The spatial-and-ethological population structure, cooperation, and the evolution of sociality in rodents

Abstract

There are two types of complex social units — aggregations and family groups — in rodent populations, with an essential difference in their social organization. The impact of ecological factors on the evolution of sociality in rodents (the transition from solitary towards family-group lifestyle) is still unclear. The inter-specific comparative analysis based on quantification of social traits related to the spatial-and-ethological population structure and cooperation allows author to propose a new conceptual approach to the assessment of differences between the species under consideration in terms of the evolution of sociality. A new conceptual model of the evolution of sociality in rodents should incorporate ecological conditions and social factors, including cooperation, operating as a complex of selective forces promoting formation of family groups.

The evolution of sociality in rodents: a family affair

“…Each family group usually occupies a defended territory that provides almost total food security for the pair of adults and their offspring in monogamous families, or a complex and also protected home range encompassing breeding territories of two or more reproducing females in extended family groups. Long-term pair bonds, biparental care, delayed offspring dispersal, cooperation in different activities, a complicated social organization related to age-based hierarchy, dif-ferentiation of behavioral roles, and suppression of reproduction in offspring are characteristic of many rodent species with family-group lifestyle, for example, beavers (Castor canadensis Kuhl, 1820, C. fiber Linnaeus, 1758 (Milne-Edwards, 1871), and some other highly social rodents (Marinelli & Messier, 1995;Lacey, 2004;Armitage, 2007;Busher, 2007;Gromov, 2008Gromov, , 2011a, as well as the naked mole-rat, H. glaber, and the Damaraland mole-rat, Cryptomys damarensis (Ogilby, 1838), regarded as the most social (eusocial) species (Jarvis, 1981;Bennett & Faulkes, 2000).…”

“…However, in female-biased populations of the yellow-bellied marmot, Marmota flaviventris Audubon et Bachman, 1841 (Armitage, 1962), prairie dogs, Cynomys spp. (Travis & Slobodchikoff 1993), beavers (Busher, 2007), musk-rats (Marinelli & Messier, 1995), tuco-tucos (Lacey et al, 1997), prairie voles, Microtus ochrogaster Wagner, 1842 (Roberts et al, 1998a, b), Brandt's voles (Zhang & Zhong, 1981), Mongolian gerbils (Ågren et al, 1989a;Gromov, 2008Gromov, , 2011a, great gerbils (Rogovin et al, 2003), and some other species, family groups include two or more reproducing females (polygynous families). Besides, in the case of the naked mole-rat, there are polyandrous families with two or three males mating with one female (Jarvis et al, 1994).…”

“…Besides, the effects related to distribution of food resources are revealed in the species with a familygroup lifestyle only (M. ochrogaster, C. gunnisoni), and no basic change of their social organization was found in either case. As for solitary foragers like the tamarisk gerbil, M. tamariscinus or the fat sand rat, Psammomys obesus Cretzschmar, 1828, uneven (patchy) distribution of food resources was found not to promote formation of polygynous social units (Daly & Daly, 1974;Gromov, 2001Gromov, , 2008Gromov, , 2011b. Thus, the RDH does not really explain the effect of habitat conditions on the evolution of different social systems in rodent populations.…”

“…As for rodents, their social organizations range from species whose young individuals disperse after weaning and live as more or less solitary dwellers, to species whose individuals exhibit delayed dispersal and in which overlapping generations share a home range and interact amicably. Besides, rodent species can be placed objectively along a continuum of social complexity , 1998, and such a criterion of sociality as group size is not quite appropriate for rodents (Burda et al, 2000;Gromov, 2008Gromov, , 2011a because the most complicated social structure is characteristic of those rodent species that form rather small family groups, with the only exception -the naked mole-rat, Heterocephalus glaber Rüppell, 1842 (Jarvis et al, 1994). In other words, a core trait defining social systems and social complexity in rodents is not group size, but group structure or group composition (Gromov, 2008(Gromov, , 2011a.…”

“…A total overlap of individual home ranges of several unrelated adults seems to be impossible for rodents, with the exception of formation of winter aggregations in some gregarious species, and further grouping that leads to a higher social complexity actually means formation of breeding pairs and, subsequently, due to reproduction and delayed offspring dispersal, restricted and extended family groups (Gromov, 2008(Gromov, , 2011a.…”