Abstract
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 lower body mass were more prone to be expelled, whereas in females no morphometrical characteristics favour the expulsion.
Zusammenfassung
Mongolische Wüstenrennmäuse (M. unguiculatus) leben in territorialen Familien, die aus einem Stammpaar und deren Nachkommen bestehen. Da das Stammweibchen und adulte Männchen die reproduzierenden Rollen einnehmen und alle anderen Nachkommen lediglich als Helfer agieren, wird diese Art als kooperativ beschrieben. Im Rahmen einer Langzeitstudie untersuchen wir Auslöser aggressiven Verhaltens und die Konsequenzen auf familiärer und individueller Ebene. Darüber hinaus wurden durch wiederholte Messungen der Körpermasse und ihrer Zusammensetzung die beschriebenen Kategorien der Familienmitglieder charakterisiert. Es wurden sechs Familien über einen Zeitraum von mindestens 1,5 Jahren beobachtet und regelmäßig ihre Zusammensetzung kontrolliert. Trat agonistisches Verhalten wiederholt auf, so wurde eine solche Aggressionsphase durch den Zeitpunkt des Auftretens, Häufigkeit, Dauer, Anzahl der verstoßenen Tiere und die Familiengröße charakterisiert. Aggression trat erstmals 247:8 ± 37:7 Tage nach der Verpaarung auf, in deren Verlauf im Mittel 2:6 ± 0:2 Tiere verstoßen wurden, wodurch die Familiengröße von anfänglich 18:7 ± 1:8 auf 17:5 ± 1:8 Tiere pro Familie reduziert wurde. Das Geschlechterverhältnis war sowohl in den Familien während der Aggressionsphasen als auch unter den Verstoßenen ausgeglichen. Alle Tiere erlebten im Mittel 2:4 ± 0:2 Aggressionsphasen. Aggressoren traten hauptsächlich (77%) allein auf und waren in 60% der Fälle Stammweibchen. Verstoßene Männchen zeichneten sich durch eine geringere Körpermasse gegenüber den integrierten Männchen aus, wohingegen bei den Weibchen keine Unterschiede gefunden wurden. Agonistisches Verhalten wirkte sich auf die Verstoßenen beider Geschlechter durch eine verringerte Körpermassezunahme im Verlauf von Aggressionsphasen aus. Zusammenfassend konnten die Konkurrenz um das weibliche Reproduktionsmonopol, die Veränderungen in der Familienzusammensetzung sowie die Familiengröße als Ursachen für das Auftreten von Aggression ermittelt werden. Weibchen wurden aufgrund ihrer Konstitution und ihrer reproduktiven Potenz von dem Stammweibchen verstoßen, während im Gegensatz dazu besonders leichte Männchen prädestiniert waren, ausgegrenzt zu werden.
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Scheibler, E., Weinandy, R. & Gattermann, R. Intra-family aggression and offspring expulsion in Mongolian gerbils (Meriones unguiculatus) under restricted environments. Mamm Biol 70, 137–146 (2005). https://doi.org/10.1016/j.mambio.2004.12.002
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DOI: https://doi.org/10.1016/j.mambio.2004.12.002