Lizard lessons: Auburn researcher uncovers how early environments shape species across continents

On Auburn’s campus, Morgan Muell conducts part of her fieldwork only steps from classrooms, using a fishing-line lasso to collect green anoles from brick walls. In Costa Rica, her field sites look very different — shaded streams, towering trees and fast-moving tropical lizards that vanish into the canopy at the slightest motion.

“I’ve been a mom to thousands of lizards,” she said. “You have to know them to be able to find them.”

Whether in the Southeast or the tropics, Muell has been trying to answer the same question. Her research as a doctoral student in the College of Sciences and Mathematics’ Department of Biological Sciences, co-advised by faculty mentors Dan Warner and Jamie Oaks, explores how early life environments, especially temperature and moisture, shape the development of reptile embryos and how those responses evolve.

She describes the goal simply: “How does the early-life environment affect the embryos of these lizards, and how do they respond to it?”

Muell studies developmental plasticity, or the degree to which embryos change in response to their surroundings.

“Some organisms are not as sensitive to the environment as others,” she said.

By comparing species across climates, she looks for patterns that reveal when those changes matter, when they do not and how they arise through evolutionary history.

One major project began when travel restrictions during the pandemic halted her planned field season in Costa Rica. Instead, she shifted her work to South Florida, the only place in the continental United States warm enough for many tropical lizards introduced through the plant and pet trades. After collecting hundreds of animals, she tested how their embryos responded to different temperatures.

The results surprised her. Sensitivity to temperature, typically considered to be adaptive, did not appear to influence survival across species.

“It seemed like a lot of the environmental sensitivity probably wasn’t making any difference at all,” she said, noting that much of the variation aligned instead with shared evolutionary history.

Once travel resumed, Muell moved the research back to Costa Rica to test how tropical species respond to moisture in addition to temperature. She worked at La Selva Biological Station, which has a long history with Auburn biologists dating back to retired professor Craig Guyer. At the same field sites where Guyer surveyed reptiles decades ago, Muell collected species from tree trunks, stream edges and the forest floor and incubated their eggs under controlled wet and dry conditions, as well as controlled warm and cool conditions.

These tests revealed a clear pattern: tropical embryos were highly sensitive to dry conditions but showed almost no response to temperature changes, consistent with the stable climates they evolved in. But the embryos’ responses were only part of the story.

Her data pointed to the influence of the mothers. These lizard species lay one egg at a time, making dozens of decisions each season about when and where to nest. Muell’s field observations showed that females avoided nesting during dry periods and held their eggs during intense rainfall, laying them only after storms passed.

“It seems like those females are evolving to cue in their reproduction with rainfall,” she said.

Those decisions matter. In tropical soils, a nest that is too dry risks desiccating the embryo, while heavy downpours can flood the egg chamber. By nesting in narrow windows of moderate moisture, mothers appear to buffer their embryos from environmental stress.

A companion project across the Southeast revealed a similar maternal role in Auburn’s native green anoles. Populations living in dramatically different climates showed little variation in embryo sensitivity, but mothers adjusted egg size and timing in ways that improved offspring survival.

“It seems like the decisions that the moms are making about their babies are what’s responsible for them surviving,” she said.

For Muell, the work underscores the connections among organisms, environments and the people who study them.

“We all benefit from being excited about who we share the earth with,” she said.

Muell said these findings show that developmental outcomes cannot be understood in isolation. By combining lab experiments, fieldwork and museum resources, she has built a comparative picture that spans continents and climates.

“By taking that sort of holistic perspective, we can learn a lot about these animals,” she said.

Seeing those pieces fit together, in the lab and in the field, is what keeps the work meaningful for her.

“Every day we could realize something that changes everything,” she said. “It is really exciting to be able to know that I have this role in generating our knowledge of how the natural world works.”

Though her time on the Plains is ending, Muell will carry that approach into her postdoctoral fellowship at the University of Texas at Austin, where she will study development in terrestrial-breeding frogs.

“Auburn has given me so much,” she said. “We do good biology here, and I am a really, really proud soon-to-be alum.”