Starting off at Palo Verde

By Finote Gijsman

It’s been two weeks since the start of the OTS Tropical Biology on a Changing Planet program in Costa Rica, and I can say that it’s already been quite a journey. The first field site we went to was the Palo Verde National Park in the Tempisque Conservation Area. Palo Verde is home to the endangered tropical dry forest ecosystem and a large marsh ecosystem that attracts many species of birds year-round.

On our first few days in Palo Verde, we familiarized ourselves with the park and its different ecosystems. Prior to the program, I didn’t really know much about the tropical dry forest, so it was interesting to learn about and experience it first-hand during our walks. Considering that this ecosystem is highly threatened due to habitat conversion by humans, I feel very privileged to have the opportunity to study one of the last significant fragments of tropical dry forests in Central America. This ecosystem is considered a “tropical dry forest”, because precipitation is highly seasonal and more limited relative to other wetter areas in the tropics. With long dry seasons that last several months, many plants shed their leaves to conserve water. We, however, were there in the middle of the wet season, so we were lucky to see many birds using the marsh. These included the green heron, boat-billed heron, and black-bellied whistling duck.

Later in the week, we hiked up to a lookout point in the park for the sunset. Along the way, we were greeted by a pair of white-headed capuchin monkeys. The two were resting in the canopy and did not seem too happy to see us, as they flared their teeth at us. Once we made it to the top of the hill, we were able to briefly watch the sunset over the wetlands, after which we rushed back to the station to avoid the evening showers.

In the evening, we got a talk by one of the researchers at the station, Davinia Beneyto, who is studying the crocodile population of the Tempisque River basin. The male: female sex ratio of American crocodiles in this region appears to be very male biased, with approximately 3.4 males for every female. The cause for this skewed sex ratio has not yet been determined, but Davinia and her colleagues suspect that the use of a hormone, 17α-methyltestosterone (MT), in nearby tilapia farms may be the reason. Tilapia farms around the national park are known to raise fish using food that is laced with the hormone to convert females into larger, more profitable males. With these crocodiles being apex predators, it may be that fish from the nearby tilapia farms are being eaten by the crocodiles at the Palo Verde National Park and in turn affecting the crocodiles’ endocrine system. Such findings may have large implications for the vulnerable American crocodile population at the Palo Verde National Park and possibly even Costa Rica as a whole. The biased sex ratio may be even more startling, because climate change is predicted to affect the crocodile sex ratio in the opposite direction, by favoring females. Crocodile nests are known to be temperature sensitive, with the sex of the baby crocodiles being determined by the temperature of their nests and not genetically. Hormones may therefore be driving a major shift away from what is naturally predicted by regional warming at Palo Verde.

I personally found this crocodile research program fascinating. I had never really heard about temperature dependent sex determination, but to learn that what was being observed was the opposite of what was expected with climate change, was even more appealing to me. Being a plant biology major, I have mainly been exposed to the way research works within that field of study, so I came into this program hoping that I would get to explore other fields in biology and learn about how the research methods within them. While the program has been field-work intensive, I have really enjoyed getting to branch out and learn about other systems. It has been very refreshing to get out of the organized lecture/classroom setting and be able to interact closely with professors specializing in many different subjects.