Copters, Darts and DNA

The crocodile raced for the muddy water of Mozambique’s Urema River, its short legs sputtering, its long tail making rapid s-curves as Ryan Long leaned from the open door of a moving helicopter aiming a dart rifle.

The dart hit the soft tissue along the side of the croc’s tail, but the animal kept going, striking the water with a splash, leaving only a wake where it left the bank and submerged. The biopsy dart, meant to grab a small piece of the crocodile’s skin and fat from which biologists would collect DNA, did not rise from the murk.

The helicopter moved on with Long, a professor of wildlife sciences at the University of Idaho, reloading and scouting ahead as the chopper skimmed the surface of the savanna.

For his ongoing research, Long spends portions of his summers in Mozambique’s Gorongosa National Park, home to herds of antelope, cape buffalo and elephants, where 16 years of civil war nearly destroyed the diverse ecosystem. During his latest research season Long chased animals with a helicopter to gather DNA that would be used in two separate projects coordinated with researchers at other universities.

“The goal for one of the studies was to collect a tissue sample from a male of as many different species in the park as we could. Those data will be part of a genomic mark-capture analysis designed to estimate population size and key demographic parameters of Gorongosa's large-animal populations as they continue to recover from near extirpation during the Mozambican Civil War,” Long said.

Long and the helo crew also chased bushbuck, attempting to collect tissue samples from a large number of the small African antelope for a Princeton University research project that explores the genetic underpinnings of predator avoidance behavior.

“We set out with those two goals in mind,” Long said. “We did all the biopsy darting from a helicopter, flying all around the park.”

Biopsy darting uses a colorful dart with a conical cutting tip that, after striking an animal and collecting tissue, falls out and is usually easily seen from the air and retrieved. The darts don’t inject anything into the animal or use any kind of drugs in any way, Long said.

The small skin plugs collected with the dart are preserved and sent to the lab to be analyzed.

“When the only thing you need from an animal is a genetic sample, this works pretty well,” Long said.

Researchers often immobilize animals using chemical darts when they need to collect other samples, including hair, blood, and various measurements.

Long said he spent about five hours last summer darting animals solely to collect DNA. Although the effort produced many dozens of samples from nearly 20 species, attempts to get crocodile tissue samples were unsuccessful.

"The challenge there was that crocs always lay up right next to the water, so it's really difficult to get close enough with the helicopter to dart them before they dive back into the river, and to have the dart bounce out in a place where we can safely recover it,” he said.

Long tried twice to dart a crocodile.

“The first time I missed. The second time I made a good shot but the dart stuck in the thick skin rather than bouncing out immediately, and so the croc was back in the water, with the dart, within about two seconds. After that we decided not to try any more crocs.”

Article by Ralph Bartholdt, University Communications.

Photos and video footage provided by Ryan Long.

Video by Rio Spiering, University Visual Productions.

Published in February 2024.