Before approaching explanatory questions by means of Tin-bergen's questions again, a brief attempt at categorization of social systems: in order to categorize the diversity of mammalian social systems, there are several variables that need to be described for each species. One is the degree of sociality. We find at least three types of social organization here: first are the solitary individuals that do not regularly have any social contact with conspecifics outside the narrow timespan of reproduction. Individuals of solitary species are commonly found alone in periods of both activity and inactivity. Examples are several species of shrews, small mustelids, and probably some other small carnivores. Next are the individuals of species with a dispersed social system that are also mostly found alone during their period of activity. They do, however, have a network of non-aggressive social relationships with neighbors (often closely related individuals) and may form sleeping groups in periods of inactivity. Examples are many prosimian species, several small possums, some wallabies and rat-kangaroos, but also brown bears, female northern white rhinos, female roe deer, and possibly many other species of ungulates formerly classified as solitary. Finally, gregarious or "social" species are those mostly found in groups, such as larger canids, zebras, or savanna-living bovids.
The second variable to consider is territorial defense. A territory is some area that is actively defended at least against members of the owner's age/sex class, where males at least do not tolerate other fully adult and reproductively active males. Territories thus cannot be "automatically" assumed as a
species' characteristic trait, from the fact that some individuals are solitary. Solitary species may well live in undefended, overlapping home-ranges, or even avoid each other actively without defending a territory, as can be seen in females of smaller cats as well as domestic cats in suburban areas (there are, however, also social feral cats). On the other hand, active defense of territories can also be found in truly social species, such as the European badger, the chimpanzee, larger canids, or the spotted hyena.
The third variable to describe mammalian social systems concerns the degree of overlap in the home range. This is, of course, something that can only be found in species with a dispersed or gregarious system. We can roughly distinguish four types here:
• Pairs are found, when one male and one female overlap in their range. This does not necessarily mean that they are found together, such as in gibbon pairs. So-called solitary ranging pairs such as tupaias, red fox, or some prosimians are a common type of mammalian social organization. Pair-living also is not necessarily connected with a monoga-
mous reproductive system, because extra-pair copulations are not uncommon.
Polygynous systems are those with one male and several females' ranges overlapping. This system is often called a "harem," or "uni-male group." Again, from looking purely at numbers of animals in the group,
we cannot fully describe the structure. "Harems" may be kept together solely by the male's herding behavior, such as in hamadryas baboons, or they may stay together even in the male's absence, such as in plains or mountain zebra, even though the mares are not related to each other. Or, they may consist of a matriline, a clan of closely related females, such as in patas monkeys, forest guenons, or Eurasian wild boar.
Polyandrous systems are those in which two or more males overlap with one female. This is found in some large canids, e.g. the African hunting dog, but is generally more common in birds than mammals.
Multi-male/multi-female systems where more than one adult of both sexes overlap are typical for many diurnal primates, large bovids, lions, or small diurnal mongooses. In these, but also in polyandrous (rarely in polygynous) systems we cannot automatically assume that all adult members are reproductively active. Helpers, such as in canids or dwarf mongoose, can be fully adult but reproductively suppressed individuals. The degree of reproductive cooperation and suppression is thus the last variable to consider, again mostly for gregarious (or theoretically at least, disperse) species. There are very few truly eusocial species of
mammals (the naked mole-rat and some other bathy-ergids), which means that reproductive suppression is irrevocable, leading to sterile worker castes and an overlap of several generations to be found. However, helpers can be found in many families (callitrichids, canids, marmots). These helpers normally are the young of previous years that remain within their parents' group and range, refrain from reproducing themselves, and help in rearing their parents' next offspring. The degree of helping often depends on the degree of relationship between helpers and the next litter (as demonstrated for the alpine marmot). Helping can be done by carrying them (callitrichids), feeding, guarding, and playing (canids), keeping the nest warm (marmots), or taking part in anti-predator vigilance or defense (dwarf mongoose).
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