Psychology

Rodents usually respond to the presence of owls by reducing overall activity, in particular foraging. In this study, a playback of recorded tawny owl, Strix aluco, calls was sufficient to induce a marked effect in the social (Gunther’s) vole, Microtus socialis. Some of the voles exposed to owl calls reduced their activity
(‘freeze’ pattern) unlike control voles exposed to a human voice. Other voles, however, dashed around the cage (‘flee’ pattern). Owl calls also increased corticosterone levels in the voles, showing that the calls induced stress. We suggest that the behavioural dichotomy to freeze or flee in voles is a result of differences in individual normal behaviour and/or in stimulus interpretation. In the common spiny mouse, Acomys cahirinus, no behavioural changes were detected after exposure to owl calls, despite
increased cortisol levels which are indicative of stress. Differences in the habitats of voles and spiny mice may explain the apparent lack of behavioural response in the latter. They are rock-dwelling rodents preferentially foraging between boulders and in rock crevices, where they are relatively protected from aerial predation, whereas voles forage in relatively open spaces.

Gerbils are a highly social species and extremely sensitive
to social manipulations. In this laboratory, separating male/
female pairs has been found to produce significant effects
on these animal’s subsequent social behaviour. The
present studies were conducted in order to examine the
effects of a short period of individual housing in females of
this species, as this may also be predicted to produce
alterations in social responding. It was found that 21 days’
individual housing induced a marked reduction in social
behaviour directed towards an untreated male placed in
the same arena. This was indicated by a highly significant
increase in immobile-in-contact, a behaviour that involves
females freezing while, and only while, they are being
socially investigated. This represents the declining of an
invitation to socially interact and so high levels of
immobile-in-contact indicate low levels of social
motivation. There was also an increase in evading, upon
another animal’s approach, and a decrease in social
investigation of other animals. The effects of 15 days of
fluoxetine were found to be highly dependent on housing
condition. In individually housed females, 10 mg/kg
increased their social investigation of other animals and
markedly reduced the duration of immobile-in-contact.
Twenty mg/kg also reduced levels of immobile-in-contact
and increased the frequency of active approaches towards
other animals. Fluoxetine therefore acts to increase social
motivation in individually housed animals. By direct
contrast, in group-housed female gerbils, fluoxetine had no
effects on social behaviour and produced clear indications
of sedation. While housing condition had no influence on
levels of corticosterone, fluoxetine produced dose-related
increases in corticosteroid levels in both group- and
individually housed animals. These findings show that:
(1) a short period of individual housing induces a
significant reduction in these animals’ motivation towards
social behaviour; (2) the effects of fluoxetine on behaviour
are greatly influenced by housing condition – prosocial
effects are seen in individually housed animals but only
sedative effects are seen in animals maintained in groups;
and (3) while housing condition has no effects on levels of
corticosterone, fluoxetine dose-dependently stimulates
corticosteroid release. It can be concluded that the effects
of fluoxetine on gerbil behaviour are independent of its
stimulatory influence on HPA axis functioning, and that the
prosocial effects of this selective serotonin reuptake
inhibitor (SSRI) can only be seen in animals with a
pre-existing social deficit.

Several authors have suggested that rather than being a disease state, depression is an evolutionary adaptation
to human social organization. Adaptations are produced in response to selection pressures and similar adaptations may easily have evolved in a range of other species. The current paper seeks to identify the social pressures that may lead to a species developing depression as an adaptation and the potential
benefits that this may confer. It also examines whether rats and mice, the species most commonly used to model depression in the laboratory, have social organizations in which selection pressures towards the development of depression as an adaptation are likely to exist. It is proposed that depression
is a useful adaptation in group-living animals, where there is competition for a social rank that gives reproductive advantage over others. The cluster of symptoms associated with depression include altered activity patterns, reduced sociability and appetite, and increased submissiveness. This combination
strongly suggests that the function of depression is to reduce the likelihood of an individual being subject to further attack once they have lost social status and so increase their chances of survival in the period immediately following this. Successful transition from high to low status may provide further opportunities to reproduce. Hence, animals that become depressed during this critical period gain a reproductive
advantage over those that do not, as these are either killed or expelled from the group. Wild mice do not have social hierarchies and are highly territorial. Wild rats do have social hierarchies however these only compete for reproductive advantage at the level of the sperm, and social status does not translate into
significantly greater access to mates. Therefore, there are no selection pressures in these species towards the development of depression and so it is most unlikely that rats and mice have this adaptation. It is concluded that current models of depression based on rats and mice are in reality models of monoamine
activity and that progression beyond this tautology requires future models to be developed using species selected on the basis of their social organizations in the wild