- For years, studying the reproductive biology of sharks has depended on capturing the animals and dissecting them.
- Scientists recently developed the Birth Alert Tag, an egg-shaped satellite transmitter that can be implanted in the uterus of pregnant sharks to document the location and timing of births.
- In another development, scientists took ultrasound readings of whale sharks and sampled their blood to detect if they were pregnant.
- The new methods aim to help researchers determine sharks’ pregnancy status and the location of shark births; one goal is to inform the establishment of corridors to protect the animals, which are among the most vulnerable vertebrates on Earth.
At first glance, the device resembles an egg-shaped game. But the birth-alert-tag is anything but a toy. Inside, it holds technology that collects data aimed at informing conservation strategies for sharks, among the most vulnerable vertebrates on Earth.
The birth-alert-tag (BAT) is a first-of-its-kind device designed to be implanted in the uterus of pregnant sharks. The mother shark ejects the device along with her babies when she gives birth; satellite technology enables it to alert researchers about the location and time of the births. A study published in March 2023 in the journal Science Advances showed the BAT works after the researchers who developed it deployed it successfully in two female sharks.
“Discovering where sharks give birth is our holy grail,” James Sulikowski, a co-author of the study and associate director of the School of Mathematical and Natural Sciences at Arizona State University, told Mongabay in a video interview. “It’s crucial information that has eluded us for so long.” Sulikowski developed the BAT along with Neil Hammerschlag, director of the Shark Research and Conservation Program at the University of Miami.
Pregnancy and infancy are animals’ most vulnerable life stages, so accurately identifying where pregnant mothers and newborns spend their time, and when, is imperative to protecting them from harmful human activities. Scientists have long used tags that implant in either the uterus or the vagina to document births in terrestrial animals. But until now, no one had adapted the devices for underwater use due to the design constraints imposed by the marine environment. This meant that studying the reproductive biology of sharks depended mainly on dissecting the animals to inspect their reproductive organs. In recent years, researchers have increasingly deployed satellite tags attached to dorsal fins to track and study sharks and other marine animals. Even so, monitoring pregnant sharks and documenting their births have remained a challenge.
However, recent technological advances have enabled the development of new tools that are less invasive for the animals and easier for researchers, the BAT among them.
Other examples are new techniques for conducting underwater ultrasonography and collecting blood samples from sharks to understand their reproductive state. A different team of researchers recently described developing and deploying these methods on whale sharks (Rhincodon typus) in the Galapagos in a study published in March in the journal Endangered Species Research. The researchers collected blood samples while they swam with the sharks and used a portable imaging system to take the ultrasound readings. All the while, they used underwater propellers attached to their oxygen tanks to keep pace with the moving sharks, study co-author Simon Pierce, co-founder and principal scientist at Florida-based nonprofit Marine Megafauna Foundation, told Mongabay in an email.
According to the study, the researchers found that the 22 female whale sharks they tested were mature but not pregnant. There was no evidence of embryos or egg cases in the ultrasound readings. And levels of steroid hormones in the blood samples were similar to, or lower than, those obtained from immature female sharks in another study.
“Now, we can start investigating their reproduction while they are in the wild,” Pierce said. “It will allow for other studies too, such as looking at stress or pollution levels in their habitats, which really improves our understanding of their conservation needs.”
In fact, performing ultrasounds on sharks was what paved the way for Sulikowski and Hammerschlag to develop the BAT. Six years back, in 2017, the duo started working on an ultrasound device to detect if a shark was pregnant. But eventually they shifted gears to develop the intrauterine tag.
Their research team implanted the tags in a tiger shark (Galeocerdo cuvier) and a scalloped hammerhead (Sphyrna lewini) — both highly migratory predators that Sulikowski and Hammerschlag selected because they had studied the species previously and could tap into their prior knowledge to corroborate the results they obtained from the tag.
According to their study, each shark was “slowly brought to and secured alongside the boat, rolled over on her back, and put into a state of tonic immobility.” Then, guided by ultrasonography (the traditional kind, because the shark’s abdomen was out of the water), the team used a specialized applicator they helped develop to guide the BAT through the shark’s cloacal opening and implant it in her left uterus. Once they’d secured the shark, they got the procedure done in five minutes; the shark was able to breathe the whole time and swam off in apparent good health when it was done.
The BAT’s egg-shaped design serves two purposes: smooth entry into the uterus and the ability to remain inconspicuous to the developing embryos. When the female sharks ejected the tags during birth, they floated to the water’s surface where drying out activated a sensor. The device then started transmitting satellite signals every 15 seconds, alerting scientists to the location and time of the births.
According to the BAT signals, the tiger shark gave birth 40 kilometers (24.8 miles) off the coast of southern Georgia, U.S., 157 days after the team implanted the tag in her uterus in the Bahamas. The scalloped hammerhead, meanwhile, gave birth 12 km (7.5 miles) off the coast of Hilton Head, South Carolina, U.S., 47 days after the team implanted the tag off Cape Hatteras, North Carolina, some 800 km (500 mi) away.
Sulikowski and Hammerschlag are now trying to gather funds to produce the device and distribute it to scientists around the world. They also intend to conduct training to ensure the device is used responsibly.
“We have to train others in the techniques so that they can use it in a way that’s not going to hurt the mom or the babies,” Sulikowski said. “We developed those skills over the last six years, and we now want to give that to the rest of the world.”
Banner image: A researcher takes an ultrasound reading from a whale shark. Photo by Simon Pierce.
Abhishyant Kidangoor is a staff writer at Mongabay. Find him on Twitter @AbhishyantPK.
Sulikowski, J. A., & Hammerschlag, N. (2023). A novel intrauterine satellite transmitter to identify parturition in large sharks. Science Advances, 9(9). doi:10.1126/sciadv.add6340
Matsumoto, R., Murakumo, K., Nozu, R., Acuña-Marrero, D., Green, J.R., … Hearn, A.R. (2023) Underwater ultrasonography and blood sampling provide the first observations of reproductive biology in free-swimming whale sharks. Endangered Species Research, 50,125-131. doi:10.3354/esr01226