Shark Sense

December 5, 2013

Scientists are finding that some animals have a sense of where they came from – a homing device of sorts – and often return to their birthplace later in life.  You may be familiar with this behavior in salmon; the migration of salmon from the ocean to their freshwater spawning habitat is one of the most extreme in the animal kingdom!  This behavior is termed “natal philopatry”, which refers to animals returning to their own birthplace to give birth to their young.  In the marine world, natal philopatry has been documented in salmon, seals, and some sea turtles; but for the first time, the phenomenon has been recognized in lemon sharks. 

What is a lemon shark?

“A lemon shark is a large coastal shark, mainly found in the western Atlantic,” says Kevin Feldheim, A. Watson Armour III Manager of the Pritzker Laboratory for Molecular Systematics and Evolution at The Field Museum, who did his dissertation on these sharks.  “They are about as sharkey-looking as they come, and are called lemon sharks because their bellies are light, almost yellow, like a lemon.”

For the past 20 years, Field Museum scientists and collaborators in the Bahamas have been tracking female lemon sharks from birth through sexual maturity, and have found that some females do return to their birthplace to give birth to their pups. 

Each year, young sharks 0-2 years old, are collected and tagged using the same microchips that pet owners can place in their dogs.  The microchip acts as a collar of sorts, and can provide information about the identity and age of the animal.  In addition, a piece of the shark’s fin is taken for genetic analysis at the Pritzker Laboratory, right here at The Field Museum. 

Using DNA from the fin, Field scientists were able to create a database and determine family relationships, based on the similarity of the DNA sequences.

“We use a program that will create a parental pedigree for us, using the DNA of the newborn sharks,” said Feldheim.  “We can tell if a baby shark belongs to a female whose DNA is already in our database, and these relationships are confirmed by catching the animals.  For example, in 2008, we caught and tagged a pregnant female, and four years later, we caught several of her babies.”

In total, the study documents six examples of natal philopatry in lemon sharks in the Bahamas. 

While it has been suggested that this may be true for many shark species, shark survival is generally low, and most sharks take several years to reach maturity. Thus, datasets like this take decades to collect, making this study remarkable, and the first of its kind.

Scientists hypothesize that there must be some evolutionary advantage for this behavior.  While it is hard to know for certain, one hypothesis predicts that females return to their birthplace because they know it is a reliable place with few predators to give birth to their young or lay their eggs, based on their own survival and reproductive success.

“These findings highlight the need for effective conservation measures for highly migratory sharks, including the protection of specific coastal habitats and the restriction of shark fishing at nursery sites,” says Feldheim.  “What is novel about our findings is that nursery areas play a much stronger role in supporting local populations.  So, conservation of local populations needs to occur at a much finer scale (in addition to the needed larger regional scale).”

Explore more about The Field Museum's study of all kinds of DNA in the Pritzker DNA Lab.

Video (c) Duncan Brake