Figures and data in The social life of Norway rats (Rattus norvegicus)

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Figures

Figure 1

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Wild and domesticated Norway rats.

Wild rats (panel [a] depicts two female wild-derived rats) differ from domesticated rats (panel [b] shows two female domesticated rats) greatly in respect to their coat colour but less so in their social life, which is illustrated by domesticated rats showing the full behavioural repertoire of wild rats. Therefore, domesticated rats can be good representatives of wild rats and *vice versa*.

Figure 2

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The social organisation of a rat colony.

Rats are social animals that live in large colonies, consisting of sometimes more than 150 female and male individuals with overlapping generations. Rats of a colony establish an underground burrow system with shared channels and chambers. In these chambers, they commonly huddle together to keep warm and often sleep like this (right chamber at the bottom and left at the top). Females establish their nest in such chambers, where they give birth to up to eight pups (middle chamber at the bottom). Colony members reproduce. Males approach females that respond defensively with sidekicks, if they are not in oestrus (left chamber at the bottom). If she is receptive, several males will copulate with her (two rats in the middle). Rats establish a dominance hierarchy, which is more pronounced in males than females. When rats meet, they inspect each other, whereby subordinate individuals show a submissive posture, crawl under the other or avoid such contacts to prevent conflict. Most conflicts, however occur between rats of different colonies. Fights typically start with a threat posture, followed by fights that are interrupted by standing upright and boxing (two rats outside the burrow system). Most commonly, however, rats show amicable behaviour with colony members. For example, they spend time in close proximity to each other (left side, middle rats) or groom each other (right chamber at the top). Drawings by Michelle Gygax.

Tables

Table 1

Ethogram of individual social behaviours in rats.

Rats show a range of social behaviour, that is behaviours that are directly related to conspecifics, which can be split into socio-positive and socio-negative contexts. The ethogram is restricted to wild rats under natural or semi-natural conditions.

Category	Behaviour	Sex	Description	Reference
Socio-positive	Allogrooming	Females and males	One individual gently nibbles or licks the fur of a conspecific, sometimes with the aid of its forepaws. All body parts of the partner may be cleaned including the tail.	Barnett, 1963, p. 77
	Huddling	Females and males	Rats lie together with direct body contact, sometimes sleeping.	Barnett and Spencer, 1951; Barnett, 1963
	Inspecting anogenital region	Females and males	One individual sniffs or licks the anogenital region of a conspecific, probably used in the context of recognition.	Barnett, 1963, p. 64
	Nosing	Females and males	One individual gently pushes another’s flank or neck with its nose.	Barnett, 1963, p. 77
	Nose-touching	Females and males	Two individuals approach each other until their noses come into contact. This possibly serves recognition and may result in socio-positive or negative behaviours.	Calhoun, 1979, p. 179
	Oral inspection	Females and males	One individual sniffs at a conspecific’s mouth. This is most common between mothers and their offspring, but takes place between adults, too.	Calhoun, 1979, p. 149
	Pioneering	Females and males	One individual leaves the burrow vigilantly and observes the surroundings for several minutes. Only then will other colony members appear from the burrow.	Barnett and Spencer, 1951; Telle, 1966
	Play fighting	Females and males	One individual attacks the nape of its opponent, which the latter tries to defend. Play fights take place only during adolescence.	Ewer, 1971; Calhoun, 1979, p. 180
	Recognition sniffing	Females and males	One individual shows enhanced sniffing at colony members and (potentially marked) objects, especially if a stranger entered its territory.	Barnett, 1967
	Scent marking	Females and males	One individual rubs the flanks or presses the anogenital region on a surface, sometimes leaving urine droplets on the surface.	Landete-Castillejos, 1997
	Sharing food	Females and males	An individual tolerates a conspecific in its close proximity, sometimes even touching each other, while feeding from the same food resource. Alternatively, one individual drops small food items that can be taken by another. Further, residues in the face or on the paws of an individual can be licked off by another.	Barnett and Spencer, 1951; Barnett, 1963, p. 36; Calhoun, 1979, p. 101
	Submissive posture	Females and males	One individual lies on its side with eyes half-closed. This posture is used to ‘greet’ more dominant individuals to prevent fights. Sometimes this posture is combined with ‘crawling under’ (see below).	Barnett, 1967
Socio-negative	Aggressive grooming	Mostly males	One individual pins down a conspecific forcefully while allogrooming it. This is often accompanied by squeaks and run-away attempts of the groomed partner.	Barnett, 2001, p. 131
	Avoiding	Females and males	One individual changes its route upon detecting another rat.	Calhoun, 1979, p. 179
	Boxing	Mostly males	Bouts of fights are typically intermitted by standing upright to box. While boxing, they hit and scratch each other’s face, which is accompanied with raised hair and ears pointing forward.	Barnett, 1963
	Chasing	Females and males	One individual runs after a second. This usually precedes fights but can also take place afterwards.	Calhoun, 1979, p. 181
	Crawling under/walking over	Mostly males	One rat crawls under, that is typically the subordinate, or walks over a conspecific, that is typically the more dominant.	Barnett, 1963
	Direct approach	Mostly males	An individual approaches an opponent to attack, often accompanied with urination and defecation and raised hair. Sometimes the individual shows tooth chattering while approaching.	Barnett, 1963
	Fighting	Mostly males	Two rats tumble, roll over the ground while holding, kicking and punching each other.	Barnett, 1963; Calhoun, 1979
	Leaping and biting	Mostly males	The attacker jumps towards the opponent with extended forelimbs and tries to bite usually its ears, limb or tail. Bites are typically very quick.	Barnett, 1963
	Passage guarding	Probably only males	One more dominant individual stands in a passage and therefore blocks the way of a second. The opponent typically waits until the first moves away or takes a detour.	Calhoun, 1979, p. 187
	Pushing away	Females and males	One individual pushes another with its forepaws or flank to move a conspecific from its current position. Sometimes pushes are accompanied with kicks or swinging the body towards the opponent.	Calhoun, 1979, p. 101
	Tail quivering	Females and males	One individual quivers its tail, which might be shown in various situations like during ‘crawling under’ or shortly before copulation.	Steininger, 1950
	Threat posture	Mostly males	An attacker adopts a posture where the back is maximally arched, all limbs are extended, and the flank is turned to its opponent.	Barnett, 1963
	Tooth-chattering	Females and males	One individual chatters with its teeth while staying immobile, most common after detecting an opponent.	Barnett, 1963

Table 2

Overview of studies showing cooperative behaviour in rats.

Rats are highly social animals that have been shown multiple times to cooperate, i.e. one individual benefits one or more other individuals (Sachs et al., 2004). Several mechanisms have been proposed to explain why they cooperate. Domesticated, wild and wild-derived rats of both sex were tested in a variety of tasks, involving various behaviours to measure their tendency to cooperate.

Proposed mechanism	Cooperative behaviour	Sex of test subjects	Origins of test subjects	Task	References
Assessing the other’s need in a helping task	Simultaneous nose-poking	Males	Domesticated (Sprague-Dawley)	Skinner box	Łopuch and Popik, 2011
	Entering one compartment, which leads to food rewards	Males	Domesticated (Sprague-Dawley)	T maze	Márquez et al., 2015
	Donating food by pulling it into the reach of a partner	Females	Wild-derived	Bar pulling task	Schneeberger et al., 2012; Schweinfurth and Taborsky, 2018b
Coordination (acting together)	Coordinating back and forth shuttling	Females and males	Domesticated (Sprague-Dawley)	T-maze	Daniel, 1942
			Domesticated (Sprague-Dawley)		Swanson and Schuster, 1987
			Domesticated (S3, Sprague-Dawley and Wistar)		Schuster et al., 1993
			Domesticated (Sprague-Dawley and S3)		Schuster et al., 1988
		Males	Domesticated (Long Evans)		Tan and Hackenberg, 2016
Division of labour (sharing of tasks)	Tolerating thefts	Females and males	Domesticated (Wistar)	Diving for food	Colin and Desor, 1986; Krafft et al., 1994; Grasmuck and Desor, 2002
Division of labour (sharing of tasks)	Donating food by pushing down a lever	Males	Domesticated (Sprague-Dawley)	Skinner box	Littman et al., 1954
Empathy (ability to perceive and care for the emotional states of others) Or: social contact seeking	Freeing trapped partners by opening a door	Females and males	Domesticated (Wistar)	Partner trapped in container	Rice and Gainer, 1962
		Females and males	Domesticated (Sprague-Dawley)		Ben-Ami Bartal et al., 2011
		Males			Ben-Ami Bartal et al., 2016
		Males			Silberberg et al., 2014
		Females and males		Partner trapped in a pool	Sato et al., 2015; Schwartz et al., 2017
		Females	Domesticated (Sprague-Dawley and Long-Evans)	Partner trapped in container	Ben-Ami Bartal et al., 2014
Inequity aversion (preference of equal outcomes)	Entering one compartment, which leads to food rewards	Males	Domesticated (Long- Evans)	T-maze	Oberliessen et al., 2016
Prosociality (preference to provide benefits to others)	Entering one compartment, which leads to food rewards	Males	Domesticated (Long- Evans)	T-maze	Hernandez-Lallement et al., 2014, Hernandez-Lallement et al., 2016
Prosociality (preference to provide benefits to others)	Entering one compartment, which leads to food rewards	Males	Domesticated (Sprague-Dawley)	T-maze	Márquez et al., 2015
Reciprocity (conditional help based on previous received help)	Allogrooming	Females	Wild-derived	Direct interactions	Schweinfurth et al., 2017b
	Allogrooming	Females	Domesticated (Sprague-Dawley)	Direct interactions	Yee et al., 2008
	Donating food by pulling it into the reach of a partner	Males	Wild-derived	Bar pulling task	Schweinfurth et al., 2019: Schweinfurth and Taborsky, 2018a
	Donating food by pushing down a lever		Domesticated (Long-Evans)	Skinner box	Li and Wood, 2017
	Entering one compartment, which leads to rewards		Domesticated (Sprague-Dawley)	T maze	Simones, 2007; Viana et al., 2010
	Donating food by pushing down a lever	Females and males	Domesticated (Long-Evans)	Skinner box	Li and Wood, 2017
	Donating food by pulling it into the reach of a partner	Females	Wild-derived	Bar pulling task	Rutte and Taborsky, 2007; Rutte and Taborsky, 2008; Dolivo and Taborsky, 2015a; Schweinfurth and Taborsky, 2016
Reciprocity between different commodities	Donating food by pulling und pushing it into the reach of a partner	Females	Wild-derived	Bar pulling and lever pressing task	Schwartz et al., 2017
Reciprocity between different commodities	Allogrooming and donating food by pulling it into the reach of a partner	Females	Wild-derived	Direct interaction and bar pulling task	Stieger et al., 2017; Schweinfurth and Taborsky, 2018c
Warning	Alarm calling	Females and males	Domesticated (Long- Evans and Wistar)	Cat exposure	Blanchard and Blanchard, 1989; Blanchard et al., 1991
		Females and males	Domesticated (Long- Evans and Wistar)	Playback	Sales, 1991
		Males	Domesticated (Wistar)	Playback	Brudzynski and Chiu, 1995