Lagged Association Rate Research Articles

Long-term associations among male sperm whales (Physeter macrocephalus)

Little is known about the social structure of male sperm whales (Physeter macrocephalus) after they leave their natal units. While previous studies found no evidence for preferred associations among males, the observation of mass-strandings consisting exclusively of males, suggest that they have strong social bonds. To investigate the social associations among male sperm whales, we used half weight index of association, permutation tests and standardized lagged association rate models on a large photo-identification database collected between 2006 and 2017 in Nemuro Strait, Japan. Our results suggest that while male sperm whales are not as social as females, they do form long term associations, have preferred companionship, and forage in social proximity to each other. The best-fitting model to the standardized lagged association rate showed that associations among males last for at least 2.7 years and as most males leave the area after 2 years, associations may last for longer. Twenty dyads were observed associating over more than 2 years, for a maximum 5 years. One dyad was observed associating on 19 different days and clustered on 7 different days. Male associations may function to enhance foraging or to fend off predators. Such relationships seem to be adapted to a pelagic habitat with uncertain resource availability and predation pressure.

Social structure of a population of bottlenose dolphins (Tursiops truncatus) in the oceanic archipelago of Madeira, Portugal

In order to investigate social structure, 11 years of individual photo-identification data of bottlenose dolphin were analysed. We examined the type of association indices between pairs of identified individuals; the patterns of affiliation between individual dolphins and the probabilities of association between individuals over time. Between 2001 and 2012, there were 272 encounters which resulted in the identification of 501 individuals. The discovery curve resulting from the photo-identification analysis indicated an open population with regular recruitment of new individuals. All individuals were found to be associated at an association index of <0.05. A total of 291 individuals recorded from 2004 to 2012 were used to assess the temporal pattern of the social structure. The model fit to the Standardized Lagged Association Rate (SLAR) that best described the studied bottlenose dolphin population was ‘casual acquaintances’, and the analysis of associations over time showed a decreasing SLAR curve that falls until reaching the null rate, confirming random associations. The decline of the SLAR curve after ~500 days (1.4 years) suggests disassociation over that time period which can be explained by demographic events such as mortality or emigration. In an open ocean habitat like Madeira this is not unexpected, as there are neither geographic boundaries nor enclosed environments. This population presented a dynamic and fluctuating social structure, where groups change in size and composition. In future conservation efforts this population should be considered as one large community, where individuals associate, disassociate and reassociate with each other over time.

The social structure of Golfo Dulce bottlenose dolphins (Tursiops truncatus) and the influence of behavioural state.

Ecological factors such as habitat and food availability affect the social structure of bottlenose dolphins (Tursiops spp.). Here, we describe the social structure of bottlenose dolphins (T. truncatus) in Golfo Dulce, Costa Rica, a semi-enclosed, fjord-like tropical embayment resembling a pelagic system. We also examine behaviour-linked social strategies by comparing social structure relative to behavioural state: feeding versus non-feeding. We analysed 333 sightings over 210 days from boat-based surveys. Despite the uniqueness of the area, the 47 analysed adults had a social structure similar to other populations: a well-differentiated fission–fusion society with sex-specific patterns of associations and aggression. These results indicate that differences in social structure relative to other populations were a matter of degree. Association strength of dyads was highly correlated across behavioural states, indicating constraints on social fluidity. Males displayed a marked difference in lagged association rate and females displayed a small difference in association homogeneity between states. We suggest this difference in population-wide social connections between behavioural states, particularly for males, was due to mating strategies, a pressure which is strongest during non-feeding behaviour and relaxed during feeding. This finding highlights the importance of considering behavioural state when examining individual bonds and the behavioural plasticity for which the bottlenose dolphin is well known.

Complex social structure of southern flying squirrels is related to spatial proximity but not kinship

Social individuals have organized relationships that affect fitness and so a species' tendency to be social has important implications for its population ecology, gene flow, and its distribution in space and time. We quantitatively examined the social structure of southern flying squirrels (Glaucomys volans) and tested for a role of kinship and prior familiarity in predicting social structure. To quantify social structure, we monitored nest group composition of southern flying squirrels. All squirrels at the study site were marked with passive integrated transponder (PIT) tags and nest cavity entrances were monitored with automated PIT tag recorders for a period of 28 months. Squirrels were genotyped at eight microsatellite loci. Permutation tests of associations suggested that individuals nested with other specific individuals more often than expected by chance. The lagged association rate indicated that relationships were stable and persisted across seasons and years. Multiple summer nest associates came together in winter to form larger nest groups which were likely important for social thermoregulation. A measure of prior familiarity, but not kinship, was related to the proportion of time individuals nested together during winter. We suggest that the evolution of sociality in southern flying squirrels is driven largely by mutually beneficial behaviors related to social thermoregulation although other, as of yet, unidentified mechanisms are needed to explain sociality in the warm season. We hypothesize that minimum group size requirements associated with social thermoregulation could explain the absence of this species in patchy landscapes and aspects of range boundary dynamics near their northern range boundary.

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.

Influence of body size and ranging patterns on delphinid sociality: Associations among Commerson's dolphins

We investigated the association patterns of 48 identifiable Commerson's dolphins (Cephalorhynchus commersonii) photographed 2 or more times over a 2-year period in Baho´a Engano, Chubut, Argentina. The associations among them were not different from random for short time periods (7 days) but were significantly different from random for longer periods. Lagged identification rate and lagged association rate models show that dolphins remained in the area on average 15 days and then moved away as the original groups dissolved. Commerson's dolphins appear to have a fission-fusion society, and the observed strength of their associations is in concordance with the predictions regarding their body size and ranging patterns. The dolphins seem to rely on Baho ´a Engano as a core area for most of their activities, but the aggregations are labile and some individuals remain associated in a more stable manner with only a few other individuals.

Selection of Models of Lagged Identification Rates and Lagged Association Rates Using AIC and QAIC

The lagged identification rate is the probability of identifying an individual given its identification some time lag earlier. The lagged association rate is the probability that two individuals are associated given their association some time lag earlier. Models of lagged identification and association rates fit by maximizing the sums of non independent log-likelihoods have approximately unbiased parameter estimates. Simulations suggest that: Akaike-Information-Criterion often selects the true model of lagged identification rate data; quasi-Akaike-Information-Criterion performs better for lagged association rates; and confidence intervals for parameters are best obtained by bootstrap methods for lagged identification rates and quasi-likelihood or jackknife methods for lagged association rates.

Nonrandom association patterns at northern long-eared bat maternity roosts

Bats are among the most ecologically diverse mammalian orders. Most species live in groups for at least a portion of their life cycle and behavioural evidence suggests that individuals of many species live within complex nonrandomly assorting societies. However, rigorous quantitative characterizations of bat societies have been rare because of the difficulties inherent in studying these highly mobile, small, nocturnal animals. Here we use an automated monitoring system (PIT tags), telemetry, and recently developed analytical techniques to investigate the social organization (size, sexual composition, and spatiotemporal cohesion) and social structure (pattern of social interactions and relationships among individuals) of a colony of free-living northern long-eared bats, Myotis septentrionalis (Trouessart, 1897). Cluster analysis of HWI (half-weight association index) for all pairs and permutation tests indicate that colonies consist of multiple, nonrandomly assorting subgroups. A plot of the temporal persistence of relationships (standardized lagged association rate) showed that roosting groups dissociate over periods of approximately 10 days after which subsets of individuals remain associated throughout the summer roosting season. A model representing a two-levelled social structure of long-term (whole summer) and short-term (up to 10 days) acquaintances best fit the lagged association rate. Subgroups were most cohesive during the lactation period, but we found no evidence for the effects of minimum nightly temperature on subgroup cohesion.

Behavioural evidence for social units in long-finned pilot whales

In general, mammal species show geographic or social dispersal by one or both sexes. Long-term behavioural observations and genetic evidence have confirmed that fish-eating resident killer whales, Orcinus orca, are a rare exception. Female and male offspring travel with their mothers for their whole lives: this is natal group philopatry. It is suspected that pilot whales, Globicephala spp., also follow this social pattern, but longitudinal data on the social structure of live long-finned pilot whales, Globicephala melas, are rare. We observed G. melas through July and August of 1998–2000 off northern Nova Scotia, Canada. Estimated group sizes ranged from 2 to 135 (mean = 20, SD = 17, median = 15, n = 249). We distinguished 322 individuals on the basis of distinctive marks on the dorsal fin, with estimated mark rates of 0.336 (proportion) (SE = 0.041) and 0.352 (mean of estimates) (SE = 0.036). Permutation testing rejected the null hypothesis of random association between individuals (p < 0.0005). The best fit model of the standardized lagged association rate suggests short-term associations of individuals over hours to days and long-term associations with a subset of those individuals over years. When scaled according to mark rate, sets of long-term associates average approximately 11–12 individuals, a much lower estimate than that presented previously from drive-fishery data from the Faroe Islands. Genetic sampling of behaviourally studied individuals is recommended.

Investigating structure and temporal scale in social organizations using identified individuals

Studies of individually identified animals can produce substantial data sets containing information on the structure and temporal scale of social organizations. However, methods of analyzing such data are not well established. Important features of a social organization are revealed by plotting the rate of persistence of the associations between pairs of individuals over a range of time lags (lagged association rate). The consistency of long-term relationships can be characterized using the rate of association of pairs of individuals between their first and last observed associations (intermediate association rate). A hierarchical series of models featuring exponentially decaying lagged association rates may be fitted to these data. This technique retrieved the essential parameters of five simulated social organizations and, when used on real data, portrayed the essential features of the patterns of temporal change in relationships between animals. The method should be especially useful for analyzing fissionfusion societies containing 10–10, 000 individually identifiable animals.