Scientists at work: Uncovering the mystery of when and where sharks give birth

Many of the large iconic shark species – like great whites, hammerheads, blue sharks and tiger sharks – cross hundreds or thousands of miles of ocean every year. Because they’re so wide-ranging, much of sharks’ lives, including their reproductive habits, remains a secret. Scientists have struggled to figure out precisely where and how often sharks mate, the length of their gestation, and many aspects of the birthing process.

Until very recently, the technology to answer these questions did not exist. But marine biologist James Sulikowski, a professor at Arizona State University and my research mentor, changed that. He developed a new satellite tag called the Birth-Tag with the help of the technology company

The Birth-Tag is a small, egg-shaped device that we insert into the uterus of a pregnant shark where it will remain dormant and hidden among the fetal sharks throughout pregnancy. This kind of tag has never before been used on sharks, but

After years of fine-tuning this new technology, we launched the first phase of the study in December of 2019 and began deploying the tags. Once the study was approved by the Institutional Animal Care and Use Committees at both Arizona State University and the University of Miami, as well as the Bahamian government, we set out to find some tiger sharks. To do this, our team of researchers from the

The high number of pregnant sharks in this small area makes finding one much easier, but actually catching and bringing a 10-foot-plus shark to the boat is no easy task. We fish for the sharks

Once we catch a female tiger shark, we first take several length and girth measurements to get an idea of her general health and to see if she is sexually mature. Then we check for bite marks, which are evidence of a recent mating event.

After we collect this baseline information, we rotate her upside down to coax her into a trance-like state called

Just like the ultrasounds used on humans, we use a mobile ultrasound machine to figure out if a shark is expecting. I put on a pair of goggles that allow me to see everything the ultrasound sees, lean over the side of the boat, and place the probe onto the upside down shark’s abdomen. The image is usually fuzzy at first as water splashes over the shark and up onto the boat. The team holds the shark still as I slowly maneuver the probe along her belly. Then, if she’s pregnant, something magical happens.

Wriggling baby tiger sharks, up to 40 of them packed tightly together inside their mother’s womb, appear in front of my eyes. The image also appears on a screen held by another team member on the boat, and everyone cheers as they gather around to take a peek into the secret world of unborn sharks. We spy on them as they pump fluid through their still-developing gills, and we watch in awe as they wiggle around, blissfully unaware that anything extraordinary is happening outside in the world. Once we have enough data on the approximate size of the offspring – which gives us an idea of how far along the pregnancy is – it’s time to tag the mama shark.

As I hold the probe as still as possible to keep a visual of the shark’s internal anatomy, Dr. Sulikowski takes the Birth-Tag and uses a custom-designed applicator to carefully insert it into the uterus through the urogenital opening. No surgery required, the tagging procedure is complete in a matter of minutes. Once the tag is inside the uterus, we rotate the shark upright to wake her and release her back to the open ocean. I am filled with hope as I watch her swim gracefully away to continue her pregnancy, with a stow-away Birth-Tag hidden among her unborn offspring.

Last December, we deployed the first Birth-Tags on three pregnant tiger sharks. For tiger sharks, pregnancy is thought to last

AWe are planning future expeditions to deploy many more Birth-Tags, but for now, we’ll just have to keep singing the “Baby Shark” song as we patiently wait for our first glimpse into the private lives of these incredible creatures.