main results

Marsupials are more social than commonly believed

Marsupials are generally believed to be more simple and ancestral than placental mammals, having a solitary living social system. Here we show that 1. Marsupial social organisation is very variable, 2. Most marsupials live in pairs or groups, and 3. There is little support to assume that the ancestor of all marsupials was solitary living. This has a great impact for our understanding of mammalian social evolution.

Elephant shrews often live in pairs but are not monogamous

Monogamy is a mating system where one male only mates with one female. Elephant shrews have been said to be the only monogamous mammalian order, as they live in pairs. Here we show that not all elephant shrew species live in pairs, one lives only solitary, others are variable and can also be solitary or form small groups. No single study ever investigated the mating system, though there is indication for very high extra-pair paternity. Thus, it is wrong to call elephant shrew "monogamous".
developed the concept of the single strategy from our results that the fitness consequences of alternative reproductive tactics can differ between generations, depending on the generation specific ecological conditions.
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Field studies need to report essential information on social
organization

Field studies are costly in time and funding, but their publications often miss to report important data on social organization. Here we recommend to include in every publication data on the composition of social units, sex of individuals, occupancy of sleeping sites, frequency of observations and trapping events, home range
overlap and the proportion of the individuals.

The Primate Ancestor was sociable flexible and mainly pair-living

Previously it was believed that the ancestor of all primates was solitary and more complex social organizations evolved later. Here we show that when taking most recent field studies and intra-specific variation in social organization into account, it becomes evident that the ancestor of all primates most often lived in pairs but had a flexible social organization, with about 15% of individuals living solitary.

shrews are not solitary!

Comparative studies need
to rely on sound
natural history data

There are 445 species of shrews and their relatives (order Eulipotyphla, previously known as insectivores), and it is generally believed that nearly all of them (98%) are solitary living. In this review we find that real field data only exist from 16 species, of which 9 were not strictly solitary. Thus, most shrews are at least sometimes social, often even pair-living, and our early shrew like ancestor from the time of dinosaurs might also have been more sociable than generally believed.

The quality of comparative studies not only depends on R programming abilities, but also on the quality of the data used. To obtain good comparative databases for theory-driven research, we need high quality field studies, including studies describing the social system of species. We should increase our efforts to describe the biodiversity of social systems if we believe social evolution is an important topic of research. In summary, to study social evolution, comparative studies must be based on reliable information and this requires many more species to be studied carefully in their natural environment.

IVSO in Prosimians

Our study supports the view that strepsirrhines are generally social (58.1% of species with another 34.9% of species sometimes living in pairs or groups) and not solitary. In sum, most strepsirrhines are sociable and show a complex and often variable social organization.

Social evolution in Artiodactyla

We found that variable social organisation occurs in 62% of artiodactyl populations and in 83% of species. Our analysis revealed that the ancestral artiodactyl population was pair-living and that changes to variable social organisation were associated with larger body size typical of present species

Comparative studies allow us to estimate traits of ancestor species. Studying social evolution, we focus on social organisation, which is the composition of social units. This can be either solitary living, pair living, or living in different forms of groups.
Social systems consist of four components:

1. Social organisation: Composition of social units (solitary, pair, 4 different kinds of groups)
2. Social structure: Interactions between conspecifics (dominance, social bonding etc)
3. Mating system: Who reproduces with whom? (Monogamy, polygyny, polyandry, polygynandry, promiscuity)
4. Care system: Who takes care of offspring? (No parental care, maternal care, paternal care, biparental care, allo-parental care).

There are more than 1000 different possible combinations of these four factors, which means more than 1000 social systems. This is why we focus on social organisation, the component of which most field data are available.
Comparative studies allow us to estimate ancestral traits and which ecological and life history factors are associated to the evolution of this trait. In our case, the trait we study is social organisation, which is the result of individual traits of individuals of a population choosing with which conspecifics to associate (or not). For the first time, we take observed intra-specific variation in social organisation into account instead of categorising each species with its mots common form of social organisation. Further, we only consider real field data and don’t accept any assumptions of social organisation in our database. Our studies indicate for mammals that the ancestor was probably not solitary living as long assumed.