Intra-family aggression and offspring expulsion in Mongolian gerbils (Meriones unguiculatus) under restricted environments

Mongolian gerbils live in families consisting of a founder pair, to which reproduction is mainly restricted, and the offspring. They are described as cooperative breeder in which males and offspring act as helpers. Family dynamics have not been systematically investigated, particularly concerning the Long-term consequences of periods of aggression. In a conceptual framework, promoting factors were investigated for the outbreak of aggression and its consequences on the families and on the individual level. Moreover, previously defined and described integrated (IFMs) and expelled family members (EFMs) were further characterized by frequent measurements of body mass and body composition. Six families were monitored for at Least 1.5 years under controlled laboratory conditions. Regularly, the family composition and the individual state of each family member were inspected. In case of agonistic interactions, aggression periods were characterized by onset, number, duration, number of expelled animals and family size. First aggression period has been occurred 247.8 +/- 37.7 days post-founding. As a consequence, family size was reduced significantly from 18.7 +/- 1.8 to 17.5 +/- 1.8 animals; the number of females decreased too from 10.6 +/- 1.8 to 9.7 +/- 1.8 females per family. ALL Mongolian gerbils experienced 2.4 +/- 0.2 aggression periods per Life. ALL EFMs had a reduced body mass increase during aggression periods compared with integrated ones. Expelled males had a Lower body mass than their integrated siblings; there was no such difference in females. In each aggression period, 2.6 +/- 0.2 adult animals per family were expelled. There was no sex-specific expulsion rate. Mainly founder females acted as aggressors (in 60% of all aggression periods). Up to three animals operated together aggressively, but mainly only one animal attacked the other family members (in 78% of all aggression periods). To conclude, animals of both sexes were excluded due to changes in family structure and an increased family size. Furthermore, females were expelled due to competition for exclusive reproduction. Males with tower body mass were more prone to be expelled, whereas in females no morphometrical characteristics favour the expulsion. (c) 2005 Elsevier GmbH. All rights reserved.