A light intensity threshold for schooling in the Atlantic mackerel, Scomber scombrus

The schooling behaviour of Atlantic mackerel was studied in a large tank at different light intensities in the range 12.6–1.8 × 10⁻¹⁰μEs⁻¹ m⁻². Variable light intensity was produced by accurately controlling the current to a green light-emitting diode (LED) 3 m above the experimental tank. Under high light levels (1.8 × 10⁻⁶μEs⁻¹ m⁻²) mackerel always formed a single school, whereas at lower levels (1.8 × 10⁻⁸μEs⁻¹ m⁻²) they swam as individuals. At light levels down to 1.0 × 10⁻⁶μEs⁻¹ m⁻² the mean nearest neighbour distance in a school remained relatively constant (0.3–0.9 body lengths), and individual mackerel swam along a path which deviated from the position of their nearest neighbours by less than 14°. As light dropped below 1.8 × 10⁻⁷μEs⁻¹ m⁻², both nearest neighbour distance and heading angle between nearest neighbours increased, with mean values of 1–1.8 body lengths and 23–92°, respectively, at 1.8 × 10⁻⁹μEs⁻¹ m⁻². The results are discussed in terms of ambient light conditions in the sea.     

Light intensity and schooling behaviour in larval gulf menhaden

Schooling behaviour was examined in larval gulf menhaden Brevoortia patronus under decreasing and increasing light intensities and related to movements of retinal photoreceptors and pigment. Nearest neighbour angles and nearest neighbour distances between fish were inversely related to light intensity and were independent of the direction of intensity change. Swimming speeds of individual fish were directly related to light intensity when intensity was decreased but did not increase when intensity was raised. The changes in nearest neighbour angles and distances more closely paralleled movements of retinal pigment cells than cone cell migration, suggesting that rod photoreceptors play a greater role than cones in determining the threshold light intensity for schooling. A general model is developed relating the threshold light intensity for schooling to eye diameter for a number of different species and the implications of this relationship are discussed.     

Presettlement schooling behaviour of a priacanthid,the Purplespotted Bigeye Priacanthus tayenus (Priacanthidae: Teleostei)

We report in situ behavioural observations of presettlement schooling in Priacanthus tayenus off Coral Bay, Western Australia collected using pelagic Baited Remote Underwater stereo-Video systems. Two groups of fish (8 and 9 individuals) were observed that aggregated into a single school. Mean total length was 24.1 mm (12.5–30.2 mm). The fish swam at a mean speed of 8.5 cm s^?1 in a group spacing themselves more or less evenly at a distance of around one body length from the nearest neighbour within the school. P. tayenus appeared to be sometimes associated with juveniles of other species. The results presented here add to the limited, but growing body of literature on the schooling behaviour of the early pelagic stages of demersal fishes.     

No evidence of kin bias in dispersion of young‐of‐the‐year Atlantic salmon Salmo salar L. in a natural stream

Ninety‐one young‐of‐the‐year Atlantic salmon Salmo salar were captured using a non‐invasive snorkelling technique in a 38 m section of Catamaran Brook, New Brunswick, Canada, to test whether related fish settle closer to one another than unrelated fish. A maximum likelihood estimate of parentage relationships assessed by genotyping eight microsatellite loci revealed five half‐sibling families in the sample of fish. Related juvenile S. salar were not found closer to one another than unrelated fish in three analyses at two spatial scales: a comparison of the relatedness of focal fish to their nearest neighbour and to their four nearest neighbours, and a correlation of the pair‐wise relatedness and distance matrices for all fish in the sample. The lack of a kin‐biased dispersion pattern may be related to the lower density of fish or the scarcity of full‐siblings at the study site compared to laboratory conditions.     

Gene dispersal and outbreeding in a philopatric mammal

Extensive mark-recapture data from banner-tailed kangaroo rats, Dipodomys spectabilis, have shown that both males and females are highly philopatric and suggest the possibility of close inbreeding. However, indirect analyses based on genetic structure appear to contradict direct observations, suggesting longer dispersal distances. Using microsatellite genotypes from most members of a banner-tailed kangaroo rat population during five successive breeding seasons, we ask how relatedness is influenced by dispersal and how it in turn influences mating patterns. The data confirm that, because of philopatry, neighbours are often close relatives. However, patterns of parentage also show that the average distance between mates is large relative to natal dispersal distances and larger than the average distance between nearest opposite-sexed neighbours. Females' mates were often not their nearest male neighbour and many were less related than the nearest male neighbour. We detected multiple paternity in some females' litters; both sexes produce offspring with multiple mates within and between breeding seasons. At the population level, heterozygosities were high and estimates of F were low, indicating that levels of inbreeding were low. Using individual inbreeding coefficients of all juveniles to estimate their parents' relatedness, we found that parental relatedness was significantly lower than relatedness between nearest opposite-sexed adult neighbours. Thus in philopatric populations, long breeding forays can cause genes to move further than individuals disperse, and polyandry may serve to reduce relatedness between mates.     

Schooling behaviour and retinomotor response of juvenile Pacific bluefin tuna Thunnus orientalis under different light intensities

Schooling behaviour and histological retinal light adaptation in juvenile Pacific bluefin tuna Thunnus orientalis were examined under various light intensities to determine the effect of light intensity on behaviour. After monitoring the schooling behaviour of juveniles 35–36 and 45–46 days post hatching, schooling variables such as nearest neighbour distance and separation swimming index were measured under different light intensities. Furthermore, retinal indices of light adaptation were investigated histologically for each experimental light intensity. Under intensities >5 lx, schooling variables in the two juvenile growth stages were nearly constant, allowing schooling. In contrast, the schooling variables indicated that the fish gradually swam more widely and randomly with decreasing light intensities <5 lx. The retinal indices also showed a shift from light adaptation to dark adaptation at light levels <5 lx. From 5 to 0·01 lx, retinal adaptation and fish schooling behaviour changed with light intensity. These data suggest that the schooling behaviour of juvenile Pacific bluefin tuna is greatly affected by retinal adaptation.     

Emergence of Oblong School Shape: Models and Empirical Data of Fish

The main benefit of the oblong shape of schools of fish is supposed to be the protection against predation. Models of self‐organised travelling groups have shown that this shape may arise as a side effect of the avoidance of collisions with group members. These models were developed for schools of fish in open water, whereas the oblong shape of schools of real fish has mostly been observed in schools in tanks. Therefore, it is not known how school shape in a tank originates neither in models nor in real fish. To find out what causes this shape, we use the combination of a theoretical and an empirical study. We test the predictions produced by our earlier models regarding the effect of school size on the school shape both in a model of self‐organised schooling in a tank and empirically. Empirically, we study the 3D positions of all individuals in the schools of 10–60 real mullets (Chelon labrosus). We calculate for each individual its distance to its nearest neighbour and its velocity and we measure per school its length and width. The relation between school shape and size in the model and in the real mullets supports our prediction and thus supports the hypothesis that school shape may be emergent from the avoidance of collisions during coordinated travelling.     

The development of shoaling behaviour in the European minnow, Phoxinus phoxinus

The effects of development and disturbance on schooling and shoaling were observed in laboratory-raised minnows. Both behaviours were present at the onset of free swimming. Shoaling increased to become the most important behaviour by 3 weeks after emergence from gravel, while the main increase in schooling behaviour occurred at 4 weeks. These changes were accompanied by rises in elective group size and in the number of following responses, and by a decrease in nearest neighbour distance. The increases in social behaviour were observed at an earlier stage in the same minnows threatened by a standard net chase. The main increase in schooling in disturbed minnows took place between 3 and 4 weeks after emergence; after this time schooling became the dominant behaviour. Disturbed fish were also more likely to follow another minnow during an encounter.     

The ontogeny of schooling behaviour in the striped jack

The ontogeny of schooling behaviour was investigated in laboratory-reared striped jack Pseudocaranx dentex larvae and juveniles. Mean separation angle between nearest individuals averaged 78 and 82° in 10- and 12-mm fish, respectively, and decreased to 57° in 16-mm fish. In addition, interindividual distance decreased from three times body length in 12-mm fish to twice the body length in 16-mm individuals; this value continued to decrease gradually to 1·3 at 19 mm to 0·79 at 30 mm. Mutual attraction due to visual recognition was observed in 12-mm fish and increased with total length. The optokinetic response ( R_o , the ability of a fish to keep station with moving patterns) first appeared in 4–6-mm larvae. In spite of the traditional emphasis of the importance of R_o in schooling behaviour, the onset of R_o was insufficient to result in school formation. The emergence of mutual attraction seems essential for the ontogeny of schooling.     

Effects of group size on school structure and behaviour in yellow‐eyed mullet Aldrichetta forsteri

Schooling behaviour in yellow‐eyed mullet Aldrichetta forsteri, a common fish species in New Zealand estuarine habitats, was investigated to identify interaction rules associated with group formation. Tank‐based three‐dimensional studies of three group sizes (15, 75 and 150 individuals) were carried out to measure the effects of these different group sizes on school structure during control, predation risk and foraging behavioural states. Increased group size positively correlated with nearest‐neighbour distance in control and foraging states. Swimming speed was the lowest in all three behavioural states in groups of 15 fish compared with 75 or 150. Immediate behavioural response following visual exposure to a simulated avian predator differed between groups resulting in loss of structure in larger groups. School shape was an oblong–oblate spheroid with a length, breadth and height ratio of 5:2:1 and the area of free space surrounding individual fish was spherical in shape with a high degree of spatial isotropy present in all size groups. These findings challenge traditional theories based on either local or global properties as key drivers of group structure. Instead, our results suggest that a more collaborative approach involving both group size and rules pertaining to nearest‐neighbour interactions affects collective behaviours in this species.     

Some geometric aspects of house sparrow flocks

We analysed 50 movie films of house sparrows (Passer domesticus) foraging on an experimental grid. The location and orientation of each bird was recorded; sex of the individual was determined in about two of every three cases. Results revealed the following: (1) Flock size exhibited a weak inverse relationship to ambient temperature, though aggression was rare in the experimental patch. (2) The frequency of males in foraging groups exceeded the frequency of males in the local population. (3) Within a given flock size, nearest neighbour distances did not differ significantly between male-male and male-female pairs. However, average nearest neighbour distance was inversely related to flock size. (4) Solitaries oriented away from safety and toward a source of disturbance. Orientation of an individual within a larger group was more variable than that of a solitary, and the orientation of nearest neighbours indicated a significant tendency to keep each other in view.     

Using biological metrics to score and evaluate sites: a nearest-neighbour reference condition approach

1. Reference (i.e. least or minimally impaired) sites can provide important information about the expected range of biological metrics and can be used to establish impairment or non‐impairment of a test site. A problem with using reference data is that biological metrics are affected by natural conditions. We present an approach that uses local information to adjust for natural conditions and a method for statistically evaluating condition at a test site using biological metrics. 2. Our method consists of four steps: selection of a distance measure to find neighbours of a test site, selecting natural variables to measure the distance, selection of the number of neighbours and calculating a scored metric. 3. We use a simulated example to illustrate when the nearest‐neighbour approach improves classification of sites as reference or not reference. 4. Using a set of data from the Mid‐Atlantic Highlands, we show that the nearest‐neighbour method improved on the ability of a regression approach to correctly classify test sites known to be from a non‐reference group without affecting the ability to correctly classify test sites known to be from the reference group.     

Influences of illumination intensity on the nearest neighbour distance in coho salmon Oncorhynchus kisutch

Effects of illumination intensity on the nearest neighbour distance (NND) of coho salmon in an experimental aquarium were studied under illumination intensities ranging from 0 to 4000 Ix. The NNDs in sighted (intact) fish were divided into three groups according to the illumination intensities; the largest [1.15 times fork length (f.l.)] at 0 1x, the medium (0.78 to 0.84 f.l.) at 0.01 to 0.4 1x, and the least (0.60 to 0.66 f.l.) at 4–4000 1x. The fact that NNDs under 4 1x or higher illumination conditions were significantly smaller than those under 0.01 to 0.4 1x conditions suggests a change from photopic to scotopic vision. Blinded fish exhibited constant NNDs irrespective of illumination intensity and the value was similar to that of sighted fish under the 0 1x condition. The NNDs of blinded fish and sighted fish exposed to 0 1x were larger than the simulated NND calculated by hypothesizing that fish distribute randomly in the experimental aquarium. These results indicate that NND in a coho school is maintained primarily by vision, however, the NND is not determined by vision alone but by integrated senses.     

Influence of social and environmental factors on nesting behaviour in captive gorillas (Gorilla gorilla gorilla)

Field studies on gorilla sleeping sites usually emphasize environmental influences on nest site selection, as individual nesting behaviours are usually not observed directly. In captivity, nesting behaviour and influencing factors are often overlooked. In the present study, the sleeping sites of members of a captive group of 16-21 gorillas were analysed, based on 152 nights over a 5-year period. Subadults tended to sleep apart from adults but clustered around the oldest blackback male. Full siblings and mothers with offspring were often nearest neighbours. Some sleeping dyads notably endured. Neither age of offspring nor rank of adult females influenced neighbour choice or distances; an interadult distance of 1.5-3 m was typical. Group dynamics were reflected in nesting arrangements. We suggest that possibilities for co-nesting and choice of sleeping sites should be provided to improve the welfare of captive gorillas and other great apes.     

Electrocommunication and Social Behaviour in Marcusenius senegalensis (Mormyridae,Teleostei)

The electric organ discharges (EODs) of Marcusenius senegalensis, a West African freshwater fish, are bipolar pulses of short duration (220 ± SE 13 μs). In males (n = 10; 10.1–13.1 cm standard length — which is around the size of getting mature), the duration of EOD pulses was of significantly greater variance than in females (n = 9; 9.8–12.8 cm standard length). Male EODs also showed a tendency for a longer duration than female EODs. Groups of three as well as of 14 M. senegalensis formed temporary schools in a ‘naturally’ equipped 720-1 tank. While swimming slowly in a loose school during their nocturnal active phase, fish discharged in irregular long-short-long inter-EOD interval patterns. Near neighbours displayed a tendency to discharge in intervals of similar duration (nearest neighbour distance < 1/2 fish length). On removal of a plastic partition that had separated a pair of fish for at least 3 days, mutual threat displays followed by fighting were observed. During threatening, the fish alternated regularly between bursts of a high discharge rate and short discharge breaks; the rate of change was 4/s. The subdominant animal in a group of two was attacked frequently and often ceased discharging when the dominant fish approached. Courtship behaviour involving gonadally mature fish was accompanied by high-discharge-rate displays with intervals of constant duration in both fish, and the reciprocal display of ‘preferred’ EOD latencies in the 12 ms range. The results demonstrate electric communication by distinct inter-discharge interval patterns in the social behaviour of this mormyrid fish.     

The number of farm mates influences social and maintenance behaviours of Japanese Black cows in a communal pasture

The objective of the present study was to investigate optimal group size in cattle in communal pastures where cattle from various farms are turned out: (1) to ascertain if cows derived from each farm form an affiliative group and (2) to investigate the effect of the number of farm mates on social and maintenance behaviours when farm mates form an affiliative group. Farm mates were defined as cows from the same farm. A total of 27 cows having zero, one, two to four, or sixteen farm mates were selected as focal animals in two communal pastures. Each focal animal was followed by an individual observer for 8 h from sunrise. The nearest neighbour and the distance to the nearest neighbour were recorded at 10-min intervals, and maintenance behaviours at 1-min intervals; social behaviours and participants were recorded continuously. Most focal animals frequently choose cows from the same farm as their nearest neighbours allogroomed with these farm mates significantly more (P<0.001). Two focal animals did not form affiliative groups with farm mates. The mean distance to the nearest neighbours, whether farm mates or non-farm mates from focal animals having one and two to four farm mates was shorter than cows turned out with no farm mate (1.4 vs. 1.9 times as long as a cow's body length, P<0.05). Cows having two to four farm mates performed (7.8 s/h/cow, P=0.06) and received allogrooming more (9.8 s/h/cow) and escaped less during agonistic encounters (0.1 frequencies/h/cow, P<0.01) than ones having other numbers of farm mates. The mean duration of grazing behaviour per bout tended to increase with the number of farm mates (5.2 to 10.3 min, P<0.05). The mean duration of recumbency behaviour of cows having two to four farm mates (44.0 min/bout) was longer than the ones having other numbers of farm mates. Social and maintenance behaviours of cows having 16 farm mates were like those having zero or one farm mates, which suggested that the social bond among them was weak. It is concluded that a group of cows having two to four farm mates, that is, a group size of three to five cows, may be optimal for a stable life in a communal pasture.     

The formation of preferential relationships at early age in cattle

Calves can develop long-lasting social relationships with peers. We examined the strength of the relationships between calves according to the time they had been together. Twenty-four female dairy calves were assigned to six groups of four animals (Type-1 partners) at 0.5 month of age. At 3.5 months of age, they were mixed with other calves (Type-2 partners) to form groups of 14. Type-3 partners were calves added to the experimental groups after 5.25 months. The calves stayed together until 1.5 years of age. Social preferences between the three partner types were examined in a Y-maze, and the position and activity of animals in the barn and pasture were followed in three periods. Behavioural synchrony, distance between animals, proximity and nearest neighbour were analysed. The calves more frequently butted Type-3 than Type-1 partners in the Y-maze (P < 0.05). They spent more time in proximity to Type-1 partners, and these were more often the nearest neighbours than other partners (P < 0.001). Synchrony and distance between animals were greater at pasture than in the barn (P < 0.01). Calves seem to form preferential relationships before 3.5 months of age. Keeping cattle together from an early age seems beneficial for them.     

Acoustically mediated individual recognition by a coral reef fish (Pomacentrus partitus)

Acoustically mediated individual recognition has been demonstrated in the field by males of the bicolor damselfish Pomacentrus partitus Poey. Playbacks of non-resident sounds from a given fish's territory elicited a greater response from its nearest neighbour than playbacks of the resident's sound—in accordance with the predictions of the ‘dear-enemy’ effect. Testing also included switching the sounds of the two nearest neighbours relative to each respective male's territory. Results demonstrated that all males in the colony individually recognized the sounds of their two nearest neighbours.     

Schooling behaviour of juvenile Pacific bluefin tuna Thunnus orientalis depends on their vision development

The effects of vision development and light intensity on schooling behaviour during growth in juvenile Pacific bluefin tuna Thunnus orientalis were investigated using both behavioural and histological approaches. The schooling behaviour of three age groups [25, 40 and 55 days post hatching (dph)] of juvenile T. orinetalis were examined under various light intensities. Subsequently, schooling variables, such as the nearest neighbour distance (D(NN) ) and the separation swimming index (I(SS) ), were also measured under different light intensities. Furthermore, retinal indices of light adaptation in juvenile fish at each experimental light intensity and visual acuities in six stages (25-55 dph) of juveniles were examined histologically. During growth, the light intensity thresholds of I(SS) decreased from 5 to 0·05 lx, and D(NN) under light conditions (>300 lx) also decreased from 9·2 times the standard length (L(S) ) to 1·2 times L(S) . The thresholds of light intensities for the light adaptation of retinas in juveniles (25-55 dph) similarly decreased from 5 to 0·05 lx with growth. In addition, the visual acuities of juveniles developed from 0·04 to 0·17 with decreasing D(NN) . These data clearly indicate that the characteristics of schooling behaviour strongly correspond to the degree of vision development. Juvenile T. orinetalis also appear to be more dependent on cone rather than rod cells under low light intensity conditions, resulting in a relatively high light intensity threshold for schooling. These results suggest that juveniles can adapt to darker conditions during growth by developing improved visual capabilities.