity.
Neonatal mortality was particularly striking in Pop B, where 30% of females showed advanced pregnancy during the last 5.5 months with no newborns surviving. About 25% of the mice in the enclosures died during the year. Highest weekly death rates occurred during the first half of the year before peak numbers were present. Autopsies of mice of Pop A revealed little in the way of abnormal findings.
Biomass either paralleled or increased more rapidly than numbers in both enclosures, contrasting with some other population: studies in which growth was impaired with crowding.
Changes of behavior included: 1. disappearance of circadian activity peaks, 2. decline in frequency of fighting per male but an increase in unusual aggressiveness, 3. aberrations of sexual behavior, 4. deterioration of maternal care, 5. cannibalism, 6. striking decrease in social responsiveness.
Cohorts in the populations were biologically distinguishable sub-units in contrast to control cohorts, which showed no such differentiation. Cohorts in Pop A and B differed with respect to reproduction physiology, mortality, and behavior, and intercohort differences persisted at all levels of population density.
Many of the properties of Pop A and B mice changes when the mice were placed in different social environments, attesting to the specificity of the influence of social factors. For example, mice of Pop A, randomly paired in control cages, showed a marked rise in reproduction, and cohorts reproductively inhibited before were most productive in the new social environment. Behavioral tests performed outside the enclosure environment revealed: 1. intercohort differences among Pop A mice contrasted with stereotyped behavior of Pop C mice, and 2. changes in behavior of Pop A mice both immediately after removal from the population and after six weeks in new social conditions. Pop B mice changed their social environment by emigrating into the empty interconnected enclosure of Pop A. Two distinctive sub-populations formed. Greater changes in reproduction, mortality, and behavior occurred in the emigrant subpopulation, which underwent more extensive social reorganization. Immediately following reunion of the two subpopulations, a population crash occurred, possibly related to the sudden changes of social conditions.
Use of genetically defined animals made feasible the study of gene frequency changes. Polymorphism of alleles at the C locus affecting coat color differed between Pop A and B on the one hand and Pop C on the other. Although the magnitude of the upward change of recessive c in Pop A and B was not large, the consistency and similarity of the change in Pop A and B and lack of change in Pop C suggested the action of systematic processes and the probable adaptiveness of the changes. There was little evidence of differential adult reproduction or mortality among the phenotypes but there were suggestions of differential neonatal survival. The relatively slow rate of change of the alleles after the first generation suggested the establishment of a state of balanced polymorphism at the C locus. Hemoglobin allele and genotype frequencies of mice of Pop A alive at the end of the year did not deviate from what might have been predicted on the basis of panmixia.
Selection for the same trait in varied environments tends to involve genetic and physiological differences. The question of adaptability to different social environments was studied; heavy body weight at sexual maturity was chosen as the trait for selection; groups of different sizes—pairs or groups of 20–30 mice—were the environmental variables. Sexes were kept separate between weaning and sexual maturity. A within-litter selection method was used.
Crowding depressed weight at sexual maturity but equal improvement with selection occurred in both social environments. Heritability was also equal in crowded and uncrowded groups. Environmental exchange carried out in the sixth and seventh generation suggested that mice selected in crowded environments performed slightly better in both crowded and uncrowded environments.
The large sizes and unusual degree of crowding attained by the freely growing populations in this study compared with previous studies may be related to the types of animals used, to the number of individuals in the founder nuclei, and to the physical structure of the enclosures. Extreme crowding was compatible with general physical health. The decline of fertility and fecundity, the decreased survival of newborns, and the appearance of behavioral aberrations—rather than disease or an increase in adult mortality—represented the major self-regulatory mechanisms that eventually limited population growth. The growth of individuals was not inhibited. Social withdrawal and the decline of social interaction rather than a rise of interaction characterized the populations. Such findings cast doubt about the generality of the so-called “Stress” theory of social ecology that emphasizes increased interaction and pituitary-adrenal hyperactivity as the principal mechanisms involved in self-regulation of vertebrate populations.
Other formulations of mammalian social ecology, such as those that focus on the importance of early development, of spatial requirements, of neurophysiological reactivity, and of communications, constitute additional explanations of the interplay of social and biological processes in crowded populations.
Although man’s potential reactions are more complex and variable than those of lower vertebrates and give prominence to the role of symbols and culture, his social environment is even more fundamental to his entire existence. This, if anything, increases the importance of the interplay of socioecological and biological processes for man.