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Book Review

Sharks Have Friends Too

Papastamatiou YP, Body TW, Caselle JE, Bradley D, Freeman R, Friedlander AM, Jacoby DMP. 2020. Multiyear social stability and social information use in reef sharks with diel fission-fusion dynamics. Proc. R. Soc. B. 287:20201063. http://dx.doi.org/10.1098/rspb.2020.1063

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2020.1063

An Unexpected Hunting Method

A grey reef shark. (Wikipedia)

Often sharks are portrayed as animals that live, hunt, and swim alone far out at sea. In reality, many shark species are associated with coral reefs in shallow water and can be seen in large groups. Grey reef sharks are one of these many shark species that spend the majority of their time near reefs. They are what are known as central place foragers. This means that they will spend a lot of their time in one, central place, then leave that place for a given period of time to hunt, and return to their original spot once they are done hunting, They do this either in small groups or individually. While central place foraging is common in many socially complex species, usually mammals and birds, it is a relatively undocumented hunting strategy for sharks. There is a lot to learn about grey reef sharks as central place foragers, and Dr. Papastamitou and fellow researchers sought to answer questions about the movement of these sharks and what motivates them to hunt the way that they do.

 

An aerial view of Palmyra Atoll. (Britannica)

Tracking and Modeling Shark Movement

The team of researchers traveled to the Palmyra Atoll in the Central Pacific Ocean, a known habitat of about 8,000 grey reef sharks.  They were able to put acoustic tracking devices on 41 of these sharks. To record the location of a shark, acoustic tracking devices will release a high pitched ping that is detected by receivers placed at different locations around the reef. The receivers can detect tagged sharks within 300 meters, or about 3 football fields away. The researchers were able to tracks these sharks as they moved around the reef for 4 years. Researchers were even able to attach cameras to 2 individuals and get a full visual account of the sharks’ behaviors for 2 days.

With all the data about the sharks’ movements, researchers were able to design a mathematical model to predict how the sharks would move under different circumstances. A model is a way to describe a system and the way it changes using a system of equations. By changing different parts of the equations, the research team was able to see how the numbers and locations of sharks would change over time depending on certain initial conditions, such as how many sharks there are, how much prey is available, and how successful they are when they hunt.

A school of grey reef sharks headed out to hunt. (Creative Commons)

What Did They Find Out?

By looking at their movement, the scientists saw that there were 5 main groups of sharks that would stick together during the day. During the night time, sharks often went off to hunt either individually or in small groups, but gradually came back to their original spot by mid-morning.  Both their real life data and their math models showed that these sharks benefited greatly from returning to their central spot. This was because they had knowledge of where food would be located in the reef, and were able to learn information from other sharks about where they had successfully hunted down a decent meal. Individuals were able to give cues to their fellow sharks about their hunt, which made the group more successful as a whole.

The research showed that grey reef sharks are strategists that plan before they head out to hunt. Such complex social behaviors are usually associated with more “intelligent” animals like mammals and birds.  Often sharks are portrayed as blood thirsty, lone-wolf killers, but the hunting strategy witnessed during this study defy those stereotypes.

Reef sharks help keep coral reef communities healthy. (Creative Commons)

Sharks as Regulators 

Sharks, especially reef sharks, are a vital part of a multifaceted ecosystem. They help keep fish populations balanced and are a key part of maintaining healthy oceans. Learning about the 5 distinct groups of grey reef sharks in the Palmyra Atoll and about shark social behaviors more generally will allow scientists to work with policy makers to protect their habitats.

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