Capture-mark-recapture methods can be applied to any population of unknown size—from flat-tailed lizards to undocumented immigrants crossing the border.
I got my first summer biological field internship when I was 20. The job was collecting data on threatened flat-tailed horned lizard populations which would help biologists conserve the flat-tails. I was thrilled.
Our summer housing was in El Centro, California—near the U.S.-Mexico border. I drove there from western Kentucky. It was a hot 28-hour drive, sticky and close until Texas, then dry and baked from New Mexico onward. To save gas money, I drove with the windows down instead of using the AC. The hot wind barely cooled me down, but I held my left arm out the window as I drove, flattening my hand and letting the air whip it up and swat it back down. I listened to the same CDs over and over.
When I crossed into California, my eyes widened at the sand dunes on I-8, just west of Yuma. I didn’t know there were sand dunes in the United States. They were like pillars lining a gateway for me. I laughed as I passed through them.
The first day on the job, our boss, Jesse, told us to all jump in a full-size white diesel truck. He said we would train for surveying flat-tails at the Yuha Desert site. There were six of us interns. I can remember all their faces but none of their names.
I was crammed against the window in the backseat, watching the city and suburbs pass. No one talked, and the air felt hot and close. Jesse flipped on the radio, and mariachi filled the truck. Jesse said this was his favorite radio station that came over the border from Mexico. I had never heard Spanish-speaking radio before, but I knew some Spanish from high school and college. I strained to understand the lyrics.
After an hour, we pulled off the highway onto a road of sand, and we began to see things I had never seen before. We passed trees wreathed in pale green wisps instead of leaves (tamarisk—an invasive species that steals water from rivers and canals I later learned). We passed black, thorny spindles rising out of a black, thorny mound. We passed dead husks of plants, half-buried in the sand, that looked like wicker bird cages. And between all these spread bare sand and golden-dead grass tufts and more bare sand.
Then my attention fell into the rows of even-spaced, tar-green bushes that passed, passed, passed. They were all five to six feet tall, had thin branches with tiny leaves, and crouched on mounds of sand. It was like watching corn or soybean fields passing on the highway: my eyes unwillingly found the rows, the diagonals, the perfect order of the bushes, and I became nauseous. So I tried to focus on the road ahead and only snuck glances at the rows and columns.
I wanted to know what everything was, but I didn’t want to show how amazed I was. Or how little I knew. I had already shaken my head many times to answer if I knew how or why or what. Did I know how to drive on sand? (I learned the trick was to go fast and not slow down or your truck would sink and get stuck). Did I know the signs of dehydration? (Once I learned, I obsessively monitored the color and clearness of my pee at every restroom and behind every bush). I was already tired, already scared by the fact that I would be working for a whole summer with five people I didn’t know.
We stopped at the bottom of a hill. We all got out. Everyone put their backpacks on, so I did too. No one looked at each other. Jesse fiddled with his GPS unit. I looked at the ground, kicked at the raised line of sand that formed the road’s curbs. Then Jesse handed the GPS to an intern with brown hair and a pale, thin face and asked her to guide us to the sampling plot.
We didn’t go far, just up a hill and down its other side. The sand made it difficult to climb. At the top of the hill, I saw again those even-spaced bushes spread out like a grid over the flatness of a valley. I asked Jesse what they were.
“Creosotes,” he said, not looking at them or me. “They’re everywhere. Sometimes the flat-tails run into holes at their bases. Got to watch out for that.”
When we arrived at the survey plot (in the middle of the creosote field), Jesse told us to make a line with three of us, each five meters apart, and walk and swing our heads slowly back and forth, scanning the ground.
Back at headquarters, Jesse had shown us pictures of the lizards, their tracks in the sand, their scat (black like shiny, ripened blackberries formed from hundreds of indigestible ant heads and limbs mashed together in the lizards’ stomachs). He had even shown us tracks that could be mistaken for flat-tail tracks, and the creatures that create them—darkling beetles scuttling over the riffled, inch-high dunes or kangaroo rats bounding into their burrows. But looking for them outside, with the tracks of everything that lived in the desert fresh from the night, their shadows in the morning sun longer and deeper than I thought beetle or mouse tracks could be, all tracks looked like letters cast over the sand—knowable one-at-a-time but illegible together.
Suddenly, the pale-faced intern to my left called out that she found one. Jesse told everyone to huddle up and take a look.
The sun had crested the hill to the east, and although it was just past sunrise, the heat pushed hard and flat on my face. I pulled my wide-brimmed straw hat down to shield my eyes as I looked at the lizard. Everyone leaned in close.
The flat-tail was smaller than I expected. It was shorter from nose to tail tip than my hand, about four or five inches. Its scales were the same gray-beige as the sand. Its body was wide and round like a sand dollar, and its four legs splayed out with short, sharp claws on each toe. It had horns in a crown along the back of its head. Its face looked like a papery-skinned, tight-lipped grandfather. The eyes were bright and round, and they were tilted up, looking at us.
Jesse told the intern good job, bent down, and scooped the flat-tail off the sand, cupping it in the palm of one hand. The lizard didn’t flinch or try to escape. One clawed foot poked out of Jesse’s fingers, but there was no thrashing, no sound.
I asked if it was always so easy to pick them up. Jesse said, yes, pretty much. Their defense strategy was almost completely based on camouflage. They often didn’t move even when growling, hot-oil-smelling OHVs and ATVs skidded through washes and ramped off old creosote mounds nearby. So the lizards, sunbathing or searching for ant mounds, would be crushed, ground inches into the sand. He told us that was part of why the flat-tails were disappearing.
Jesse flipped the lizard onto its back and pointed to its white, flat belly. He drew two parallel lines down the center, a half or quarter of an inch from its rounded sides. Jesse said all its ribs and innards were in there. The outsides of the belly were just air and water reserves contained by a separate set of “ribs” that stretched its skin flat and taut over the sand. Looking at it face-first, the flat-tail had the profile of a low desert mountain in the distance—its spine the peak and its wide sides the gentle slopes. Jesse told us the geometry of this profile did not let a shadow form on the sand, and along with the lizard’s coloration and stillness, it was almost invisible to predators.
“This one doesn’t seem to have a number on its belly, but we’ll have to check it for a PIT tag to make sure.”
Then he unzipped the lunch cooler and pulled out a flat blue plastic rectangle with a digital screen and some buttons. It was twice as long as the lizard. He tapped a button, the screen blinked on, and then he passed the device over the lizard’s belly. Nothing happened, and Jesse put the device away and told us that meant this lizard hadn’t been tagged.
Handing the lizard to another intern, Jesse bent back over the cooler and pulled out a large needle-like instrument, a bag of what looked like rice, a bottle of iodine, and a small tube of superglue. He dabbed the right outer belly with a cotton swab soaked in iodine, staining half the belly a sickly brown.
He pulled on the gloves and asked one of us to dip the needle in alcohol. Holding one gloved hand up in front of him and pointing to the rice bag with the other, he told another intern to open the bag of PIT tags. Jesse reached into the proffered bag and selected a grain with his gloved thumb and index finger.
Holding the grain up, Jesse told us it was a PIT tag. Each tag has a unique number identifier. We would use the needle to insert the tag into the lizard so we would be able to identify it if we recaptured it. That would allow Jesse and the other biologists researching the lizards to estimate the how many flat-tails were out there.
Jesse took the needle, still slick with alcohol that hadn’t evaporated, and slid the rice grain into the tip. The tip was hollow and just big enough to hold the grain in place. Its end was diagonally-cut so that one side came to a sharp, fine point and the other was rounded, making a tear-shaped opening. Slowly, Jesse turned the needle so all of us could see how the PIT tag fit in.
“Now this is the hard part. It gets easier with practice, but you got to do it right the first time.”
Jesse took the lizard from the technician. He positioned the needle over the outer left side of its belly, above the iodine blotch. He told us to remember the empty spaces on the outside of the belly, with nothing but air and water. The needle had to get in there.
Then he slowly but firmly pushed the needle into the lizard. The lizard made a small kick, but was still again. Jesse depressed the top of the needle and the PIT tag disappeared into the lizard’s belly, like it was being swallowed, like it was swallowing a pill.
When Jesse pulled the needle out, the wound, a dark gash in the murky iodine, did not bleed. I could see the lizard breathing, but I couldn’t tell if it breathed harder or faster. One of the interns had turned away and was holding his abdomen. I felt the blood draining from my face.
Jesse took the superglue and squeezed a bead onto the hole. He bent over the lizard and blew on the glue once, twice, again. He unbent and watched the lizard for a moment, then took a sharpie pen and wrote a number “1” on the unstained side of the belly. He got up, walked over to a nearby creosote, and placed the lizard on the sand under the bush. He turned and walked back to us, not looking back at the lizard. I could hardly see the lizard now, even from just a few meters, even though I saw exactly where Jesse had placed it.
We went back to searching. Eventually, I found one, and Jesse walked me through the PIT tag insertion.
It was easier than I expected. I tried to think of why I was doing this—to help flat-tails. I didn’t flinch when I broke its belly, although I shivered on the inside, near where I thought it would be if a giant hand held me and was inserting a needle the size of a PVC pipe into my abdomen, just above my pelvis, where there would be room to eject a tracker the size of two thumbs wrapped together. The lizard didn’t flinch either.
I put the lizard down, near an anthill under another creosote, with the number 4 on its belly. I stepped back to watch it. I wanted to make sure it was okay. It held its head up, just as it had in my hand. It elbows and palms pressed into the sand. But it still didn’t look at me. After several moments with no movement, I turned to resume the survey.
I said I was half Mexican. I didn’t tell them that half was from my biological father who left before I was born and that I didn’t know his name.
Two weeks into the summer, I was very tan. The back of my neck darkened into the color of wet sand. My arm hairs bleached in the sun. After a few weeks, I didn’t wear sunscreen except on my nose and the tops of my ears.
My coworkers had to apply sunscreen every day. They kept a communal bottle by the front door of the house we shared. In the morning, they would stand by the door and rub it into their foreheads, their fingers and forearms while I yawned and twirled my straw hat, waiting for them with the sky still dark, the sprinklers hissing and clicking in neighboring yards, the air almost cold, granola and yogurt still heavy in my stomach.
One morning, my skin color came up, and I said I was half Mexican. I didn’t tell them that half was from my biological father who left before I was born and that I didn’t know his name. I had never asked, and my mother—who was white—didn’t tell.
They said it must be nice to be so dark and not have to worry about the sun. I smiled, looked away, rubbed the back of my neck. Yes, it was, I said. I didn’t know what else to say.
Three weeks into the summer, we finished with the Yuha, and moved on to our next site, right on the border. Our GPS marked the line down to the centimeter and how the gravel road we ground down followed in perfect parallel.
We were casually proficient in our jobs: we pulled up to the site, got our equipment out, and began the survey. Our first sweep went within a few meters of the border. There was no fence or marker—the desert on both sides was rocky and dirty and pocked with creosote.
Thinking it would be funny, not thinking of what it meant, I threw a rock over the border, yelling “Take that, Mexico!”
My crewmates laughed, but none of us crossed the border with our bodies.
On our second sweep, we heard vehicles approaching.
Two pastel green trucks pulled up, hissing to a halt on the sand road.
“Border Patrol,” a coworker said.
But I was from Kentucky and had never seen the Border Patrol. I didn’t even know they had trucks. Without saying we should or would, we all walked to our truck. Everyone else stood with their arms crossed near the truck, but I leaned on the corner near the tailgate.
The agents got out of the trucks and sauntered toward us. One looked Hispanic, but the rest were white. They all wore ballcaps with the Border Patrol insignia and wore jackets even in the heat.
“What are you all doing out here?” one of the white agents asked.
We told him we were with the BLM, surveying for lizards.
I didn’t know why they asked. Our truck had federal tags, and they must have seen it as they passed. I shifted around to the back of the truck and put my shoulder pits over the tailgate wall and clasped my hands.
“Okay, well, you’ll need to get out of here. We’ve got a group of illegals trying to cross the border here, but with you here, they’re going to keep hidden and not move. And we don’t want them to get heat stroke and die out there. If you leave, they’ll cross, and we’ll take them.”
Without looking at each other, we climbed back into the truck and we drove off. The agents did the same.
As we drove, I looked to see if anyone crossed the border. I wanted to know they would cross and be okay, maybe that they would not be caught. But the farther we drove, the harder it was to see where the borderline might actually be, let alone people darting like lizards through the desert.
Five weeks into the summer, I finally asked Jesse how it was we were actually helping the flat-tails. Sitting at his desk, he handed me a one-sided printout of someone’s master’s thesis. I read it that night by headlamp, lying on my cot in the stuffy upstairs room I shared with another guy.
I read that capture-mark-recapture—or mark-recapture for simplicity—is a statistical method for estimating the size of a population of interest. It requires researchers to capture subjects, release them, and note when subjects are recaptured or if new subjects are caught—exactly what we were doing with the flat-tails. It’s a standard technique for estimating wildlife population sizes, particularly endangered species. My coworkers and I were part of a multi-year effort to monitor flat-tail population sizes and make sure they weren’t disappearing.
That made me feel better. Catching flat-tails was getting repetitive, and I still didn’t like sticking them. The day before, I was pretty sure I hurt one with the needle. It had bled a lot when I pulled the needle out, but like the others, it didn’t flinch. The superglue had turned reddish-pink when I sealed up the hole. I folded the thesis, clicked off my headlamp, threw them under my cot, and tried to go to sleep.
Later, I learned mark-recapture methods can be applied to any population of unknown size—including undocumented immigrants crossing international borders.
The F-14 fighters pulled apart with a thunderclap—a wave that riffled my shirt and filled my chest.
Nine weeks in, we began the Superstition Hills site. Jesse warned us about the Superstition site: there was an Air Force base on the eastern slope (we were surveying the western slope), so we needed to watch for unexploded ordnance, like white phosphorus cannisters that when disturbed would burn through clothing, shoes, and flesh—and water couldn’t put it out. Sometimes the military trained in the area, too.
The site was in a low wash, and the Superstitions—low, brown-rocked, bare-backed desert mountains—rose to the east. Their pinnacles were like fingers, and when the sun rose, they looked like they were cupping the rays. As the morning went on, the saddles would redden like flesh when strong light pushes through it. Then the sun would crest and spill out onto the wash and our shadows would appear, moving through the identical checkerboard of the creosotes’ shadows.
But we saw few flat-tails at the Superstitions, the fewest of the sites we surveyed. When we asked, Jesse said flat-tails had been getting rarer and rarer there.
A week into our Superstition site surveys, we had only recorded three flat-tails, and I was getting a little desperate. I wanted to find as many as I could—to know they were still there and would survive. I searched harder, concentrated more on every track and movement in the sand than I did at any other site.
So when I heard whining thunder one morning, I started and looked up. The sky, partly cloudy that morning, had turned overcast. Everyone had stopped and was scanning the hills. I waited for a jet to arc over the hills, but nothing.
I continued my survey. The whining grew again, but I tuned it out so I could focus.
Even so, I almost stepped on a baby flat-tailed horned lizard. My boot toe was an inch from it, but it hadn’t moved. It was lighter than the sand. My heart was thumping. The whining reached a crescendo.
I called out and two of my coworkers joined me. We didn’t tag babies; the PIT tags were longer then their heads and wouldn’t fit between their rib sets. I picked it up and laid it in my palm. It didn’t try to escape.
As one of my coworkers recorded the baby on our datasheet, I pulled my digital camera from my pocket and took a photo. I was ecstatic to have found it. I felt it meant the flat-tails might make it at least one more season in the Superstitions. When my coworker finished the data entry, I put the baby on the ground, next to the boot print where I’d found it, where the sand had formed about its tiny hands and belly and tail, and we continued on.
A few minutes later, the whine became thunder, and we snapped our heads back to see two jets drop from the clouds. They were F-14 fighters, with gray bodies, black cockpits, and needled noses. They pulled apart with a thunderclap—a wave that riffled my shirt and filled my chest.
The jets flew low, weaving around each other and coming apart. They flew away and came back. They showed us their white bellies and black cockpits. I was afraid and wished for shelter.
One of the girls waved at the jets, jumping up and down, laughing. When they passed overhead again, she shouted back at us that she thought they were looking at us.
Then the jets shot back into the clouds and the thunder echoed into silence. The wind had died too.
By the end of the summer, I was ready to return to the green humidity of Kentucky.
I felt good driving in the early morning, the car packed, sandwiches in the cooler. The sand dunes, the same ones I had passed in the beginning of the summer, felt like an honor guard as I passed and left them behind.
A storm hung black over the oncoming desert mountains, and I saw lightning miles away. I was excited to pass through the storm. The hairs on my arm rose when I heard and felt the rain torrent begin pounding against my car.
I was at the California-Arizona border in a few hours, my car beaded with water and gleaming in the newly visible sun. The desert had already dried out except for the ditches, brown and green with sitting water.
Then I hit my breaks. I had noticed both lanes were nearly stopped in front of me. I jerked forward and felt my seat jolt as the cooler smacked into its back. I skidded to a stop just a few yards behind the next car and let out my breath.
The other vehicles were beginning to merge into a single lane, red brake lights flashing with the stops and goes, the ones far ahead tinted purplish through the glass of the cars ahead of me.
The highway had been blocked with cones, but I couldn’t see any construction. Then I saw vehicles exiting down and to the right. I got onto the ramp, one foot tapping to the radio and the other tapping the brake. I turned left a bit and craned my neck to see what was happening. Then I saw that pastel green and the symbol of the Border Patrol.
I wondered how long this checkpoint would hold me up, though at least the cars ahead of me seemed to move through quickly. I looked at the ramp wall. It was a decorative mosaic, depicting layers of soil and fossils and trees and plants and animals that had lived here millions of years ago. I didn’t know any of their names.
Soon I was two cars away from the checkpoint, and I could see three agents, two blondes and a black-haired, mustachioed pale man. One of the blondes approached the sedan’s window with a clipboard. He was smiling as he spoke, seemed to laugh, and wrote nothing down as he waved the driver on. The mustachioed man waved me forward.
My heart was beating in my ears and my stomach felt heavy. I had my driver’s license in my wallet in my front pocket and nothing illegal in the car. I put on a smile.
The blonde agent stepped up to my door and waited as I rolled my window down, looking at me, looking away, looking back at me. His uniform was neat: shirt tucked in, ball cap with the Border Patrol symbol pointing straight forward. He was cleanshaven, maybe in his early 30s. But he didn’t smile at me like he had with the sedan driver. He looked at me closely, then peered into the backseat.
“How we doing today?” he said, eyes searching the backseat, the passenger seat floorboard, the car roof.
The other agents were walking behind my car; I could see them in the rearview mirror.
“I said, how we doing today?”
My eyes snapped back to the agent. His eyes were fixed on mine.
“Uh, fine. I’m fine,” I said.
I felt afraid. My voice sounded strange and scratchy. I wondered if he thought I had an accent.
The agent looked down at his clipboard. He wrote or scratched something out.
“Okay, you’re good,” he said, waving me forward without another look.
I pulled forward, drove up the ramp, and merged back onto the highway. The Arizona desert accelerated around me, cholla and creosote passing as distinct shapes, then doubles and triples, then blurring together between blinks of the eye. I was no longer shaking, and my heart had stopped racing. But I kept trying to track individual cactus, each creosote, watching, picking them out, and then following them as I passed. Doing this, though, I got nauseous, so instead I looked ahead at the white line that seemed to tie me to the road like a train track.
I turned the radio back up and rolled down the window. The wind was hot. It didn’t make me feel better, but I hoped it would distract me. I hoped there would be no more checkpoints for the rest of the drive. I was pretty sure there wouldn’t be.
Caleb Roberts is an ecologist studying ecological resilience, landscapes, and invasive species. He has published scientific articles in Nature Climate Change and Biological Conservation and creative nonfiction at The Fourth River. Caleb is from Kentucky but lives in Arkansas with his wife, daughter, and cat.