Sexual Segregation Research Articles

Intra-specific Niche Partitioning in Antarctic Fur Seals, Arctocephalus gazella

Competition for resources within a population can lead to niche partitioning between sexes, throughout ontogeny and among individuals, allowing con-specifics to co-exist. We aimed to quantify such partitioning in Antarctic fur seals, Arctocephalus gazella, breeding at South Georgia, which hosts ~95% of the world’s population. Whiskers were collected from 20 adult males and 20 adult females and stable isotope ratios were quantified every 5 mm along the length of each whisker. Nitrogen isotope ratios (δ15N) were used as proxies for trophic position and carbon isotope ratios (δ13C) indicated foraging habitat. Sexual segregation was evident: δ13C values were significantly lower in males than females, indicating males spent more time foraging south of the Polar Front in maritime Antarctica. In males δ13C values declined with age, suggesting males spent more time foraging south throughout ontogeny. In females δ13C values revealed two main foraging strategies: 70% of females spent most time foraging south of the Polar Front and had similar δ15N values to males, while 30% of females spent most time foraging north of the Polar Front and had significantly higher δ15N values. This niche partitioning may relax competition and ultimately elevate population carrying capacity with implications for ecology, evolution and conservation.

Sexual segregation in European cyprinids: consequence of response to predation risk influenced by sexual size dimorphism

Fish respond to predation threat by size/cohort-dependent presence in particular habitats and this may contribute to sexual segregation between habitats in species with sexual size dimorphism (SSD). The present study examines the validity of the “predation risk hypothesis” and importance of SSD on habitat (pelagic/inshore) segregation and dietary differences between sexes in three cyprinids with various magnitudes of SSD-roach (Rutilus rutilus), freshwater bream (Abramis brama) and bleak (Alburnus alburnus). Fish were sampled using gillnets over five consecutive years in the Řimov Reservoir. Habitat segregation with female overrepresentation in the pelagic habitat was found in the most sexually dimorphic species with 26% bigger females—the roach. When analysis of habitat segregation was size-controlled, this segregation ceased to confirm the importance of SSD. Freshwater bream sexes (2% SSD) differed in diet but did not differ in habitat occupation. Bleak sexes were not segregated. Larger roach individuals (predominantly females) are less threatened by gape-limited predators and consequently they can occupy the risky but optimal (for zooplankton acquisition) pelagic habitat. Our results demonstrate that habitat segregation is present in the species with the most pronounced SSD.

Defining Sex-Specific Habitat Suitability for a Northern Gulf of Mexico Shark Assemblage

Understanding the factors that influence species’ distributions is crucial for implementing effective management and conservation practices, yet difficult for highly vagile species like sharks. Many shark species demonstrate either spatial and/or temporal sexual segregation, further confounding accurate quantification of habitat suitability. Given the importance of understanding spatiotemporal patterns in the distribution of coastal shark assemblages, we sought to quantify sex-specific abiotic factors that influence seasonal variation in a coastal shark assemblage using data from a long-term fisheries-independent bottom longline program in the northern Gulf of Mexico. Catch data (individuals/100 hooks/hour) were coupled with a suite of potentially predictive variables: surface and bottom values for temperature (C) and salinity (psu), sea surface height (m), three-dimensional surface and bottom current velocity (u, v, w, in m/s), bottom dissolved oxygen (mg/l), depth (m), substrate grain size (mm), daylength (min), and distance from shore (km). Data were analyzed using boosted regression trees (BRT) to describe the relationships between catch data and environmental factors potentially influencing sex-specific species distribution and abundance. Between May 2006 and November 2018, we conducted 1,226 bottom longline sets and caught 13,742 individuals encompassing 67 species. The majority of the animals captured (74%) were elasmobranchs, primarily sharks. Two species from each of the following three categories were selected for further analyses: small coastal sharks (Atlantic sharpnose shark Rhizoprionodon terraenovae and blacknose shark Carcharhinus acronotus), large coastal sharks (blacktip shark C. limbatus and sandbar shark C. plumbeus), and shelf-associated sharks (smoothhound sharks, Mustelus spp. and scalloped hammerhead Sphyrna lewini). Depth and distance from shore were the strongest predictors of distribution and relative abundance, followed by longitude and bottom salinity; other factors (e.g. temperature, daylength, substrate grain size) were less predictive. For the six species examined, predictive factors were often the same for males and females, although the range of preferred values varied. Surprisingly, the importance of these predictors varied little across seasons. Collectively, our findings demonstrate that sexual segregation is the norm for sharks in the north-central Gulf of Mexico. Long-term fishery-independent monitoring to further quantify these sex-based differences in habitat use should be prioritized, particularly in light of impending climate change.

Population structure and movement patterns of blackbelly rosefish in the NE Atlantic Ocean (Azores archipelago)

AbstractBlackbelly rosefish Helicolenus dactylopterus (Delaroche, 1809) is one of the most important species of the demersal/deep‐water assemblages fished in the NE Atlantic Ocean (Azores archipelago). However, there is insufficient information about the stock structure of this species to support analytical resource assessments and it is currently managed with a precautionary approach. Here, we examine the depth and temporal distribution, size composition, sex, and movement patterns of blackbelly rosefish in order to test the hypothesis that the stock structure in the Azores region comprises different local management units (MUs). We also evaluate annual abundance indices and size composition from scientific surveys and commercial landings over the past c. 25 years for an assessment of the species' vulnerability to bottom longline fishing pressure. The greatest abundance occurred in seamount areas at 350–800 m depth. Larger individuals were mainly found in deeper waters, and no sexual segregation by depth was observed. Tagging results indicated a strongly sedentary behavior and an intraregional separation among populations. Although the abundance pattern and size composition were very similar over time, this species is particularly vulnerable to overfishing due to its biological characteristics (long life, large size, late maturity, slow growth, and low mortality rate). Proper monitoring and management of its exploitation should be a priority. Exploratory analysis for analytical assessment should be performed under the assumption of local MUs. Further oceanographic research is needed to understand larval transport and mixing of populations.

Social dynamics and sexual segregation of Australian humpback dolphins (Sousa sahulensis) in Moreton Bay, Queensland

AbstractUnderstanding the social organization of coastal dolphins, particularly those that reside adjacent to major cities and urbanized regions, is important for understanding the vulnerability of populations to human impacts. We investigated the sexual segregation and social dynamics in a population of Australian humpback dolphins (Sousa sahulensis) inhabiting the near‐urban embayment of Moreton Bay, Queensland. Over 160 vessel‐based surveys, 148 adults were photographically identified. This included 116 assumed females and 32 assumed males. The population was sexually segregated (SSAS = 0.52, p < .001) and individuals preferred to associate with others of the same sex occupying the same spatial area (p < .025). Association indices between paired males were almost double those of females (, SD = 0.04 and , SD = 0.02, respectively). However, females were more gregarious than males and maintained a larger network of associates based on preferred companionships. The social network was also modular with five clusters identified, and a limited number of individuals had a disproportionate influence on connectivity between clusters. This highly fragmented social network implies that the population may be vulnerable to social disruptions from human impacts.

Sexual differences in the foraging ecology of 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic

AbstractMarine mammals often exhibit significant sexual segregation in their diet and habitat use but these differences have not been studied systematically in historic or ancient populations due to the difficulties associated with determining the sex of skeletal elements based on gross morphology. Using a combined ancient DNA and stable isotope approach, we document a sexual difference in the foraging ecology of late 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic. Using two PCR assays that coamplify fragments of the Y‐linked SRY and X‐linked ZFX genes, we assigned reproducible sex identities to 35 beluga specimens. This provided a basis for investigating sex‐specific differences in foraging ecology using stable carbon and nitrogen isotope analyses of bone collagen. These isotopic data demonstrate that although both males and females primarily consumed Arctic cod, males utilized a wider range of prey than females, feeding on high trophic level benthic prey (sculpins) to a greater extent. Because bone collagen integrates prey isotopic compositions over the course of several years these sex‐based differences in beluga bone collagen isotopic compositions reflect long‐term and sustained sexual differences in foraging.

Overlooked sexual segregation of habitats exposes female migratory landbirds to threats

Male and female animals often segregate spatially among habitats and landscapes outside of breeding seasons, but it is unclear to what extent conservation efforts account for sexual segregation. Overlooking this phenomenon may result in conservation plans that don’t meet the needs of both sexes, especially when resources or threats vary spatially. We assessed the prevalence of sexual segregation and degree to which current conservation efforts account for it with a review of nonbreeding ecology and conservation planning literature for 66 North American migratory landbirds of conservation concern. Sexual segregation was common in the group—reported for one-third of all species and two-thirds of those with published nonbreeding sex ratios—and did not differ with dimorphism or conservation status. Despite this, only 3% of species distribution models and 8% of conservation recommendations considered sexual segregation, indicating the pattern is widely overlooked. Next, we used the declining Golden-winged Warbler (Vermivora chrysoptera) as a case study to test for sex-bias in habitats prioritized by current conservation efforts. By modeling nonbreeding occupancy and forest cover loss for males and females, we show that females lost twice as much nonbreeding habitat as males from 2000 to 2016, yet existing conservation focal areas remain heavily biased towards male-dominated landscapes. Furthermore, female-dominated habitats face higher rates of conversion than male habitats for multiple species of Neotropical migrants, suggesting the failure to address sexual segregation may compromise the effectiveness of migratory landbird conservation. We recommend greater effort to report sex ratios and create sex-specific habitat assessments and distribution models.

Roost Composition and Sexual Segregation in a Lowland Population of Daubenton's Bats (Myotis daubentonii)

Using data from 1100 day roosts containing a minimum of 9424 Myotis daubentonii bats, we compare the distribution of adult males, adult females, and juveniles, within SchweglerTM box roosts distributed across 394 ha of woodland adjacent to a river corridor in lowland England. Five social groups, containing both males and females (each comprising 55–230 adult bats per annum, using 37–127 known roosts per colony), occupy discrete roosting areas to which individuals show high inter-annual fidelity. Natal philopatry is also high, compared to between colony movements, for both sexes in our study population. Despite considerable spatial overlap and variability in roost composition over the summer (April to October), related to temporal changes in reproductive status, there is sexual segregation within day roosts. Bachelor (adult male dominated) roosts are situated significantly further from water than maternity (adult female or juvenile dominated) roosts on average. This spatial partitioning between maternity and bachelor roosts operates within rather than between colony home roost ranges, however, with adult males from some social groups found roosting closer to water than are adult females from adjacent colonies. The co-occurrence of maternity and bachelor groups roosting in close proximity, including extensive spatial overlap in roosts occupied, and temporal overlap during roost sharing between the sexes, provides novel insights into social organisation and potential drivers of sexual segregation and mating strategies for this widespread and common species within lowland habitats.

Migratory strategies of juvenile northern fur seals (Callorhinus ursinus): bridging the gap between pups and adults

In species exhibiting differential migration by sex and age, understanding what differences exist, and the adaptive reasons for these differences is critical for determining how demographic groups will respond to environmental variability and anthropogenic perturbations. We used satellite-telemetered movement and diving data to investigate differential migration and its ontogeny in a highly migratory North Pacific Ocean predator, the northern fur seal (Callorhinus ursinus; NFS), with a focus on understudied juvenile (1- to 2-year-old) animals. We instrumented 71 juvenile NFS in two years (2006–07 and 2007–08) at three major North American breeding sites and compared their migratory strategies with pups and adults. Although sexual dimorphism is strong in adult NFS, only weak differences in body mass between sexes were found in juveniles, which had similar body mass to pups (~3–4 months). However, unlike widely-dispersed pups, juvenile male and female NFS dispersed in different directions, and used different habitats characterized by distinct hydrography and prey assemblages during migration, similar to breeding adults. Juvenile diving behavior differed only modestly among habitats and between sexes, consistent with weak differences in body mass. Evidence of habitat sexual segregation by juvenile NFS contradicts previous hypotheses that physiological differences predominantly drive the ontogeny of differential migration.

Reproductive biology of the protandric hermaphrodite fat snook Centropomus parallelus Poey 1860 in a tropical estuary, northeastern Brazil

ABSTRACTThe aim of the present study was to analyze the reproductive aspects of the fat snook, Centropomus parallelus, in a tropical estuary. During the study period, 187 specimens were collected, including 73 females and 114 males. The length at first sexual maturity was estimated to be 224 mm for the females and 141 mm for the males. An extensive reproductive period, January to August, was observed for the females based on the gonadosomatic index. For the males, the highest values were from July to October. Sexual segregation was observed in length classes, with predominance of males and females in the smaller and larger sizes, respectively. This pattern, observed in protandric hermaphrodites, has been confirmed with the identification of an individual in sexual transition. The average relative fecundity was 14,260 oocytes. The average absolute fecundity was 491,036 oocytes/female, and the average size of the vitellogenic oocytes was 435 µm. Management actions are suggested for better conservation of the species.

Does sexual segregation occur during the nonbreeding period? A comparative analysis in spatial and feeding ecology of three Calonectris shearwaters.

Sexual segregation (SS) is widespread among animal taxa, with males and females segregated in distribution, behavior, or feeding ecology but so far, most studies on birds have focused on the breeding period. Outside this period, the relevance of segregation and the potential drivers of its persistence remain elusive, especially in the marine environment, where animals can disperse over vast areas and are not easily observed. We evaluated the degree of SS in spatio‐temporal distribution and phenology, at‐sea behavior, and feeding ecology during the nonbreeding period among three closely related shearwaters: Scopoli's, Cory's, and Cape Verde shearwaters (Calonectris diomedea, C. borealis, and C. edwardsii, respectively). We tracked 179 birds (92 males and 87 females) from 2008 to 2013 using geolocation‐immersion loggers and collected the 13th secondary remige (molted in winter) for stable isotope analyses as a proxy of trophic level and diet. The global nonbreeding distribution did not differ between sexes for the three species, but one specific nonbreeding area was visited only by males. Cory's shearwater males remained in areas closer to the colony in a larger proportion compared to females and returned earlier to the colony, probably to defend their nests. Males presented a slightly lower nocturnal flying activity and slightly (but consistently) higher isotopic values of δ13C and δ15N compared to females. These differences suggest subtle sexual differences in diet and a slightly higher trophic level in males, but the extent to which sexual dimorphism in bill size can determine them remains unclear. Our study showed that SS in ecological niche in seabirds can persist year‐round consistently but at a different extent when comparing the breeding and nonbreeding periods. Based on our findings, we propose that SS in these seabird species might have its origin in an ecological specialization derived from the different roles of males and females during reproduction, rather than from social dominance during the nonbreeding period.

A battle of the sexes? Sex ratio and performance at small scales in dioecious Mercurialis perennis

Mercurialis perennis is a dioecious clonal woodland herb. Its male-biased shoot sex ratio may result from differential selection (with skewed genet sex ratio) and/or differential performance (fairly even genet sex ratio but skewed shoot sex ratio since male clones grow more vigorously or have a higher flowering propensity). To test both scenarios, a total of 1024 shoots were mapped and excavated in five 1 m2 plots in an old-growth forest in northeastern Germany. SSR genotyping revealed a less skewed intrinsic male:female shoot sex ratio (1.6) and a nearly even genet sex ratio. The sexes showed intermingling rather than sexual niche segregation. Most of the 36 genets found split into numerous independent units (merigenets). A slight sexual size dimorphism (SSD) was found: male clones possess on average more shoots, have longer stolons and a higher proportion of flowering shoots. Analyses of resource allocation patterns showed 1.5–2.8 times higher costs for sexual reproduction (dry mass, nitrogen) for females. Males invest surplus resources into leaves and stolons, accentuating the SSD; although for genets the difference may be smaller due to the higher flowering rate of males (53% vs. 34%). Photosynthetic performance, deduced from rapid light curves measured with a portable fluorometer, was similar for both sexes, especially in spring; the main challenge being the transformation from light to shade plant with tree foliation. Our data indicate that differential performance accounts for the male bias in shoot sex ratio; competition between sexes is unlikely given the strongly intermingled clones.

Inter-annual isotopic niche segregation of wild humboldt penguins through years of different El Niño intensities

The Humboldt Current System presents high interannual variability, influenced by the El Niño Southern Oscillation (ENSO), whose implications in wildlife are not fully understood. We studied the isotopic niche of wild Humboldt penguins at Punta San Juan (Peru) during the pre-moult foraging trip in 4 consecutive years (2008–2011) under known oceanographic (ENSO) conditions. Our results show that there is a clear isotopic niche segregation (on both δ13C and δ15N values) of wild Humboldt penguins among all years. Besides isotopic niche segregation, niche width also varied significantly among years. The larger isotopic niche displayed in 2008 reflected the opportunistic feeding behaviour of Humboldt penguins when oceanographic conditions were unfavourable (i.e. El Niño of strong intensity). In contrast, and despite strong segregation, penguins displayed a more specialist behaviour in years of mild environmental conditions (i.e. 2009 “warm-weak”, 2010 “neutral” and 2011 “warm-moderate”). No evidence of sexual segregation in wild Humboldt penguins during the pre-moulting foraging trip was found. This study highlights the coping mechanisms of an endangered species to changes in environmental conditions (i.e. overall, from strong to neutral El Niño events), which should have important ramifications in the management of the marine ecosystem in Peru, particularly the one related to the anchovy industry.

Rainfall and habitat interact to affect the condition of a wintering migratory songbird in The Bahamas.

On the subtropical and tropical wintering grounds of migratory birds, variation in moisture levels and habitat can influence the availability of food resources and subsequently impact overwintering birds. Using stable carbon isotopes in blood samples as a measure of moisture, we assessed the interactive effects of rainfall, vegetation, and moisture on the demographics and condition of Prairie Warblers (Setophaga discolor) wintering in The Bahamas. Carbon isotopes in Prairie Warbler blood were more depleted in taller, wetter habitats; we additionally detected novel temporal effects of rainfall on isotope values. During a winter with more rainfall, most birds maintained mass and pectoral muscle regardless of the habitat type occupied. In a winter with less rainfall, birds lost mass and pectoral muscle, and this effect was more pronounced in birds with enriched isotope values and birds that occupied drier, shorter habitat. Prairie Warblers exhibited strong patterns of sexual habitat segregation with males disproportionately observed in areas with taller vegetation and females in shorter vegetation. During the drier winter, older males had better maintenance of pectoral muscle compared to females and younger individuals. Also in the drier winter, daily rainfall patterns explained more of the variation in body condition compared to the date of capture; pectoral muscle was best explained by recent precipitation (during the previous 30 days), while size‐corrected mass was more a function of longer‐term (90‐day) rainfall and habitat moisture. Our findings along with other studies suggest that Prairie Warblers and other migratory birds are sensitive to interactions between annual variation in winter rainfall, within‐season daily rainfall patterns, and habitat quality. Increasing drought and habitat loss in the Caribbean may be having a negative impact on wintering bird populations. To best conserve Nearctic–Neotropical migratory passerines in the region, we recommend prioritizing the protection of the least drought‐prone wintering areas.

Sexual segregation in a sexually dimorphic seabird: a matter of spatial scale

A recommendation of: Christophe Barbraud, Karine Delord, Akiko Kato, Paco Bustamante, Yves Cherel Sexual segregation in a highly pagophilic and sexually dimorphic marine predator https://doi.org/10.1101/472431

Distribution, body size and biology of the megamouth shark Megachasma pelagios.

We compiled historical reports of megamouth sharks Megachasma pelagios (mostly fishery by-catch and strandings) from 1976 to 2018 (n = 117) and found that they are distributed globally (highest latitude, 36°) with three hotspots: Japan, Taiwan and the Philippines. Despite possible biases due to variability in fishing effort, more individuals were reported at higher latitudes in the summer, suggesting seasonal, latitudinal migrations. Sex ratios were female-biased in Japan, but more even in Taiwan and the Philippines, suggesting some sexual segregation. Females (total length, LT = 3.41-7.10 m) were larger than males (LT = 1.77-5.39 m) and matured at a larger LT (5.17 m) than males (4.26 m). Also, we reviewed the systematics, feeding ecology and swimming behaviour of Megachasma pelagios based on the literature. Our review shows that, compared with their morphology, anatomy and genetics, behavioural ecology of this species remains largely unknown and electronic tagging studies are warranted.

Roost selection by male northern long-eared bats (Myotis septentrionalis) in a managed fire-adapted forest

Wildlife conservation in multi-use landscapes requires identifying and conserving critical resources that may otherwise be destroyed or degraded by human activity. Summer day-roost sites are critical resources for bats, so conserving roost sites is a focus of many bat conservation plans. Studies quantifying day-roost characteristics typically focus on female bats due to their much stronger influence on reproductive success, but large areas of species’ ranges can be occupied predominantly by male bats due to sexual segregation. We used VHF telemetry to identify and characterize summer day-roost selection by male northern long-eared bats (Myotis septentrionalis) in a ponderosa pine (Pinus ponderosa) forest in South Dakota, USA. We tracked 18 bats to 43 tree roosts and used an information-theoretic approach to determine the relative importance of tree- and plot-level characteristics for roost site selection. Bats selected roost trees that were larger in diameter, more decayed, and under denser canopy than other trees available on the landscape. Much like studies of female northern long-eared bats have shown, protecting large-diameter snags within intact forest is important for the conservation of male northern long-eared bats. Unlike female-specific studies, however, many roosts in our study (39.5%) were located in short (≤3 m) snags. Protecting short snags may be a low-risk, high-reward strategy for conservation of resources important to male northern long-eared bats. Other tree-roosting bat species in fire-prone forests may benefit from forest management practices that promote these tree characteristics, particularly in high-elevation areas where populations largely consist of males.

Predation risk and space use of a declining Dall sheep (Ovis dalli dalli) population.

The abundance of ungulate populations may fluctuate in response to several limiting factors, including climate, diseases, and predation. In the northern Richardson Mountains, Canada, Dall sheep (Ovis dalli dalli) have undergone a major decline in the past decades and predation by grizzly bears (Ursus arctos) and wolves (Canis lupus) was suspected as a leading cause. To better understand the relationship between these three species located in this rugged and remote ecosystem, we relied on a combination of indirect methods. We investigated the apparent role of predation on the Dall sheep population using spatial ecology and stable isotopes. We examined seasonal variation in predation risk, focusing on how it may affect Dall sheep habitat use and sexual segregation, and we evaluated the proportion of Dall sheep in the diet of both predators using stable isotopes. The movements of the three species were monitored by satellite telemetry. Dall sheep habitat use patterns were analyzed using topographical features, greenness index, land cover, and apparent predation risk. The diets of grizzly bears and wolves were examined using a Bayesian mixing model for carbon and nitrogen stable isotopes. We found that Dall sheep habitat use varied seasonally, with different patterns for ewes and rams. Exposure to grizzly bear risk was higher for rams during summer, while ewes were further exposed to wolf apparent predation risk during winter. The importance of safe habitats for ewes was reflected in space use patterns. Stable isotopes analyses suggested that the diet of grizzly bears was largely from animal sources, with mountain mammals comprising about one quarter. Wolves mostly fed on both aquatic browsers and mountain mammals. Diet variation between individual predators suggested that some individuals specialized on mountain mammals, likely including Dall sheep. We conclude that grizzly bear and wolf apparent predation risk are important in driving Dall sheep habitat use and play a role in sexual segregation. Overall, this study presents an innovative combination of indirect methods that could be applied elsewhere to better understand predator-prey dynamics in remote ecosystems.

Grouping patterns and social organization in Siberian ibex (Capra sibirica): feeding strategy matters

A study on group size and composition in Siberian ibex (Capra sibirica) was conducted in the Eastern Tien-Shan Mountains. We found that small groups (1-5 individuals) were most common for this species. Generally, mixed-sex groups were larger than male and female groups. Since males have more ability for fiber digestion, because of their larger body size than females, they are less selective in food quality. Due to this, males gathered into larger single-sex groups in August-September, when forage was at its maximal biomass, and into smaller groups when the amount of food was limited (winter-early-spring). During birthing period, pregnant females typically separated from groups to give birth, returning to the female groups in May-June, when high-quality food was at its maximal availability, making female groups largest in this period. Similar to other sexually dimorphic species, Siberian ibex females and males stayed together in mixed-sex groups during rutting season, demonstrating sexual segregation outside of the rutting season, particularly during summer. Nevertheless, there are still a significant number of males and females that stay in mixed-sex groups throughout the year. In conclusion, the grouping pattern and social organization of Siberian ibex were closely correlated to the availability and quality of their food supply and yearly breeding cycle. Accordingly, Siberian ibexes changed their feeding strategy: being mixed feeders they behaved similar to concentrate selectors when food was scant and preferred small sized groups; while they enlarged their group sizes during seasons when the enriching of forage conditions allowed them to do that. However, group enlarging happened in different months depending on group typology (i.e. mixed-sex, male or female groups) depending on the feeding strategy of their individuals.

Assortative interactions revealed in a fission–fusion society of Australian humpback dolphins

Abstract Understanding individual interactions within a community or population provides valuable insight into its social system, ecology, and, ultimately, resilience against external stimuli. Here, we used photo-identification data, generalized affiliation indices, and social network analyses to investigate dyadic relationships, assortative interactions, and social clustering in the Australian humpback dolphin (Sousa sahulensis). Boat-based surveys were conducted between May 2013 and October 2015 around the North West Cape, Western Australia. Our results indicated a fission–fusion society, characterized by nonrandom dyadic relationships. Assortative interactions were identified both within and between sexes and were higher among members of the same sex, indicating same-sex preferred affiliations and sexual segregation. Assortative interactions by geographic locations were also identified, but with no evidence of distinct social communities or clusters or affiliations based on residency patterns. We noted high residency among females. Models of temporal patterns of association demonstrated variable levels of stability, including stable (preferred companionships) and fluid (casual acquaintances) associations. We also demonstrated some social avoidance. Our results point to greater social complexity than previously recognized for humpback dolphins and, along with knowledge of population size and habitat use, provide the necessary baseline upon which to assess the influence of increasing human activities on this endemic, Vulnerable species.