Aggressive motivation in the rat

Pro-social behavior in rats is modulated by social experience

In mammals, helping is preferentially provided to members of one's own group. Yet, it remains unclear how social experience shapes pro-social motivation. We found that rats helped trapped strangers by releasing them from a restrainer, just as they did cagemates. However, rats did not help strangers of a different strain, unless previously housed with the trapped rat. Moreover, pair-housing with one rat of a different strain prompted rats to help strangers of that strain, evidence that rats expand pro-social motivation from one individual to phenotypically similar others. To test if genetic relatedness alone can motivate helping, rats were fostered from birth with another strain and were not exposed to their own strain. As adults, fostered rats helped strangers of the fostering strain but not rats of their own strain. Thus, strain familiarity, even to one's own strain, is required for the expression of pro-social behavior. DOI: http://dx.doi.org/10.7554/eLife.01385.001.

Pro-social behavior in rats is modulated by social experience

In mammals, helping is preferentially provided to members of one’s own group. Yet, it remains unclear how social experience shapes pro-social motivation. We found that rats helped trapped strangers by releasing them from a restrainer, just as they did cagemates. However, rats did not help strangers of a different strain, unless previously housed with the trapped rat. Moreover, pair-housing with one rat of a different strain prompted rats to help strangers of that strain, evidence that rats expand pro-social motivation from one individual to phenotypically similar others. To test if genetic relatedness alone can motivate helping, rats were fostered from birth with another strain and were not exposed to their own strain. As adults, fostered rats helped strangers of the fostering strain but not rats of their own strain. Thus, strain familiarity, even to one’s own strain, is required for the expression of pro-social behavior.

DOI:http://dx.doi.org/10.7554/eLife.01385.001

Humans help family members and friends under circumstances where they may not help strangers. However, they also help complete strangers through both direct actions, such as helping someone who has stumbled, and indirect actions, such as giving to charity. Ben-Ami Bartal et al. have now explored the biological basis of such socially selective helping by testing whether rats help strangers, and if so, under what circumstances.

In the experiments a free rat was exposed to another rat trapped inside a plastic tube with an outward-facing door for 12 one-hour sessions. When tested with a cagemate trapped inside the tube, most free rats learned within a few days to release the trapped rat by opening the door. Ben-Ami Bartal et al. then exposed the free rats to strangers they had never met or seen before. Remarkably the rats consistently released the trapped stranger, acting toward strangers just as they had acted toward familiar cagemates. This result suggested that individual familiarity is not required for helping to occur.

To test the limits of rat benevolence, Ben-Ami Bartal et al. tested free rats (always white albino rats) with trapped rats from a different outbred strain (black-hooded rats). The rats helped cagemates of a different strain but not strangers of a different strain. These results could be explained by a requirement for strain familiarity or individual familiarity. To distinguish between these possibilities, albino rats were housed for 2 weeks with a rat of a different strain, and then re-housed with another albino rat before being tested with a trapped rat belonging to a different strain. Consistent with a requirement for strain but not individual familiarity, the free rats now helped stranger rats from the different, but now familiar, strain.

To explore if there is any role for genetics or relatedness in socially selective helping, Ben-Ami Bartal et al. tested whether rats will help strangers of their own strain based on genetic relatedness alone. To do this albino pups were transferred to litters of a different strain on the day they were born, and never saw or interacted with another albino rat until testing. Remarkably, the albino rats helped strangers from the different strain that they were raised with, but they did not help strangers of their own strain because this strain was unfamiliar to them. The fact that the motivation to help other rats has its origins in social interactions rather than genetics provides the flexibility that is needed to navigate their way through social environments that often change unexpectedly.

DOI:http://dx.doi.org/10.7554/eLife.01385.002

Pro-social behavior in rats is modulated by social experience

In mammals, helping is preferentially provided to members of one's own group. Yet, it remains unclear how social experience shapes pro-social motivation. We found that rats helped trapped strangers by releasing them from a restrainer, just as they did cagemates. However, rats did not help strangers of a different strain, unless previously housed with the trapped rat. Moreover, pair-housing with one rat of a different strain prompted rats to help strangers of that strain, evidence that rats expand pro-social motivation from one individual to phenotypically similar others. To test if genetic relatedness alone can motivate helping, rats were fostered from birth with another strain and were not exposed to their own strain. As adults, fostered rats helped strangers of the fostering strain but not rats of their own strain. Thus, strain familiarity, even to one's own strain, is required for the expression of pro-social behavior. DOI: http://dx.doi.org/10.7554/eLife.01385.001.

Mouse killing, insect predation, and conspecific attack by rats with differing prior aggressive experience

Mouse-killing, cockroach predation, and conspecific attack were examined in male Long-Evans rats with a history of intraspecific aggression (n = 20), defeat (n = 20), or no aggressive experience (n = 20). Roaches were more likely to be attacked during 30 min tests, and were attacked more rapidly than mice or rats regardless of previous social experience of subjects. Rats with aggressive experience attacked conspecifics more readily than subjects with defeat or no experience. There was no effect of prior experience on mouse-killing. These results indicate that mouse-killing does not correspond closely to either predation or intraspecific attack.

Dissociation of prey killing and prey eating by naloxone in the rat

Mouse killing rats matched for killing latency and prey eating were injected (IP) with 0.5, 2.0, or 5.0 mg/kg naloxone or 0.9% saline. Naloxone did not significantly inhibit prey killing or alter prey killing latency at any dose but did reduce prey eating by 50% at the two higher doses. The dissociation of prey killing and prey eating by naloxone is consistent with other evidence that these two behaviors are separate components of predation in rats.

Relations between muricide, circadian rhythm and consummatory behavior

Three forms of behavior--muricide, eating, and drinking--have been studied at six photic periods during a 12/12 hr light/dark circadian cycle to which the subjects have been habituated. One hundred and eight rats served as subjects, 18 per photic period. The frequency of muricide was recorded for each period and subsequent food and water intakes were measured during a 1 hr test period. Results show a significantly higher frequency of muricide during the dark than during periods of light. Food intake covaried significantly with the incidence of muricide rs = 0.89, p less than 0.05), while no such relationship was found between muricide and water intake (rs = 0.17, p less than 0.05). The findings are consistent with reports of circadian changes in other rodent behaviors, including rhythmicity in home-cage and in shock-induced aggression. Covariation of muricide and eating does not establish a causal relation between the two. Three models of physiological mechanisms which might provide substrates for the covariance are discussed.

The inhibitory modulation of agonistic behavior in the rat brain: a review

Neural regions which exercise an inhibitory influence on agonistic behavior are identified by the enhancement of agonistic behavior that follows their removal. The specific kinds of agonistic behaviors altered by each region are then examined. Increased reactivity to the experimenter and enhanced shock-induced fighting are produced by lesions of the region ventral to the anterior septum, the lateral septum, the medial hypothalamus, and the dorsal and median raphe nuclei. It is argued that the increased reactivity and shock-induced fighting correspond to an enhancement of defensive behavior. Mouse killing is induced by lesions of the anterior olfactory nucleus, the region ventral to the anterior septum, the lateral septum, the medial hypothalamus, the dorsal and median raphe nuclei, and the medial amygdala. Because the lesion-induced mouse killing is similar to that emitted by spontaneous mouse killers, it is argued that these regions normally exert an inhibitory control over predatory killing. The available evidence on social attack behavior has not convincingly identified regions exerting an inhibitory control over this dimension of behavior. Our conclusion is that separate brain systems exert an inhibitory control over defensive behavior, predatory killing, and social attack behavior. To a substantial extent, the regions modulating these behaviors appear to act independently of one another. The only neurotransmitter that is clearly active in these inhibitory systems is serotonin, and has only been directly implicated in the control of mouse killing by neurons originating in the dorsal and median raphe nuclei.

Mouse-killing in the rat: effects of sensory deficits on attack behaviour and stereotyped biting

Sensory transections were performed in rats that killed mice after food-deprivation and subsequent satiation. Following partial maxillary or combined maxillary and optic deafferentation, but not after optic lesions alone, the number of mouse-kills declined sharply. This decline was accounted for by approximately a 50 per cent decrease in the number of mouse-presentations in which attack-behaviour took place and in the occurrence of attack-behaviour that did not always lead to a kill because rats frequently failed to bite. Furthermore, examination of killed mice revealed a shift in the distribution of body-bites away from the rostral regions normally seized by rats to more caudal areas of the mouse's body. Results indicate that certain stimulus-characteristics of mice mediated by the maxillary and optic nerves are important determinants of attack-behaviour and stereotyped biting in the rat.

Aggression as a reinforcer: operant behavor in the mouse-killing rat

Two experiments examined mouse killing as a reinforcer of key pressing by rats that killed mice. In Experiment I, mouse-killing rats performed the key-pressing response when each press was reinforced with presentation of a mouse. Offered a choice between a key that yielded presentation of mice and one that did not, the rats preferred the key that yielded mice. When the contingency was reversed, the rats preferred the other key and continued to kill mice. In Experiment II, mouse-killing rats that did not kill rat pups performed a key-pressing response reinforced with presentation of mice on a variable-interval schedule. In tests for responding reinforced on that schedule with presentation of normal mice, anesthetized mice, dead mice, or rat pups, these rats that killed mice but not rat pups exhibited a decline in response rate when rat pups were the reinforcer. Altering the condition of the mice did not significantly affect performance.

Lateral hypothalamic control of killing: evidence for a cholinoceptive mechanism

In rats that would not ordinarily kill mice, lateral hypothalamic injection of crystalline carbachol, a cholinomimetic, elicited killing. Norepinephrine, amphetamine, serotonin, and sodium salts were ineffective at the same site. Carbachol was ineffective when injected into the medial, dorsal, or ventral hypothalamus. As additional evidence for a cholinoceptive mechanism, neostigmine elicited killing, and, in spontaneous killers, methyl atropine blocked it. The results indicate that the lateral hypothalamus contains a cholinoceptive component of an innate system that activates killing, and anticholinergic treatment can be used as a means of suppressing killing.