Half-weight Index Research Articles

397 Young Scholar Award Talk: Detecting Livestock Welfare Concerns and Production Issues Using Accelerometer and Global Positioning Systems

Abstract Monitoring behavior changes of livestock may help producers expedite intervention and treatment in both intensive and extensive systems. Sensor technologies can identify behavioral trends, offering the potential to detect abnormal behavior associated with health status and animal welfare changes. The aim of recent studies was to investigate behavior deviations using tri-axial accelerometers and global positioning systems (GPS). Accelerometers were effective tools in determining behavior changes due to the onset of illnesses and parturition. Eight Brahman-cross heifers were fitted with accelerometers to study water consumption behavior. Two of the heifers were diagnosed with Bovine Ephemeral Fever, which symptoms include lameness and stiffness. Movement Intensity (MI), a metric calculated from accelerometer axis data, was lower (P ≤ 0.01) during the day of disease diagnosis compared to 48 h prior. Ten ewes were each fitted with HerdDogg biometric accelerometers, with five ewes also fitted with accelerometers to investigate behavior before and after illness caused by the unintentional feeding of mold-contaminated feedstuffs. Activity levels were lower for days 1 to 4 after feeding (P < 0.05) than day -4 to 0 before feeding. Thirteen Debouillet ewes, with accelerometer ear tags, were used to determine the ability of random forest machine learning techniques to accurately identify and predict parturition-related behavior. Prediction models were able to predict seven mutually-exclusive behaviors and active vs. inactive behavior with 66.7% and 87.2% accuracy, respectively. The use of GPS tracking can help identify proper pasture rotation timing. Thirty-two and twenty-nine cattle were fitted with GPS collars in central Arizona to evaluate the temporal changes in association patterns on two herds of Corriente cattle managed under two different stocking densities: high stocking density (0.417 animal ha-1) and low stocking density (0.123 animals ha-1), respectively. Half-weight index (HWI) value was calculated for each pair of GPS-tracked cattle to determine the time cattle spent within 75 m and 500 m of others. Cattle within both pastures exhibited low mean association values (HWI < 0.25) at both spatial scales at the beginning of the study with increasing levels of independence throughout the study. Cattle dispersed throughout the study (P < 0.01) and traveled farther from water (P < 0.01), likely in search of forages. The combination of GPS and accelerometers can improve detection of welfare issues. Seventeen cattle were tracked with both sensors and sixteen cattle tracked with only GPS across two years to determine the potential of detecting water system failures. Movement intensity was greater (P = 0.03) and cattle remained closer to water (P = 0.01) on the days of simulated water failure compared to control days. Real-time sensors have the potential to provide caretakers notification of changes in livestock behavior that can occur due to disease, parturition, or infrastructure failure.

Resources and population traits modulate the association patterns in the common bottlenose dolphin living nearby the Tiber River estuary (Mediterranean Sea)

Sociality and ecological drivers that can influence individual association patterns are infrequently considered in wildlife management, although they are essential aspects affecting animals’ responses to both human-related pressures and conservation strategies. In common bottlenose dolphins (Tursiops truncatus), sex-specific social dynamics and interactions with anthropogenic activities may affect grouping and induce changes in relationships between individuals. Out of a total of 347 individuals, we assessed the level of association among 68 bottlenose dolphins that have been sighted more than five times near the Roman coast (central Mediterranean Sea, Italy). The half-weight index (HWI) of dyadic associations, their network relations, and stability over time were investigated by using the SOCPROG software. Outcomes showed that females were more strongly associated than other individuals, with both preferred constant short-term associations and random long-term associations, possibly resulting in greater success in rearing young. Individuals interacting with the bottom trawl fishery showed weaker and short-term associations. Temporary disruption of individual associations during interaction with fishery and the relatively low number of females with calves participating in depredation seem to denote both the opportunistic nature of interactions with fishing vessels and the offspring-related protection strategy. The results show that the dolphins in this region maintain a complex but flexible social structure that varies with local biological requirements and is resilient to anthropogenic pressures.

Associates from infancy influence postweaning juvenile associations for common bottlenose dolphins (Tursiops truncatus) in Florida

AbstractIn many long-lived mammalian species, association patterns between individuals have been found to influence sociality, behavioral traits, survival, and longevity. In common bottlenose dolphins (Tursiops truncatus), the early stages of development are of particular importance as associations experienced as dependent calves may influence future association patterns. While behavioral characteristics associated with the transition from a dependent calf state to an independent juvenile state have been documented, there are limited studies that examine associations between these time periods. This study aims to document association longevity for bottlenose dolphins as they transition from calves to juveniles and determine the extent to which kinship plays a role in the development of these associations. Using social network analysis, a generalized linear mixed model (GLMM), and a tiered association scale, we found 53.7% of associations were retained from the calf to the juvenile phase. GLMM results indicated that preferred associates (half-weight index [HWI] > 0.178) from the calf state were 3.6 times more likely to associate in the juvenile state (0.178 > HWI > 0) and 5.67 times more likely to be preferred associates in the juvenile state compared to nonpreferred calf associates. The majority of juveniles, 76.92%, maintained a low–moderate to moderate level association (0.089–0.54) with their mother, and a few retained their mother as their top associate. Kin were preferred associates in 46.15% of cases and found to be the top juvenile associate in 26.92% of cases. Identifying continuity in associations, particularly from the calving state to the juvenile state, is imperative as mammalian association patterns may influence community structure, disease transmission, reproductive success, and predict survival.

Temporal Changes in Association Patterns of Cattle Grazing at Two Stocking Densities in a Central Arizona Rangeland

Simple SummaryMonitoring changes in the utilization of forages across rangelands can be time consuming and difficult with untrained personnel. The use of real time positioning for cattle is becoming commercially available with the improvements in technology. The objective of this case study was to identify the changes in livestock social associations and spatial location at two stocking densities throughout a six-week grazing period. Both pastures used similar sized herds with 35 and 29 animals tracked with global positioning systems set at 30-min intervals. A half-weight index value was calculated for each pair of tracked cattle to determine the proportion of time that cattle were within 75 m and 500 m of each other. Throughout the study, forage utilization increased from 5 to 24% and from 10% to 20% and forage mass decreased from 2601 kg ha−1 to 1828 kg ha−1 and 2343 kg ha−1 to 1904 kg ha−1, in the high stocking density pasture and low stocking density pasture, respectively. Utilization of forages throughout the trial forced cattle to disperse and travel further from water sources to find new feeds. Real-time GPS tracking has the potential to remotely detect changes in animal spatial association, identify when cows disperse, and improve recognition for the need of pasture rotation to avoid rangeland degradation.Proper grazing management of arid and semi-arid rangelands requires experienced personnel and monitoring. Applications of GPS tracking and sensor technologies could help ranchers identify livestock well-being and grazing management issues so that they can promptly respond. The objective of this case study was to evaluate temporal changes in cattle association patterns using global positioning system (GPS) tracking in pastures with different stocking densities (low stocking density [LSD] = 0.123 animals ha−1, high stocking density [HSD] = 0.417 animals ha−1) at a ranch near Prescott, Arizona. Both pastures contained similar herd sizes (135 and 130 cows, respectively). A total of 32 cows in the HSD herd and 29 cows in the LSD herd were tracked using GPS collars at location fixes of 30 min during a 6-week trial in the summer of 2019. A half-weight index (HWI) value was calculated for each pair of GPS-tracked cattle (i.e., dyads) to determine the proportion of time that cattle were within 75 m and 500 m of each other. Forage mass of both pastures were relatively similar at the beginning of the study and forage utilization increased from 5 to 24% in the HSD pasture and increased from 10 to 20% in the LSD pasture. Cattle in both pastures exhibited relatively low mean association values (HWI < 0.25) at both spatial scales. Near the end of the study, cattle began to disperse likely in search of forages (p < 0.01) and travelled farther (p < 0.01) from water than during earlier periods. Real-time GPS tracking has the potential to remotely detect changes in animal spatial association (e.g., HWI), and identify when cows disperse, likely searching for forage.

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.

The first assessment of social organisation of the Indian Ocean humpback dolphin (Sousa plumbea) along the south coast of South Africa

The Indian Ocean humpback dolphin (Sousa plumbea (G. Cuvier, 1829)) is the most endangered marine mammal species in South Africa, and the overall decline of its abundance and group size may affect the social organisation of the species, potentially accentuating its vulnerability. Understanding the social organisation is therefore particularly relevant to conservation efforts. From photo-identification surveys along the south coast of South Africa from March 2014 to June 2015, we quantified association patterns and investigated the social organisation of Indian Ocean humpback dolphins using the half-weight index, social cluster, and network analyses. During the 101 surveys conducted and 553 h of survey effort, 98 sightings of dolphins were recorded and 65 individuals identified. Using individuals seen at least three times, this study reveals that the social network is well differentiated, as strong social divisions exist between individuals that seem to be split into four distinctive social clusters. Network strength was low; approximately half of the associations were low to moderate, whereas some individuals were strongly associated, especially between four pairs of individuals. Although this study is based on a limited number of individuals, our study nevertheless suggests that the atypical strong social bonds recorded here could result from behavioural responses following the decline in group size and abundance.

Commensal bacterial sharing does not predict host social associations in kangaroos.

Social network analysis has been postulated as a tool to study potential pathogen transmission in wildlife but is resource-intensive to quantify. Networks based on bacterial genotypes have been proposed as a cost-effective method for estimating social or transmission network based on the assumption that individuals in close contact will share commensal bacteria. However, the use of network analysis to study wild populations requires critical evaluation of the assumptions and parameters these models are founded on. We test (a) whether networks of commensal bacterial sharing are related to hosts' social associations and hence could act as a proxy for estimating transmission networks, (b) how the parameters chosen to define host associations and delineate bacterial genotypes impact inference and (c) whether these relationships change across time. We use stochastic simulations to evaluate how uncertainty in parameter choice affects network structure. We focused on a well-studied population of eastern grey kangaroos (Macropus giganteus), from Sundown National Park, Australia. Using natural markings, each individual was identified and its associations with other kangaroos recorded through direct field observations over 2years to construct social networks. Faecal samples were collected, Escherichia coli was cultured and genotyped using BOX-PCR, and bacterial networks were constructed. Two individuals were connected in the bacterial network if they shared at least one E.coli genotype. We determined the capacity of bacterial networks to predict the observed social network structure in each year. We found little support for a relationship between social association and dyadic commensal bacterial similarity. Thresholds to determine host associations and similarity cut-off values used to define E.coli genotypes had important ramifications for inferring links between individuals. In fact, we found that inferences can show opposite patterns based on the chosen thresholds. Moreover, no similarity in overall bacterial network structure was detected between years. Although empirical disease transmission data are often unavailable in wildlife populations, both bacterial networks and social networks have limitations in representing the mode of transmission of a pathogen. Our results suggest that caution is needed when designing such studies and interpreting results.

Association patterns of bottlenose dolphins (Tursiops truncatus) in Bahía San Antonio, Argentina

AbstractThis study aims to investigate the association patterns of a small and resident population of bottlenose dolphins (Tursiops truncatus) in Bahía San Antonio (Argentina), and assess any seasonal variation in view of the reported seasonal variation in the population's grouping behavior. The estimation of social differentiation (S) was 0.29 (SE = 0.08), suggesting a homogenous society. The half‐weight index (HWI) averaged 0.23 (± 0.06), with evidence of long‐term preferred associations. However, data showed seasonal variation in the association patterns. In winter, when large groups were reported in this population, HWI values averaged 0.30 (± 0.09) with no indication of preferred or avoided associations. However, during summer, when group size was generally small, HWI values averaged 0.14 (± 0.07), with an indication of preferred/avoided associations. This study indicates that the social structure of the bottlenose dolphins in Bahía San Antonio seems relatively homogenous and flexible over time, with the formation of a random social network at times when large aggregations are formed, and a more disconnected network made up of strongly connected components when the cost of grouping is high.

Do Movement Patterns of GPS-Tracked Cattle on Extensive Rangelands Suggest Independence among Individuals?

In behavioral studies, cattle within the same pasture are not considered as independent experimental units because of the potential confounding effects of the herd’s social interactions. However, evaluating cattle behavior on extensive rangelands is logistically challenging for researchers, and treating individual animals as independent experimental units may be beneficial for answering specific research questions. The objective of this study was to evaluate the association patterns among global positioning system (GPS)-tracked cattle at six different study sites in the western United States. A Half-Weight Index (HWI) association value was calculated for each pair of GPS-tracked cows (i.e., dyad) to determine the proportion of time that cattle were within 75 m and 500 m of each other. Cattle at two study sites exhibited relatively low mean HWI-association values (i.e., less than 0.23 HWI); whereas, cattle at other study sites tended to have greater mean HWI associations (i.e., greater than 0.35 HWI). Distinguishing features between study sites with low and high association values were the management of cattle prior to the study, herd size, pasture size, and the number of watering points. However, at all ranches except one, at least 75% of all dyadic associations had HWI values of less than 0.5 at 500 m, indicating that most of the GPS-tracked cows were greater than 500 m from each other for over 50% of tracking period. While interactions among cattle in the same pasture are often inevitable, our data suggests that under some situations, movement patterns of a sub-set of individual GPS-tracked cows may have levels of independence that are sufficient for analysis as individual experimental units. Understanding the level of independence among GPS-tracked cattle may provide options for analysis of grazing behavior for individual cattle within the same pasture.

Identifying the Relevant Local Population for Environmental Impact Assessments of Mobile Marine Fauna

Environmental impact assessments must be addressed at an appropriate scale of biological organization for the species affected. It can be challenging to identify the relevant local wildlife population for impact assessment for those species that are continuously distributed and highly mobile. Here, we document the existence of local communities of Indo-Pacific bottlenose dolphins (Tursiops aduncus) inhabiting coastal and estuarine waters of Perth, Western Australia, where major coastal developments have been undertaken or are proposed. Using sighting histories from a four-year photo-identification study, we investigated fine-scale, social community structure of dolphins based on measures of social affinity and network (Half-Weight Index - HWI, preferred dyadic association tests, and Lagged Association Rates - LAR), home ranges, residency patterns (Lagged Identification Rates - LIR), and genetic relatedness. Analyses revealed four socially and spatially distinct, mixed-sex communities. The four communities had distinctive social patterns varying in strength, site fidelity and residency patterns. Overlap in home ranges and relatedness explained little to none of the association patterns between individuals, suggesting complex local social structures. The study demonstrated that environmental impact assessments for mobile, continuously distributed must evaluate impacts in light of local population structure, especially where proposed developments may affect core habitats of resident communities or sub-populations. Here, the risk of local extinction is particularly significant for an estuarine community because of its small size, limited connectivity with adjacent communities, and use of areas subject to intensive human use. In the absence of information about fine-scale population structure, impact assessments may fail to consider the appropriate biological context.

Long-term consequences of mother-offspring associations in eastern grey kangaroos

Abstract: Close behavioural association between mothers and offspring should enhance survival and growth of the young. Eastern grey kangaroos Macropus giganteus are gregarious and live in fission-fusion societies where adult females do not form strong bonds with other females but associate closely with their juvenile offspring. We aimed to determine whether the strength of these mother-offspring associations correlated with offspring size, survival and reproduction. We observed 129 marked offspring, aged 10 to 21 months, and their mothers in a high-density population at Wilsons Promontory National Park, Australia. We used half-weight indices to quantify mother-offspring associations and determined the proportion of time offspring spent with their mother, but isolated from other kangaroos, while foraging. We found strong cohort effects on size, mass, body condition, survival and reproduction. Mother-offspring sociability indices were not correlated with offspring body condition as 2-year-olds or reproduction as 3-year-olds. Juveniles that spent proportionally more time with their mothers at 18–21 months, however, were 6% larger and 19% heavier as 2-year-olds than those that did not associate closely with their mothers. In addition, juveniles that were often found alone with their mothers were more likely to survive than those that were more often found in larger groups. Stronger mother-offspring associations before weaning likely reflected nutritional maternal care in sons but non-nutritional care in daughters and had a beneficial effect on juvenile growth and survival. Significance statement: The possible fitness consequences of mother-offspring behavioural associations can affect reproductive decisions by mothers. These fitness consequences affect population dynamics and are relevant to conservation when mothers may be harvested or killed by vehicles, as is the case for many large herbivores. We show that variability in these associations in kangaroos affects correlates of offspring fitness. Juveniles that spent proportionally more time with their mothers between 18 and 21 months of age were larger and heavier as 2-year-olds. In addition, juveniles that spent proportionally more time with their mothers but isolated from other kangaroos experienced improved survival. This study is among very few to examine the relationship between mother-offspring sociability and reproductive success in a non-primate mammal. Improved juvenile growth in sons appears to result from nutritional maternal care through prolonged nursing. Isolation of the mother with her young-at-foot occurs in all large macropod marsupials, and adaptive benefits of this behaviour should therefore occur in other species of macropods.

Social Differentiation in Common Bottlenose Dolphins (Tursiops truncatus) that Engage in Human-Related Foraging Behaviors.

Both natural and human-related foraging strategies by the common bottlenose dolphin (Tursiops truncatus) have resulted in social segregation in several areas of the world. Bottlenose dolphins near Savannah, Georgia beg at an unprecedented rate and also forage behind commercial shrimp trawlers, providing an opportunity to study the social ramifications of two human-related foraging behaviors within the same group of animals. Dolphins were photo-identified via surveys conducted throughout estuarine waterways around Savannah in the summers of 2009–2011. Mean half-weight indices (HWI) were calculated for each foraging class, and community division by modularity was used to cluster animals based on association indices. Pairs of trawler dolphins had a higher mean HWI (0.20 ± 0.07) than pairs of non-trawler dolphins (0.04 ± 0.02) or mixed pairs (0.02 ± 0.02). In contrast, pairs of beggars, non-beggars, and mixed pairs all had similar means, with HWI between 0.05–0.07. Community division by modularity produced a useful division (0.307) with 6 clusters. Clusters were predominately divided according to trawler status; however, beggars and non-beggars were mixed throughout clusters. Both the mean HWI and social clusters revealed that the social structure of common bottlenose dolphins near Savannah, Georgia was differentiated based on trawler status but not beg status. This finding may indicate that foraging in association with trawlers is a socially learned behavior, while the mechanisms for the propagation of begging are less clear. This study highlights the importance of taking into account the social parameters of a foraging behavior, such as how group size or competition for resources may affect how the behavior spreads. The positive or negative ramifications of homophily may influence whether the behaviors are exhibited by individuals within the same social clusters and should be considered in future studies examining social relationships and foraging behaviors.

Using temporal associations to determine maternal parentage in extensive beef herds

The assignment of maternal parentage, although time-consuming and expensive using traditional methods, is essential for genetic improvement. Within the sheep industry the recording of time-based (temporal) associations without human intervention has been routinely used to derive maternal parentage, however it has not been researched in extensive beef production systems. To determine whether temporal associations could be used to assign maternal parentage, cows and calves had their identity recorded as they walked to water over a 27-day trial. Two methods of association were investigated, being the half-weight index and the time difference between a cow and calf having their identity recorded. The half-weight index, which is a measure of the number of times two individuals are recorded together, correctly assigned greater than 90% of maternal pairs. When investigating the duration of data recording it was shown that 85% of maternal parentage could be achieved within only 21 days. Further work is required to determine the effect of calf age, herd and paddock size; however, the results showed that the half-weight index method of determining maternal associations is a labour-saving and accurate alternative to traditional methods used to identify maternal parentage.

Association patterns of visually-observed cattle on Montana, USA foothill rangelands

Cattle on extensive rangelands typically graze in smaller groups rather than as one large herd. The objective of this study was to examine the strength of social associations among cattle in different sized herds. Associations among individual pairs of cows (i.e., dyads) on rangelands were determined from 12 to 15 early-morning visual observations during the summer months from 1997 to 2001. Strength of associations was assessed at group (i.e., cows that were within 30m of other cows) and pasture section (i.e., cows within the same general pasture area) spatial scales by determining Half-Weight Index (HWI) association values for all dyads with the association software program SOCPROG 2.4. Herds with 40 or less cows exhibited association patterns that were not different (P<0.95) from what would be expected by chance suggesting that cattle in herd sizes of 40 or less did not exhibit preferential or avoidance associations. In contrast, cattle in larger herds (i.e., 53–240 cows) tended to form associations with other cows that were stronger or weaker than what would randomly be expected. Strengths of associations at the group scale in larger herds were typically low with most pairs of cattle only being observed together in the same group in less than 30% of the observation periods. At the pasture section scale, cattle appeared to have relatively strong associations, but it was unclear if this was an affinity to the same pasture area or a strong social bond with herd mates. Differences in association patterns at different herd sizes suggests that social behaviours observed in small herds may not fully reflect the natural associations that occur with larger herds of cattle on rangelands.

The role of weighted and topological network information to understand animal social networks: a null model approach

Network null models are important to drawing conclusions about individual- and population-(or graph) level metrics. While the null models of binary networks are well studied, recent literature on weighted networks suggests that: (1) many so-called ‘weighted metrics’ do not actually depend on weights, and (2) many metrics that supposedly measure higher-order social structure actually are highly correlated with individual-level attributes. This is important for behavioural ecology studies where weighted network analyses predominate, but there is no consensus on how null models should be specified. Using real social networks, we developed three null models that address two technical challenges in the networks of social animals: (1) how to specify null models that are suitable for ‘proportion-weighted networks’ based on indices such as the half-weight index; and (2) how to condition on the degree- and strength-sequence and both. We compared 11 metrics with each other and against null-model expectations for 10 social networks of bottlenose dolphin, Tursiops aduncus, from Shark Bay, Australia. Observed metric values were similar to null-model expectations for some weighted metrics, such as centrality measures, disparity and connectivity, whereas other metrics such as affinity and clustering were informative about dolphin social structure. Because weighted metrics can differ in their sensitivity to the degree-sequence or strength-sequence, conditioning on both is a more reliable and conservative null model than the more common strength-preserving null-model for weighted networks. Other social structure analyses, such as community partitioning by weighted Modularity optimization, were much less sensitive to the underlying null-model. Lastly, in contrast to results in other scientific disciplines, we found that many weighted metrics do not depend trivially on topology; rather, the weight distribution contains important information about dolphin social structure.

Female mountain goats, Oreamnos americanus, associate according to kinship and reproductive status

In fusion–fission social systems, associations are generally weak, but animals can associate preferentially, although the ultimate and proximate causes for such associations can be multiple and depend on the species. We applied a social network approach to a population of marked female mountain goats, Oreamnos americanus, characterized by a fusion–fission social system in which associations between individuals are assumed to be weak. We analysed the composition of nursery groups of mountain goats from the Caw Ridge (Alberta, Canada) population from 2005 to 2008 to test whether kinship, reproductive status and differences in body mass affect associations. We hypothesized that close kin should associate more than unrelated individuals, and we expected females of similar reproductive status, age or body mass to associate more than dissimilar individuals. We used the half-weight index correcting for differences in gregariousness (HWIG) to calculate associations between dyads of females. Females did not associate at random. Immature females associated strongly with their mothers, whereas adult females associated mainly with females of similar reproductive status. Reproductive females may associate because they share similar needs during lactation. Nonreproductive adult females and females with different reproductive status associated according to their relatedness. Body mass did not influence associations between females. To our knowledge, this is the first time that a multivariate approach has been used to describe association preferences. Our results reveal how this approach can be relevant when studying the social structure of a population because it highlights the interactions between factors influencing associations.

Adoption in eastern grey kangaroos: a consequence of misdirected care?

Adoption is rare in animals and is usually attributed to kin selection. In a 6-year study of eastern grey kangaroos (Macropus giganteus), 11 of 326 juveniles were adopted. We detected eight adoptions by observing behavioural associations and nursing between marked mothers and young and three more by analysing the relatedness of mothers and young using microsatellite DNA. Four adoptions involved reciprocal switches and three were by mothers whose own pouch young were known to subsequently disappear. Adoptive mothers were not closely related to each other or to adoptees but adoptive mothers and young associated as closely as did biological pairs, as measured by half-weight indices. Switch mothers did not associate closely. Maternal age and body condition did not influence the likelihood of adoption but females were more likely to adopt in years with high densities of females with large pouch young. Adoption did not improve juvenile survival. We conclude that adoptions in this wild population were potentially costly and likely caused by misdirected care, suggesting that eastern grey kangaroos may have poorly developed mother-offspring recognition mechanisms.

Social structure and abundance of coastal bottlenose dolphins,Tursiops truncatus, in the Normano-Breton Gulf, English Channel

A large, but poorly studied, bottlenose dolphin community, Tursiops truncatus, inhabits coastal waters of Normandy (Normano-Breton Gulf, English Channel, France). In this study, the social structure and abundance of this community were assessed using photo-identification techniques. Like other bottlenose dolphin communities worldwide, this resident community has a fission—fusion social structure with fluid associations among individuals (half-weight index = 0.10). Association patterns were highly variable as indicated by a high social differentiation (S = 0.95 ±0.03). The majority of associations were casual, lasting days to months. However, individuals exhibited also a smaller proportion of long-term relationships. A mean group size of 26 was large compared with other resident coastal communities, and variable, ranging from 1 to 100, which could be the results of ecological conditions, in particular resource predictability and availability. Analyses also showed that the community was organized in 3 socia...

Changes in the group associations of free-ranging beef cows at calving

Dyadic association between individuals forms the basis of group structures for herding animals. Group associations and social bonds are dynamic and can result in the establishment of new subgroups. The onset of parturition and the introduction of an offspring create a social change for a mother that is part of a herd. There is a need to nurture the young, develop and maintain a strong maternal bond, and build or maintain social networks within the larger herd. The present study explored associations within a herd of cattle that included pregnant cows and cows with calves (maternal cows). Group dynamics were determined by daily observations of group associations over an 11-week period. During the period, some pregnant cows calved and it was possible to quantify their associations before and post-calving. The associations between individual cows were quantified using a half-weight index (HWI). The HWI data for the maternal and pregnant class of cows were compared. The overall HWI data and the individual class data (pregnant or maternal) were tested against a random model, using data that were generated using permutation methods. There were significant differences in the associations of the pregnant and maternal cows; the maternal cows had stronger associations with other maternal cows while the pregnant cows showed evidence of weaker associations with other pregnant cows and the maternal cows. As pregnant cows calved, they developed stronger associations with other maternal cows. The present study provided evidence that pregnant cows prefer to maintain a degree of isolation, but strengthen their social bonds with other mothers as they enter a maternal phase.

Correcting for the impact of gregariousness in social network analyses

The social network approach provides a set of statistical tools to analyse associations between individuals. The ‘half-weight index’ (HWI), the association index most commonly used in social network analyses, does not take into account differences between the gregariousness of individuals. Thus, the HWI may not be a good measure of relationships between individuals: it could indicate strong affinities that do not exist and vice versa. Here we present a new index, the HWIG, that corrects the association index between two individuals for their respective levels of gregariousness. We compared the HWIG to the HWI by simulating populations in which individuals varied in their gregariousness and their affinities for each other. Unlike the HWIG, the estimation of associations made by the HWI was strongly influenced by the gregariousness of individuals: the HWI was systematically less strongly correlated with the true (input) affinity than the HWIG and this discrepancy increased when variation in individual gregariousness increased. We recommend using the HWIG, or similar variants of other common association indices, as unbiased measures of association between individuals.