Catching Squirrels to Study Them

by Jane McMillan-Brown

Why would we spend a couple of days placing, baiting, and setting 80 traps along six-tenths of a kilometer of trail? What will we do with whatever is trapped? How do we know that only the animals we want will enter the traps? Will they get hurt when the trap springs shut? Will they ever be free again? When? How? Where?

These were some of the questions I had when I learned that this year we would be trapping squirrels. Maybe you have some of the same questions as you come along with me and learn some of the hows and whys of mammal trapping. If you have different questions, maybe you'll find time to send them to me along with your thoughts and I'll share everyone's responses.

Now for some of those earlier questions: First of all, it is squirrels we want to catch for the purpose of marking them for future observation and study. Giving them each their own identification makes it possible to keep track of when they are seen again, where they are seen, (for example, have they moved to another part of the island or are they feeding around the same area?), how healthy they appear to be, and so on. Jackie and Greg have been doing this every other year and observing the squirrels every January, gathering and recording the data. Trapping and marking animals is a complex job that requires great commitments of time, patience, and diligence. Once the traps are actually set, they must be checked frequently to safeguard the well-being of the animals. But, here I am getting ahead of myself.

One of the first things we have to do in preparation is to check the traps to be sure they are in good working order. No sense baiting a trap if the door won't shut because the metal parts are rusted. Many of the traps have been left outside wired to tree branches for several years. They are wired open so that no animals will get caught after we end the trapping season. This helps get the animals used to seeing them and smelling them in the forest so that they are not afraid of them. When we go out to check the traps, we know we will be leaving the trails and walking through brush and vine tangles, so we remember to tuck our pants tightly into our socks and tie our boots securely to help protect our legs from the ticks and chiggers. The snakes usually feel the vibrations we make from walking on the ground or see us coming and they get out of our way. They really aren't very interested in us if we aren't a threat to them.

We also have a map that Jackie drew which shows where the traps were the last time they were used. (Have you ever drawn a map, and then given it to someone to find something?) That helps us locate the traps after we've been away from BCI for a long time. Traps sometimes get covered with vines or fallen leaves, and coatis can rip them off the trees and drop them on the ground. Or some traps get crushed by falling trees and need to be repaired or removed from the forest and put in the trash. We can mark on the map any new traps we put out or any that we move to different locations. Once the traps are set so that they will catch animals, it's also very important to have that diagram showing where the baited traps are so that we can be sure to check every one for animals and not miss any. (However, Jackie and Greg usually have all trap locations memorized by the second day!)

While there are no guarantees that other small animals, besides squirrels, won't get into the traps, one way to attract the squirrels is to put some of their favorite fruit in the trap, especially if it's a fruit that they can't get right now, like royal palm fruit. Jackie had some stored in the freezer for just such an occasion so that's what we used. The traps are about 50 cm in length by 20 cm wide by 20 cm high. They are made of steel wire and have a trap door at one end that will spring shut after the squirrel is well past the door. The door won't hurt the animal when it is closing. We place the bait at the far end of the trap so that the squirrel enters the trap to get the fruit, which smells very good to a hungry squirrel.

The trap is set so that once the squirrel walks across a small pan of metal on the bottom of the trap, the squirrel's weight causes the pan to tip, which releases a spring that closes the trap door. Some squirrels get more upset than others and will try to escape. That is why it is important to check the traps frequently once they are set. We don't want squirrels upset and trying to get through the small openings in the wire cage. They could bump their noses and get hurt if they were left too long. We also worry that a predator like a coati might harrass a trapped squirrel and harm it. One of us must walk through the forest several times in the morning and again in the late afternoon when the squirrels are most active and likely to get into the traps. (They often nap in the afternoon heat of the day.)

Okay, by 11 AM on our second day of preparation, we've methodically checked, baited, and set every one of those 80 traps and marked our map indicating each location. After a refreshing drink of water from our portable water bottles, it's time to start back down the trail, wash up, and get to the dining hall for lunch by noon. We figure that we'll come back for the first check right after lunch. But wait a minute! What's that movement over by the trap near the big Dipteryx tree? Looks like something brown moving in the trap! 'Let's go over for a closer look.' 'Wow, a squirrel has already found the fruit.' Our lunch break will have to wait. Squirrel business has to come first now.

We hadn't expected to get one so quickly and we've been caught off guard. Usually, we'd be carrying an old shirt or towel to place over the trap after we remove the trap from the tree branch. (It's fastened on with thin wire.) The covering helps keep the squirrel calm as we carry it in the trap back to the lab for marking. Well, we all laugh about who will have to sacrifice their shirt for the cause. This time, Greg drew the short straw and agreed to take off his T-shirt and cover the trap for the remainder of our walk to the lab Jackie has set up in one of the buildings near the trailhead. I made a mental note to carry an extra, old shirt with me next time so I wouldn't have know.

Once inside the lab, the squirrel is left in the covered trap on a work table while Jackie and Greg get the marking materials ready. This squirrel sits quietly and feeds on the bait while we prepare. I stand ready with my camera to photograph the marking procedure. The entire trap, containing the squirrel, is placed into a large plastic bag and a small cotton ball wetted with an anesthetic is added. Very quickly, the squirrel goes to sleep. This is the only safe way to handle the squirrel and keep from being bitten or scratched, while at the same time preventing the squirrel from being extremely upset. The sleeping squirrel is removed from the trap and placed in a mesh bag, like the ones often used in laundries. The bag is then gently laid on a scale. The squirrel's weight and length are recorded, along with its gender, and any significant markings. Now it is time to give the squirrel its own special identification marking. So guess what we use to mark them? Special jewelry! Yeah, really.

Designer jewelry. Nothing but the best for our BCI squirrels! Colored ceramic beads threaded onto a stainless steel neck chain, custom measured, is carefully fastened around the squirrel's neck and a numbered tag is attached to its ear. Each squirrel gets a different color combination and its own tag number. The ceramic beads won't crack and rot like plastic ones would and the stainless steel chain won't rust and cause problems for the squirrel. Careful notation is made of the tag number and color combination of the beads. From now on, for the rest of its life, this squirrel will be identifiable.

Almost before the tagging is complete, the squirrel begins to stir. We don't use ether, but instead an anesthetic that both acts quickly and wears off quickly, with few side-effects. This first one today is a young male who hadn't been previously caught. Before he is fully awake, he is placed back into the trap for his return trip to the forest. He will be taken back to the same location, the trap will again be attached to the branch, the trap door will be opened and the squirrel will leave it when he's ready. We reset that trap and check all of the others. Believe it or not, sometimes the same squirrels go back into the same trap another day. We only mark them once. After that, we just record their activity and location.

Several squirrels got caught before lunch, so Jackie and Greg have to finish marking and releasing the squirrels before they can take the time to eat. We always return squirels to the exact places where they get caught, especially young ones that could get lost. I agree to go quickly to the dining hall to see if I can save us some lunch before the food is all put away or eaten by hungry researchers just back from their own morning adventures.

Now, you might ask, don't other species of mammals besides squirrels like royal palm fruit? And don't they get caught in these traps? Yes! But this is limited somewhat by using squirrel-sized traps, so big coatis and big monkeys can't fit into them. Occasionally a very pesky and determined coati will squeeze into a trap-- then we have what looks like a cubical coati, all jammed in with its long nose mashed up against the end of the trap, fur sticking out in all directions, and no way to turn around. That's another reason why we check the traps frequently--trapped coatis want to get out NOW! We have caught a whole variety of mammals over the years: baby coatis and white-faced monkeys, and even a baby anteater (they don't eat fruit, but can just walk into a trap by accident). We have also caught 'common' possums, 'four-eyed' possums, pygmy possums, spiny rats, climbing rats, and a porcupine. Sometimes we catch doves. We let them go, take a picture sometimes, and record that they were caught, and where.

This year, after 8 days of trapping, we caught more squirrels than we have in any year since 1979. We certainly don't ever catch ALL of the squirrels: some are too cautious to enter a trap. We left BCI this year with 23 marked squirrels living in an area of about 10 hectares. We saw many of those same animals in our census walks and are using those data to estimate how many squirrels live in the trapping area, and on all of BCI. We've also learned about the lifespans of squirrels, that a female might live eight to ten years and raise many broods of babies, but males often live only three or four years and rarely make it to eight years. Mapping the locations where we have seen or caught a marked squirrel also helps us understand how big a piece of forest they need for their daily activities of feeding and raising a family. Next week we will give you a quick summary of our mammal census results, and sign off for this season! Bye!