Half-weight Index Research Articles

Dolphins restructure social system after reduction of commercial fisheries

Although human activities are known to affect the social behaviour of group-living animals, the resilience of animals' social structure to disturbance is poorly understood. In the 1990s, bottlenose dolphins, Tursiops aduncus , in Moreton Bay, Australia, formed two distinct social communities (‘trawler’ and ‘nontrawler dolphins’) based on foraging interactions (or lack thereof) with commercial prawn trawlers. Members of the two communities almost never associated, despite overlapping home ranges. Since then, changes to fisheries legislation have substantially reduced trawling in Moreton Bay. We used association analyses and social network metrics to compare patterns of sociality among bottlenose dolphins across two periods: 1997–1999 (during trawling) and 2008–2010 (post trawling). Over this decade, their social network became less differentiated and more compact (average geodesic distance between individuals decreased), with significantly more and stronger associations between individuals (mean and maximum half-weight indices increased). The previously described partitioning into two communities has disappeared, with former ‘trawler’ and ‘nontrawler dolphins’ now dispersed over the entire social network and associating with each other. This restructuring suggests that although fisheries can influence the social behaviour of bottlenose dolphins, their social structure represents a complex adaptive system that is resilient to disturbance.

Site fidelity and association patterns of Indo-Pacific humpback dolphins off the east coast of Zhanjiang, China

This study examined the population of Indo-Pacific humpback dolphins, Sousa chinensis, inhabiting the waters off the east coast of Zhanjiang, China. A total of 116 dolphins were identified during 147 boat-based surveys, completed between June 2005 and June 2007. Abundance estimates indicated that a small population of 268 dolphins (95% CI = 189–413) inhabited this coastal area. The sighting frequencies of identified dolphins varied between one and five when the 2006 and 2007 data were organized into six occasions. Twenty-three percent (27 individuals) of the cataloged dolphins were identified in more than one calendar year. The two animals (ZJ001, ZJ011) with the highest degree of site fidelity were present in the study area for a period of 23 months. The most frequently (15 times) sighted dolphin (ZJ046) was recorded in nine (56%) of the 16 months surveyed. This evidence indicates the interannual site fidelity of S. chinensis to the area off the eastern coast of Zhanjiang City. Range sizes of eight identified individuals calculated by the minimum convex polygon varied from 2.07 to 331.20 km2. Associations between 34 individuals sighted at least three times and more than three times were measured by the half-weight index and SOCPROG program. The majority of these dolphins (80.57%) were not seen together during surveys and the mean association rate between dyads was low at 0.05. Temporal analyses for all the individuals were conducted using a lagged association rate. The results indicated a non-random social structure made by constant companions.

Guiana dolphins (Sotalia guianensis) displaying beach hunting behavior in the Cananéia Estuary, Brazil: social context and conservation issues

To elaborate a fuller description of the beach hunting behavior displayed by Guiana dolphins (Sotalia guianensis) in the Cananéia Estuary (CE) (25ºS, 48ºW), a photo-identification study was conducted from May 2000 to July 2003. Three land platforms were chosen at the main entrance of the above-mentioned estuary, where scientists enjoy a unique opportunity to observe dolphins at greater proximity, undisturbed. Observations were opportunistic and unevenly distributed in time and space. To enhance the identification efforts, an 80-pound crossbow was used to collect skin samples in order to determine the sex of the eight individuals thus monitored. In 67 days of surveys, rendering approximately 80h of direct observations, 4,102 photographs were taken, of which 1,098 (26.8%) were considered useful for identification purposes. A total of 103 groups were reported (3.7 ± 2.6 individuals), ranging from solitary dolphins to 15 individuals. Female-calf pairs were observed in 92.4% of all the groups investigated. On the first occasion, a calving interval was observed for the species in the CE: approximately 3 years and 9 months. Of the 40 individuals identified at least once, eleven used the sloping beaches regularly: females KN #s 10, 30, 255, 268 and 279; males KN #s 86 and 257; and four individuals of unknown sex. The characteristics of the associations among beach hunters were evaluated by the analyses of association indices. A total of 83 possible associations between dyads were analyzed using the half-weight index and two selective criteria (5+ and 8+ sightings/individual). Mean association indices varied from 0.12 to 0.16, evidencing weak bonds among beach hunters. Most regular beach hunters were females, showing that the beach hunting behavior should probably be considered more typically female. Beach hunting in the CE might be considered another example of cultural transmission in cetacean societies.

Social structure of bottlenose dolphins,Tursiops truncatus, in Panama City, Florida

Social organization is an important attribute of the animal society. We describe the social structure of a bottlenose dolphins population living in Panama City, a seaside resort located on the north-west coast of Florida. Study was conducted with 46 individuals. Dolphins are associated on average half weight index of 0.11. Preferred long-term associations are observed. The proportion of the non-zero association indices suggests that some dolphins seem to avoid others. Associations between and within sex-classes were investigated using only dolphins of known sex and observed at least 4 times. Highly significant differences are found in associations between and within sex-classes (Mantel test,t = 3.7987;P = 1); indeed, male associations are stronger than between inter-sexual associations or between females only. Sociogram of males reveals a complex network with strong associations between pairs or trios that reach up to 0.97, whereas female associations are lower than males. The cluster analysis shows no clear division in the social organization of bottlenose dolphins in Panama City, except for dyads, triads and their multiple networks. The population structure seems to be temporally stable over the study and constant companionships are observed in the dolphin population in Panama City.

Absence of sexual differences in aggregation behavior and association patterns in South American sea lion pups (Otaria flavescens) at Isla de Lobos, Uruguay

In the South American sea lion (Otaria flavescens), a polygynous species with a high degree of sexual dimorphism, pups form groups during maternal foraging trips. The aim of this study was to investigate the existence of sexual differences in aggregation behavior and association patterns in O. flavescens pups at Isla de Lobos, Uruguay, during the first 2 mo of life. Scan samplings of marked pups were conducted every hour from 0700 to 1800 to register behavior. Association patterns were analyzed in SOCPROG (using the Half-Weight Index of Association). We did not find significant sex differences in any variable (time spent in groups, average group size, time interacting with other pups, and time in groups at sea). Each pup showed preference for at least one partner. Associations between individuals of the same and different sex were not significantly different. As expected, during the first month, pups associated more strongly with pups born in the same zone than with those born in a different zone. This research provides new evidence on the development of social behavior in otariids and serves as a basis for future studies focusing on sexual differences in pup behavior and association patterns among individuals (e.g., related with kinship).

The bottlenose dolphin community of Doubtful Sound features a large proportion of long-lasting associations

More than 12 studies of different bottlenose dolphin populations, spanning from tropical to cold temperate waters, have shown that the species typically lives in societies in which relationships among individuals are predominantly fluid. In all cases dolphins lived in small groups characterised by fluid and dynamic interactions and some degree of dispersal from the natal group by both sexes. We describe a small, closed population of bottlenose dolphins living at the southern extreme of the species' range. Individuals live in large, mixed-sex groups in which no permanent emigration/immigration has been observed over the past 7 years. All members within the community are relatively closely associated (average half-weight index>0.4). Both male–male and female–female networks of preferred associates are present, as are long-lasting associations across sexes. The community structure is temporally stable, compared to other bottlenose dolphin populations, and constant companionship seems to be prevalent in the temporal association pattern. Such high degrees of stability are unprecedented in studies of bottlenose dolphins and may be related to the ecological constraints of Doubtful Sound. Fjords are low-productivity systems in which survival may easily require a greater level of co-operation, and hence group stability. These conditions are also present in other cetacean populations forming stable groups. We therefore hypothesise that ecological constraints are important factors shaping social interactions within cetacean societies.

Association patterns of bottlenose dolphins (Tursiops aduncus) off Point Lookout, Queensland, Australia

The social structure of animal communities is usually measured through interactions or associations of individuals within the community. However, investigating and identifying association patterns for large communities of social animals can be difficult, given the logistical difficulties of identifying a large number of individuals within a given area and time period. In this study, over 550 individuals were identified within a large community of bottlenose dolphins (Tursiops aduncus) sampled intensively during the winters of 1998 and 1999 off Point Lookout, Queensland, Australia. Association patterns within this community were analysed using the half-weight index of association, including seven criteria for selecting individuals for inclusion in the analysis. Selection criteria were based on the number of times an individual was sighted during the entire study period. Overall, the community showed a highly fluid association pattern, with only two selection criteria showing association patterns that differed significantly from random. This type of association pattern is commonly reported for large communities of cetaceans. However, without the inclusion of other population information such as estimates of the proportion of identifiable individuals in the community and of community size, it appears that association patterns for these large communities may not be accurately assessed.

Association patterns in three populations of Hector's dolphin, Cephalorhynchus hectori

Most delphinids are very social and spend much time in the presence of conspecifics. The proportion of co-occurrences of any two individuals is usually measured by means of association indices. In this study, the "half-weight index" was used to describe the association patterns in three previously unstudied populations of Hector's dolphin, Cephalorhynchus hectori, around the South Island of New Zealand. For inclusion in this analysis individuals were photoidentified repeatedly over a 2-year study period. Only about 1% of the individual associations of dolphin pairs (dyads) tested were found to be significantly different (p less than or equal to 0.01) from random distributions derived from a new constrained Monte Carlo randomisation. The overall association patterns in the two study populations off Kaikoura ( n = 48 individuals) and Moeraki (n = 24) did not differ significantly from what could be expected by chance (p = 0.07 and 0.09, respectively), whereas those in the Jackson Bay population (n = 38) were significantly different from chance (p < 0.0002). Hector's dolphins appear to have a typical fission-fusion society, with many weak associations within the wider envelope of the local population.

A method for testing association patterns of social animals

Association indices were originally developed to describe species co-occurrences, but have been used increasingly to measure associations between individuals. However, no statistical method has been published that allows one to test the extent to which the observed association index values differ from those of a randomly associating population. Here, we describe an adaptation of a test developed by Manly (1995,Ecology,76, 1109–1115), which uses the observed association data as a basis for a computer-generated randomization. The observed pattern of association is tested against a randomly created one while retaining important features of the original data, for example group size and sighting frequency. We applied this new method to test four data sets of associations from two populations of Hector’s dolphin,Cephalorhynchus hectori, using the Half-Weight Index (HWI) as an example of a measure of association. The test demonstrated that populations with similar medianHWIvalues showed clear differences in association patterns, that is, some were associating nonrandomly whereas others were not. These results highlight the benefits of using this new testing method in order to validate the analysis of association indices.

Associations among photographically identified Hector's dolphins

Associations among Hector's dolphins photographically identified in the waters off Banks Peninsula, New Zealand, were studied using cluster analyses of simple and half-weight association indices. In addition, we carried out a temporal analysis of association patterns, plotting changes in the standardized reassociation rate over time. Some individually identified Hector's dolphins were photographed in the study area in successive seasons and years, and appeared to be resident in the area. Social organization was characterized by relatively fluid association patterns, with little stability over periods longer than a few days. Both male and female Hector's dolphins interacted with a large number of other individuals, males more so than females. Association patterns and other evidence support the hypothesis that Hector's dolphins have a promiscuous mating system.