The Social Network and the Sand Tiger Shark, by Don Orth

Long before the movie, The Social Network, scientists have been investigating social networks – first in humans and, more recently, in wild animals.  A social network is a group of individuals interconnected by social ties between them; these interactions may be sexual, cooperative, learning, disease transmission, or others (Krause et al. 2015). Understanding the social network structure provides insights that the study of simpler, two-individual interactions cannot provide. Recently, a press release highlighted the social network of the Sand Tiger Shark Carcharias taurus (Carchariniformes; Odontaspididae). 

Most applications of social network theory to the study of animals have focused on more easily observable subjects, such as captive mammals, so this work on a free-living fish is innovative. Danielle Haulsee and her colleagues tracked 300 Sand Tiger Sharks over a one-year period, in order to provide the first evidence that this apex predator displays a more complex social structure than previously thought. The same research team had described the habitat selectivity of migrating Sand Tiger Sharks with acoustic telemetry and an autonomous underwater vehicle. Therefore, they knew where this coastal shark was migrating, but understanding their social network is a more challenging problem. In order to solve the first problem of obtaining behavioral data on a large mobile predator in the open ocean, the investigators used archival tags that recorded detections from nearby telemetered animals.  

Sand Tiger Shark amidst shoal of prey fish. Photo by  Tanya Houppermans/Caters News Agency

The Sand Tiger Shark is one of the best-studied sharks, and, therefore, a good fish to begin to examine social networks.  Populations are concentrated along the mid Atlantic coast and uncertainty over population declines prompted NOAA to continue to list the Sand Tiger Sharks as a species of concern.   Their low net reproductive rate means the populations will be slow to recover after prohibition of harvest by commercial and recreational fisheries in 2006.   

 

Close up of head of Sand Tiger Shark. Photo National Aquarium.

One look at the mouth of a Sand Tiger Sharks will tell you this fish is a specialized piscivore.  Consequently they are frequently captured by recreational anglers as well as longline and gill net fisheries.  Pioneer shark-watcher, Russell J. Cole, observed schools of Sand Tiger Sharks surrounding and herding schooling prey in order to feed on them. The systematic and coordinated movement of hundreds of Sand Tiger Sharks has the hallmarks of cooperative feeding behavior, a phenomenon more associated with dolphins and birds.        

Close up of teeth of Sand Tiger Shark.  Photo from National Aquarium.

 

While most sharks provide buoyancy with a large liver, the Sand Tiger Sharks also gulp air at the surface and store it in their stomach to provide buoyancy. These sharks generally mate in the fall after a courtship that involves the male aggressively nipping his potential mate.  Females give birth to only one or two large pups every two years and gestation lasts for nine months. Pups hatch and develop in the female.  By the time they reach 17 cm, they already possess prominent teeth and feed on new eggs and embryos produced by the female, a form of nutrition called ovophagy and embryophagy.  It’s hard to say how many eggs are eaten before the pups are born at a size of almost one meter.   Demian Chapman and colleagues did genetic tests on embryos in the uterus of a number of Sand Tiger Sharks.  As expected, the females mated with multiple males, but 60% of the females were carrying only babies from the same father.  Perhaps females have multiple mates in order to feed the offspring from the first father.  For the Sand Tiger Shark, the uterus is a safe place from predation from other sharks, however, it is not a safe place from being cannibalized by your sibling womb-mates.

Small and embryos from same uterus of Sand Tiger Shark. Photo by D.L Ambercrombie

Social networks are of interest to scientists studying social animals; social animals have a variety of strategies that individual animals use in groups. How can scientists understand how cooperation, aggression, information flow or dominance at the individual level translates to group phenomenon? Animals are not robots whose behavior is programmed by genes; rather they are individuals and their behavior is influenced by genetics, the environment, and social interactions.  Animals learn and they remember.   The group, or the network, is also important since information flows between some members in the group.  Social network analysis is needed to understand processes such as pathogen transport, feeding, movements, mating opportunities, and teaching survival skills.  Importantly, certain individuals play a larger role in the well-being of the group (Dugatkin and Hasenjager 2015).

The social network methods originated largely from psychologists and anthropologists.  For a brief history, click here. Scientists today study groups of individuals and monitor the many types of individual interactions in order to provide a network of social structure.  For example, which individuals are more likely to be affiliated in space or time and which individuals are avoided, or pursued as mates?  Although there are many applications, the analysis of social networks permits scientists to investigate the role of individual variation in social behavior on population structure.  Sophisticated methods for analyzing social networks are emerging.  These novel methods allow for  the study of networks of genetic, affiliative, agonistic, cooperative, dominant, and other relationships that form the social system (Wey et al. 2008; Farine and Whitehead 2015)

Social networks are most studied in social mammals, such as dolphin  (Lusseau and Newman 2004), bats, and other mammals, such as Zebras.  Social networks have also be applied to explore how the information-sharing networks contribute to fishing success in the Northumberland lobster fishery (Turner et al. 2014). Why should we study social networks?  The new investigation of the Sand Tiger Shark tells us that there is much more to learn about fish in social networks.  Social networks matter in contributing to the well-being of the group.  We need to consider the possibilities and open our minds to study possibilities of these hidden networks. 

References

Chapman, D.D., S.P. Wintner, D.L. Abercrombie, J. Ashe, A.M. Bernard, M.S. Shivji, and K.A. Feldheim. 2013.  The behavioral and genetic mating system of the sand tiger shark, Carcharias taurus, an intrauterine cannibal.  Biology Letters.   

Dugatkin, L.A., and M. Hasenjager. 2015.  The networked animal.  Scientific American 312:50-55.   

Farine, D.R., and H. Whitehead. 2015. Constructing, conducting and interpreting animal social network analysis.  Journal of Animal Ecology 84:1144-1163.

Lusseau, D., and M.E.J. Newman.  2004.  Identifying the role that animals play in their social networks.  Proceedings of the Royal Society of London B (Supplement) 271:S477-S481.  DOI 10.1098/rsbl.2004.0225

Krause, J., R. James, D.W. Franks, and D.P. Croft, editors.  2015. Animal social networks.  Oxford University Press.  Oxford, United Kingdom.  288 pp.

Turner, R.A., N.V.C. Polunin, and S.M. Stead. 2014.  Social networks and fishers’ behavior: exploring the links between information flow and fishing success in the Northumberland lobster fishery.  Ecology and Society 19(2):38.  

Wey, T., D.T. Blumstein, W. Shen and F Jordan.  2008.  Social network analysis of animal behaviour: a promising tool for the study of sociality. Animal Behaviour 75:333-344.