April 2022 – Two new papers in Current Biology on bottlenose dolphin social complexity.

We are lucky enough to have two papers out in Current Biology this week on bottlenose dolphin social complexity.

In the first paper, led by Bristol MSc student Emma Chereskin, we show that vocal exchanges can function as a replacement of physical bonding in dolphin alliances. This is the first evidence for Robin Dunbar’s social bonding hypothesis outside of the primate lineage:

Chereskin E, Connor RC, Friedman WR, Jensen FH, Allen SJ, Sørensen PM, Krützen M, King SL (2022). Allied male dolphins use vocal exchanges to ‘bond-at-a-distance’. Current Biology https://doi.org/10.1016/j.cub.2022.02.019

In the second paper, led by Zürich PhD graduate Livia Gerber, we show that ‘popular’ allied male dolphins enjoy higher reproductive success:

Gerber L, Connor RC, Allen SJ, Horlacher K, King SL, Sherwin WB, Willems E, Wittwer S, Krützen M (2022). Social integration influences fitness in allied male dolphins. Current Biology. https://doi.org/10.1016/j.cub.2022.03.027

A selection of press coverage below:
Science: https://www.science.org/content/article/dolphins-whistle-keep-touch-distant-friends?utm_campaign=NewsfromScience&utm_source=Social&utm_medium=Twitter
The Independent: https://www.independent.co.uk/news/uk/university-of-bristol-shark-bay-bristol-b2043243.html
Daily Mail: https://www.dailymail.co.uk/sciencetech/article-10648125/Dolphins-whistle-male-bonding-ritual.html

June 2020 – New paper on shelling transmission pathways in Current Biology

From small beginnings involving a series of photographs of an intriguing behaviour captured in our inaugural western gulf field season in 2007, followed by our first sighting of the now-famous “William the Concheror” with a shell on his beak in 2009, through thousands upon thousands of hours of fieldwork and subsequent data analysis… to this:

Full citation: Wild S, Hoppitt WJE, Allen SJ, Krützen M 2020. Integrating genetic, environmental and social networks to reveal transmission pathways of a dolphin foraging innovation. Current Biology 30: 1-7.

Check out the video abstract here: https://www.youtube.com/watch?v=f9nc3-pz7Tc&t=2s

Get your piece of this science here: https://www.cell.com/current-biology/fulltext/S0960-9822(20)30756-9

Highlights:

Network-based diffusion analysis revealed that ‘‘shelling’’ spreads among associates
Dolphin foraging innovations can spread socially outside of the mother-calf bond
First quantification of a non-vertically learned foraging tactic in toothed whales

In brief: Dolphins use empty gastropod shells to trap prey. Wild et al. integrate genetic, behavioural, and environmental data in a network-based diffusion analysis to show that ‘‘shelling’’ spreads socially among associates. This study provides the first quantitative evidence for non-vertical cultural transmission of foraging behaviour in toothed whales.

Summary: Cultural behaviour, which is transmitted among conspecifics through social learning, is found across various taxa. Vertical social transmission from parent to offspring is thought to be adaptive because of the parental generation being more skilled than maturing individuals. It is found throughout the animal kingdom, particularly in species with prolonged parental care. Social learning can also occur among members of the same generation or between older, non-parental individuals and younger generations via horizontal or oblique transmission, respectively. Extensive work on primate culture has shown that horizontal transmission of foraging behaviour is biased toward species with broad cultural repertoires and those with increased levels of social tolerance, such as great apes. Vertical social transmission has been established as the primary transmission mechanism of foraging behaviours in the Indo-Pacific bottlenose dolphins (Tursiops aduncus) population of Shark Bay, Western Australia. Here, we investigated the spread of another foraging strategy, ‘‘shelling’’, whereby some dolphins in this population feed on prey trapped inside large marine gastropod shells. Using a multi-network version of ‘‘network-based diffusion analysis’’ (NBDA), we show that shelling behaviour spreads primarily through non-vertical social transmission. By statistically accounting for both environmental and genetic influences, our findings thus represent the first evidence of non-vertical transmission of a foraging tactic in toothed whales. This research suggests there are multiple transmission pathways of foraging behaviours in dolphins, highlighting the similarities between cetaceans and great apes in the nature of the transmission of cultural behaviours.

Funding and acknowledgements: We thank Shark Bay Salt Pty Ltd and the community of Useless Loop for their logistic support, as well as all research students and volunteers who have contributed to data collection for the Dolphin Innovation Project and Shark Bay Dolphin Research (http://www.sharkbaydolphins.org). This study was supported by a Swiss National Science Foundation grant (31003A_149956), Seaworld Research & Rescue Foundation (SWRRFI) Australia, National Geographic Society, AH Schultz Stiftung, Claraz-Schenkung, Julius-Klaus Stiftung, and WV Scott Charitable Trust, all to MK.

Author contributions: MK, SJA, and SW conceived the study. SW and WJEH conducted statistical analyses. SW collected data and drafted the manuscript. MK secured funding. SJA collected data. All authors contributed to writing.

Watch this space for forthcoming findings on another of the Shark Bay dolphins’ fascinating foraging specialisations…

Apr 2020 – Paper on alliance habitat selection published in Animal Behaviour

Late April sees another paper stemming from our dolphin research off Monkey Mia in Shark Bay. This manuscript represents the fruition of one of PI Richard Connor’s many ideas based on years of field observations, followed by the application of solid analytical work and an excellent literature review by Orla O’Brien to obtain her Master’s thesis through UMASSD.

Title: Alliance-specific habitat selection by male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia

Authors: O’Brien O, Allen SJ, Krützen M, Connor RC.

Highlights:

Niche specialization can occur among individuals and groups within populations.
Male dolphin alliances (each of 3–14 members) cooperate to secure oestrous females.
Alliances inhabit overlapping ranges with similar proportions of available habitats.
Alliances with overlapping ranges and similar habitats forage in different habitats.
Thus food, as well as females, may explain marked differences in alliance tactics.

Abstract: Individuals and groups within the same population may differ in their use of resources. Also referred to as niche specialization, such differences can be documented through direct or indirect observation of resource or habitat use. Here, we examined selective habitat use in alliance-forming male Indo-Pacific bottlenose dolphins, Tursiops aduncus, in Shark Bay, Western Australia. Using 6 years of sighting data (2001-2006), we calculated the home ranges of 17 male alliances (comprising 3-14 individuals each). We defined five habitat types in the study area and measured the proportion of each habitat type in the home range of each alliance. Habitat selectivity was examined using selection ratios of used and available habitat within alliance home ranges. Since home range is also a form of habitat selection, we also examined selection ratios of alliances within defined subareas within the study area. Subareas were the combined home ranges of subsets of alliances that were linked by general location, distinctive habitat features and seasonal movement patterns. During each sighting, the predominant group behaviour was recorded. We analysed data from all sightings (which included all behavioural types: resting, travelling, socializing and foraging), as well as a restricted set of foraging groups only. For all sightings, alliances were selective of habitat within each of three subareas (northern, central, southern), and 14 alliances were selective of habitat within their home range. For foraging sightings, alliances within all three subareas were selective with respect to foraging habitat, and seven alliances showed significant selection of foraging habitat within their respective home ranges. Some alliances with broadly overlapping ranges foraged in different habitats. Alliance-specific habitat use may be influenced by ecological and social factors. Our results raise the possibility that the considerable variation in alliance size within this population may be influenced by the distribution of food as well as the distribution of females.

Full citation: O’Brien O, Allen SJ, Krützen M, Connor RC 2020. Alliance-specific habitat selection by male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia. Animal Behaviour 164: 39-49. https://www.sciencedirect.com/science/article/abs/pii/S0003347220300828?via%3Dihub

Funding and acknowledgements: This study was supported by grants from the Australian Research Council (A19701144 and DP0346313), The Eppley Foundation for Research, SeaWorld Research and Rescue Foundation, W. V. Scott Foundation, The National Geographical Society’s Committee for Research and Exploration and the National Science Foundation (NSF 1316800). Accommodation was very generously provided by the Monkey Mia Dolphin Resort. Many generous people helped make this project possible.

Supplementary Material: Supplementary material associated with this article is available, in the online version, at https://doi.org/10.1016/j.anbehav.2020.03.014.

Apr 2020 – Popping allies published in Proceedings B

Pop goes the dolphin… at the same tempo as his ally!

Early April sees another paper out of our long-term dolphin research off Monkey Mia in the eastern gulf of Shark Bay. This research represents the fruition of an excellent Masters thesis by Bronte Moore under the primary supervision of PI Stephanie King.

Harking back to the sweet days of our Current Biology cover shot in 2018, we scored another front cover of this latest issue of the Proceedings of the Royal Society B (our 3rd for this journal):

Title: Acoustic coordination by allied male dolphins in a cooperative context.

Authors: Moore BL, Connor RC, Allen SJ, Krützen M, King SL.

Abstract: Synchronous displays are hallmarks of many animal societies, ranging from the pulsing flashes of fireflies, to military marching in humans. Such displays are known to facilitate mate attraction or signal relationship quality. Across many taxa, synchronous male displays appear to be driven by competition, while synchronous displays in humans are thought to be unique in that they serve a cooperative function. Indeed, it is well established that human synchrony promotes cooperative endeavours and increases success in joint action tasks. We examine another system in which synchrony is tightly linked to cooperative behaviour. Male bottlenose dolphins form long-lasting, multi-level, cooperative alliances in which they engage in coordinated efforts to coerce single oestrus females. Previous work has revealed the importance of motor synchrony in dolphin alliance behaviour. Here, we demonstrate that allied dolphins also engage in acoustic coordination whereby males will actively match the tempo and, in some cases, synchronize the production of their threat vocalization when coercing females. This finding demonstrates that male dolphins are capable of acoustic coordination in a cooperative context and, moreover, suggests that both motor and acoustic coordination are features of coalitionary behaviour that are not limited to humans.

Full citation: Moore BL, Connor RC, Allen SJ, Krützen M, King SL. 2020 Acoustic coordination by allied male dolphins in a cooperative context. Proc. R. Soc. B 287: 20192944. http://dx.doi.org/10.1098/rspb.2019.2944

Ethics: Permits for the scientific use of animals were obtained from the Department of Biodiversity, Conservation and Attractions (DBCA), Western Australia. The University of Western Australia, University of Zürich and University of Bristol granted animal ethics approvals. Data accessibility. Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.r2280gb9h [58].

Funding: S.L.K. was supported by the Branco Weiss Fellowship—Society in Science. S.L.K. and R.C.C. were supported by a grant from the National Geographic Society (050R-17). M.K. was supported by the Swiss National Science Foundation (31003A_149956).

Acknowledgements: We thank RAC Monkey Mia Dolphin Resort, Shark Bay Resources, Monkey Mia Wildsights, and the DBCA’s Shark Bay Rangers for their continued support and assistance. We thank all field assistants for their help during this study. We thank three anonymous reviewers and the handling editor for their constructive comments.

Watch this space for forthcoming updates on this remarkable dolphin population…

Mar 2020 – From playmates to wingmen, new paper on alliance formation

The most recent issue of Behavioral Ecology features our latest paper, tracking the ‘careers’ of more than 50 sub-adult males through early adulthood as they form second-order alliances.

Title: Affiliation history and age similarity predict alliance formation in adult male bottlenose dolphins

Authors: Gerber L, Connor RC, King SL, Allen SJ, Wittwer S, Bizzozzero MR, Friedman WR, Kalberer S, Sherwin WB, Wild S, Willems EP, Krützen M.

Abstract: Male alliances are an intriguing phenomenon in the context of reproduction since, in most taxa, males compete over an indivisible resource, female fertilization. Adult male bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia, form long-term, multilevel alliances to sequester estrus females. These alliances are therefore critical to male reproductive success. Yet, the long-term processes leading to the formation of such complex social bonds are still poorly understood. To identify the criteria by which male dolphins form social bonds with other males, we adopted a long-term approach by investigating the ontogeny of alliance formation. We followed the individual careers of 59 males for 14 years while they transitioned from adolescence (8–14 years of age) to adulthood (15–21 years old). Analyzing their genetic relationships and social associations in both age groups, we found that the vast majority of social bonds present in adolescence persisted through time. Male associations in early life predict alliance partners as adults. Kinship patterns explained associations during adolescence but not during adulthood. Instead, adult males associated with males of similar age. Our findings suggest that social bonds among peers, rather than kinship, play a central role in the development of adult male polyadic cooperation in dolphins.

Full citation: Gerber et al. 2020. Affiliation history and age similarity predict alliance formation in adult male bottlenose dolphins. Behav Ecol 31: 361-370.

Supplementary material and acknowledgements: Supplementary material can be found at Behavioral Ecology online. We thank the RAC Monkey Mia Dolphin Resort, Monkey Mia Wildsights, and the DBCA’s Shark Bay Rangers for their continued support and assistance. We also thank all field assistants for their help during this study. We thank the anonymous reviewers and the editor for their insightful comments on the manuscript.

Funding: This study was supported by a Swiss National Science Foundation grant (31003A_149956) to M.K. Further financial assistance was provided by grants from the Australian Research Council (A19701144, DP0346313), National Science Foundation (1316800, BNS 8601475), Eppley Foundation for Research, National Geographic Society, W.V. Scott Foundation, SeaWorld Research and Rescue Foundation, A.H. Schultz Stiftung, Ann & Gordon Getty Foundation, Francis V.R. Seebie Charitable Trust, US National Institutes of Health Fellowship, Fulbright Fellowship to Australia, Rackham pre-doctoral Grant, University of Michigan, and the University of Zurich. S.L.K. was supported by The Branco Weiss Fellowship – Society in Science. W.R.F. was supported by a Graduate Fellowship in Anthropogeny from the University of California, San Diego.

Data Accessibility: Analyses reported in this article can be reproduced using the data provided by Gerber et al. (2019) at Behavioral Ecology online.

Watch this blog space for a couple more forthcoming papers. 🙂

Nov 2019 – New paper on aggression between male dolphins

Earlier this month we had another paper published in the journal Behavioral Ecology and Sociobiology about the rates of aggression between adult male dolphins. It follows on nicely from our 2017 paper in Scientific Reports.

Title: Male aggression varies with consortship rate and habitat in a dolphin social network

Authors: Hamilton RA, Borcuch T, Allen SJ, Cioffi WR, Bucci V, Krützen M, Connor RC.

Abstract: Coalitions and alliances exemplify the core elements of conflict and cooperation in animal societies. Ecological influences on alliance formation are more readily attributed to within-species variation where phylogenetic signals are muted. Remarkably, male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia, exhibit systematic spatial variation in alliance behavior, not simply within a species or population, but within a single social network. Moving SE-NW along Peron Peninsula in Shark Bay, males ally more often in trios than pairs, consort females more often, and exhibit greater seasonal movements. Ecological models predict more male-male conflict in the north, but sufficient observations of aggression are lacking. However, dolphins often incur marks, in the form of tooth rakes, during conflicts. Here we report that the incidence of new tooth rake marks varies systematically in the predicted pattern, with greater marking in the north, where males form more trios and consort females at a higher rate. While our previous work demonstrated that alliance complexity has an ecological component, we can now infer that ecological variation impacts the level of alliance-related conflict in Shark Bay.

Significance statement: To understand ecological influences on animal societies, researchers have focused on differences within species, where confounds due to evolutionary history are minimized. Such differences are usually found among geographically separated populations, but in Shark Bay, Western Australia, male dolphin alliance size and access to females increase along a spatial axis within a single social network. Here we report that aggression levels, evidenced by tooth rake marks, increase along the same axis. Alliances are of particular interest as they represent a complex kind of relationship, often implicated in the evolution of social intelligence. Our discovery of spatial variation in alliance behavior and aggression within a social network provides a unique opportunity to investigate the intersection of cognition, social structure, and ecology.

Full citation: Hamilton et al. 2019. Male aggression varies with consortship rate and habitat in a dolphin social network. Behavioral Ecology and Sociobiology 73: 141. https://doi.org/10.1007/s00265-019-2753-1

Ethics: Data were collected under permits from the Western Australian Department of Parks and Wildlife. The University of Massachusetts at Dartmouth and the University of New South Wales provided animal ethics approvals for this study. All applicable international, national, and/or institutional guidelines for the use of animals were followed.

Funding: Funding information This study was supported by grants from the Australian Research Council (A19701144 and DP0346313), the Eppley Foundation for Research, the Seaworld Research and Rescue Foundation, the W. V. Scott Foundation, the National Geographical Society’s Committee for Research and Exploration, and NSF (1316800).

Acknowledgements: We thank the RAC Monkey Mia Dolphin Resort for their generous and ongoing support. Many generous people, including field volunteers, helped make this project possible. Stephanie King provided statistical advice. We express gratitude to the reviewers for their constructive suggestions.

Nov 2019 – Tursiops aduncus coming out party!

Need a solid summary or good source of references about one of the most intriguing animals on the planet, the Indo-Pacific bottlenose dolphin Tursiops aduncus? Then this book chapter is for you, and it’s peppered with pretty pictures for the visually aroused…

Title: The Indo-Pacific bottlenose dolphin Tursiops aduncus

Authors: Connor RC, Sakai M, Morisaka T, Allen SJ

Abstract: The behavioral ecology of Tursiops aduncus (Indo-Pacific bottlenose dolphin) is usually reviewed alongside the much more widely studied T. truncatus (common bottlenose dolphin). However, the smaller, typically shallow water T. aduncus has been closely scrutinized in Australian and Japanese waters. As a result, there now exists a robust body of information spanning all three of Hinde’s levels of social analysis—interactions, relationships, and social structure—that may be unmatched in any other cetacean. Research on T. aduncus has contributed significantly to the social complexity hypothesis of large brain evolution and our understanding of delphinid mating systems, communication, and individual differences in foraging tactics within populations. Here, we focus on behavioral research at two primary sites, Shark Bay in Australia and Mikura Island in Japan, with additional observations of importance from other locales in each region.

Full citation: Connor RC, Sakai M, Morisaka T, Allen SJ 2019. The Indo-Pacific bottlenose dolphin Tursiops aduncus. Ch16 in Ethology and Behavioral Ecology of Odontocetes (ed. Würsig). Springer Nature. Pp 345-368. DOI: 10.1007/978-3-030-16663-2_16.

Jul 2019 – Another new paper in Animal Cognition

This week sees another paper out of our long-term research in the eastern and western gulfs of Shark Bay. It includes science AND pretty pictures!

Title: Vocal behaviour of allied male dolphins during cooperative mate guarding.

Authors: King SL, Allen SJ, Krützen M, Connor RC.

Get your piece of this open-access action right here: https://doi.org/10.1007/s10071-019-01290-1

Abstract: Coercive mate guarding, where males use aggression to control female movements, is a form of sexual coercion which functions to constrain female mate choice. Non-human primates, for example, herd females to keep them away from competing males, but male bottlenose dolphins (Tursiops aduncus) also herd females to keep them close to their alliance partners. Indeed, pairs and trios of male dolphins work together to sequester single estrus females and defend them from competing alliances. Yet how males facilitate such coordination remains unknown. Here, we investigate the vocal behaviour of allied male bottlenose dolphins during the herding of individual females, examining how the production of whistles and ‘pops’ (a threat vocalisation) varied with behavioural state and inter-animal distances. Allied males produced both whistles and pops significantly more often and at higher rates during social interactions, though they differed in function. Whistle rates increased significantly when new individuals joined the consorting group, consistent with previous work showing that whistles are part of a greeting sequence for this species. Whistle matching also appeared to play a role in within-alliance coordination. Pop vocalisations increased significantly when the nearest male to the female changed, likely inducing the female to remain close as the males coordinate a guard switch. Building upon prior research examining female movements in response to pops, we show that males approach the female and current guard whilst popping, leading to a guard switch. Our results provide new insights into the use of vocal signals during cooperative mate guarding between allied male dolphins.

Full citation: King SL, Allen SJ, Krützen M and Connor RC 2019. Vocal behaviour of allied male dolphins during cooperative mate guarding. Animal Cognition https://doi.org/10.1007/s10071-019-01290-1

Ethics: Permits for the use of animals for scientific purposes were obtained from the Department of Biodiversity, Conservation and Attractions. The University of Zurich and University of Western Australia granted animal ethics approvals.

Funding: SLK was supported by The Branco Weiss Fellowship – Society in Science. SLK and RCC were supported by the National Geographic Society (050R-17). MK was supported by the Swiss National Science Foundation (31003A_149956).

Acknowledgements: We thank RAC Monkey Mia, Shark Bay rangers (Department of Biodiversity, Conservation and Attractions), Shark Bay Resources, the Useless Loop community and all field assistants for help in this study. We thank David Allen of Wolf Design Limited (www.wolfd esign ltd.co.uk) for the dolphin template in Fig. 3b. Lastly, we thank two anonymous reviewers and the handling editor for their valuable comments on the manuscript.

Jul 2019 – New paper on sponging out in Biology Letters

Highlights: This new research incorporates over ten years of data on dolphin behaviour, genetics and habitat use. We show how ‘sponging’ is a learned behaviour, transmitted socially between mother and (primarily female) calves. Meanwhile, habitat use and genetics do not appear to influence if a dolphin learns sponging or not. These findings build on and complement previous research, providing strong quantitative evidence for the existence of dolphin culture.

Title: Multi-network-based diffusion analysis reveals vertical cultural transmission of sponge tool use within dolphin matrilines.

Authors: Wild S, Allen SJ, Krützen M, King SL, Gerber L, Hoppitt WJE.

Abstract: Behavioural differences among social groups can arise from differing ecological conditions, genetic predispositions and/or social learning. In the past, social learning has typically been inferred as responsible for the spread of behaviour by the exclusion of ecological and genetic factors. This ‘method of exclusion’ was used to infer that ‘sponging’, a foraging behaviour involving tool use in the bottlenose dolphin (Tursiops aduncus) population in Shark Bay, Western Australia, was socially transmitted. However, previous studies were limited in that they never fully accounted for alternative factors, and that social learning, ecology and genetics are not mutually exclusive in causing behavioural variation. Here, we quantified the importance of social learning on the diffusion of sponging, for the first time explicitly accounting for ecological and genetic factors, using a multi-network version of ‘network-based diffusion analysis’. Our results provide compelling support for previous findings that sponging is vertically socially transmitted from mother to (primarily female) offspring. This research illustrates the utility of social network analysis in elucidating the explanatory mechanisms behind the transmission of behaviour in wild animal populations.

Full citation: Wild S, Allen SJ, Krützen M, King SL, Gerber L, Hoppitt WJE. 2019 Multi-network-based diffusion analysis reveals vertical cultural transmission of sponge tool use within dolphin matrilines. Biol. Lett. 15: 20190227. http://dx.doi.org/10.1098/rsbl.2019.0227

Ethics: Permits for the use of animals for scientific purposes were granted by the Department of Biodiversity, Conservation and Attractions (SF002958; SF010888; SF10388; SF002958; SF010774; 08-000920-1; 08-000706-3) and the Department of Agriculture and Food (U 10/2015-2018). The animal ethics committees of the University of Western Australia, Murdoch University and the University of Zurich provided approvals for the ethical treatment of animals in scientific research (R2649/14; RA/3/100/1449; RA/3/100/1464).

Funding: This research was funded by: Swiss National Science Foundation (31003A_149956), Seaworld Research & Rescue Foundation Inc. (SWRRFI), National Geographic Society, A.H. Schultz Stiftung, Claraz-Schenkung, Julius-Klaus Stiftung and W.V. Scott Foundation, all to M.K. S.L.K. was supported by The Branco Weiss Fellowship—Society in Science.

Acknowledgements: We thank Shark Bay Resources and the Useless Loop community for logistic support, and all researchers and volunteers who have contributed to data collection for the Dolphin Innovation Project.

Jun 2019 – Social homophily in male dolphins, new paper in Proc Roy Soc B

Blurb: The beautiful and remote Shark Bay World Heritage Area (and Marine Protected Area) is home to the only known communities of tool-using dolphins on Planet Ocean. Only certain matrilines of dolphins engage in the tool-using behaviour, wearing basket sponges over the beaks/rostra as they forage. It’s thought to act as a protective ‘glove’ against sharp rocks, stingray barbs and the dorsal spines on some fish as the dolphins probe the sea floor.

There’s a general female-bias to this behaviour, with a much greater proportion of female calves adopting the behaviour her mum performs than do young males. This difference was long thought to be explained by the incompatibility of such a solitary and time-consuming behaviour with a male lifestyle – who should instead be investing time and energy into formulating alliances with other males.

Well, after a fateful survey in the western gulf of Shark Bay in May 2011, however, we discovered the existence of male alliances of sponging dolphins. About 8 years and two weeks later, here is the paper explaining this phenomenon.

Highlights: Diving into the behaviour of male dolphins that engage in tool-use for the first time and comparing them to non-tool-using males, we found that:

The male spongers spend more time associating with other male spongers than they do non-spongers, irrespective of relatedness or other factors.
They also devote more time to foraging and less time to resting and travelling than do non-sponging males.
Nevertheless, they spend just as much time socialising as do non-spongers.
Our study indicates social homophily in the second-order alliance composition of tool-using bottlenose dolphins.

Title: Tool use and social homophily among male bottlenose dolphins.

Authors: Bizzozzero MR, Allen SJ, Gerber L, Wild S, King SL, Connor RC, Friedman WR, Wittwer S, Krützen M.

Abstract: Homophilous behaviour plays a central role in the formation of human friendships. Individuals form social ties with others that show similar phenotypic traits, independently of relatedness. Evidence of such homophily can be found in bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia, where females that use marine sponges as foraging tools often associate with other females that use sponges. ‘Sponging’ is a socially learned, time-consuming behaviour, transmitted from mother to calf. Previous research illustrated a strong female bias in adopting this technique. The lower propensity for males to engage in sponging may be due to its incompatibility with adult male-specific behaviours, particularly the formation of multi-level alliances. However, the link between sponging and male behaviour has never been formally tested. Here, we show that male spongers associated significantly more often with other male spongers irrespective of their level of relatedness. Male spongers spent significantly more time foraging, and less time resting and travelling, than did male non-spongers. Interestingly, we found no difference in time spent socializing. Our study provides novel insights into the relationship between tool use and activity budgets of male dolphins, and indicates social homophily in the second-order alliance composition of tool-using bottlenose dolphins.

Full citation: Bizzozzero MR, Allen SJ, Gerber L, Wild S, King SL, Connor RC, Friedman WR, Wittwer S, Krützen M. 2019 Tool use and social homophily among male bottlenose dolphins. Proc. R. Soc. B 286: 20190898. http://dx.doi.org/10.1098/rspb.2019.0898

Funding: This study was supported by a Swiss National Science Foundation grant 31003A_149956) to M.K. Further financial assistance was provided by grants from the National Geographic Society, W. V. Scott Foundation, Sea World Research and Rescue Foundation Inc., A. H. Schultz Stiftung, and the University of Zurich. S.L.K. was supported by The Branco Weiss Fellowship—Society in Science. W.R.F. was supported by a Graduate Fellowship in Anthropogeny from the University of California, San Diego.

Acknowledgements: We thank Shark Bay Resources and the Useless Loop community for their generous, long-term, in-kind and logistical support. We also thank all field assistants for their help during this study.