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The rat catchers: Researching animal migration in Mexico

July 29, 2015
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Mexico---Bea

TAPACHULA, Mexico – Most people wouldn’t dare step off the trail and climb several feet down a steep ravine covered in thick foliage  – excellent cover for the poisonous snakes common here in the mountains of southern Mexico.

But it doesn’t seem to bother Beatriz Otero-Jimenez. Wearing tall rubber boots, she fearlessly wades into the weeds to do something that many would find just as terrifying or repulsive: catching rats and mice.

“I worked with frogs before I came here, listening to their calls. This is my first experience handling animals in the field,” said Otero-Jimenez, a third-year doctoral student in ecology and evolutionary biology at the University of Michigan.

This summer, Otero-Jimenez is in the field studying how the rodents move in this coffee-growing region in southern Chiapas state, close to the border with Guatemala. Specifically, she’s interested in whether the coffee fields inhibit or help the migration of the animals. She’s wondering if the mice living in the fields are different from the ones in the forest.

Most of the tropical rain forests across the world are fragmented, with patches of forest separated by farm fields. This poses a serious problem for organisms because moving across farm fields – especially those with pesticides – can be challenging for them. If the migration stops, species can experience inbreeding and eventually become extinct.


Every morning at 6 a.m.,  Otero-Jimenez sets out to check the 120 traps she has set in the forest and coffee fields. The traps look like small metal shoe boxes. Raw oatmeal is used as bait, and a spring-loaded door traps the rodents after they enter the box.

With her hair pinned back with a thick head band, she quickly moves down the slippery muddy trails with a sure-footed shuffle step. Little orange ribbons tied to tree branches mark the spots where she has placed her traps.

The first few traps are empty, so she dumps out the oatmeal and puts the box in her backpack.

“Out of 100 traps, I usually catch three to four animals,” said Otero, whose hometown is Bayamon, Puerto Rico.

One trap set a few feet off the trail is rocking slightly – a sure sign that something is inside. Otero-Jimenez’s assistant, Angel Vasquez, picks up the trap, opens the door and quickly puts the animal in a large Ziploc plastic bag. The animal puts up a fight, running in place, trying to get out, but it eventually gives up and stays still.

It’s a deer mouse, similar to the ones Otero-Jimenez practiced catching in a Michigan barn before she came to Mexico.

She weighs the animal and uses the flame from a cigarette lighter to sterilize a pair of surgical scissors and tweezers.

As she slips on a pair of light blue latex gloves, Vasquez gently grabs the rodent by the loose skin behind the neck. With the speed and precision of an expert surgeon, Otero-Jimenez snips off a tiny piece of ear and uses the tweezers to put the DNA sample in a labeled vial. She uses GPS to record the coordinates of each sample.

When the mouse is released, it scampers down the trail before taking a hard right turn and disappearing into the bushes.

On a different part of the mountain, another U-M student, Zu Dienle Tan, is checking the 72 traps she has set for rodents. She is researching the diversity of rodents  – how communities of animals change in composition and abundance.

After she traps an animal, Tan records its weight and gender. She also clips some fur off its back for DNA analysis.

Graduate student Zu Dienle Tan cleans scores of rat traps each day.

Graduate student Zu Dienle Tan cleans scores of rat traps each day.

Handling the rodents doesn’t bother her. “I don’t mind them at all,” said the student from Kuala Lumpur, Malaysia. “I think they’re pretty cute.”

Tan traps her rodents at two coffee-growing sites. One has thick vegetation while the other has half as much foliage because workers clear the land and use herbicides.

So far, Tan has trapped more rodents at the site with less vegetation – something that surprises her.

“We suspect that the site is closer to the forest,” she said.

Otero-Jimenez thinks that there might be more mice because there might be fewer snakes. When workers clear the land, they destroy the reptiles’ habitat.

Before the graduate students left for Mexico, they got a rabies vaccine – a three-injection process required by U-M for people handling animals in university-funded research projects.

It was a wise precaution because one rat bit Otero-Jimenez.

“I went to get checked and the doctors said mice are rare carriers of rabies and there haven’t been any cases in the area,” said Otero-Jimenez, who also consulted with U-M Health Systems.

“I told my parents I got bit, and they freaked out a bit,” she said.

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