Horoscope haters beware: a turtle’s birthday may decide more than we think

Illinois State University contributor

Featured Scientist: Haley M. Nichols, B.S. 2017, School of Biological Sciences, Illinois State University

A selfie of Haley Nichols in her home.

Birthplace: Okinawa, Japan 

My Research: I studied the role of nesting season and maternal investment on the behavior of juvenile freshwater turtles. 

Research Goals: In the future, I would like to study how maternal nutrition or nesting conditions affect animal development and behavior. 

Career Goals: I would like to work in managing invasive reptile species, particularly in sensitive areas such as the Florida wetlands. 

Hobbies: I enjoy road trips with my fiancé and taking care of our cold-blooded pets and house plants! 

Favorite Thing About Science: My favorite thing about science is learning new things to better understand the world we live in. I think it allows us to appreciate each moment a little more. 

Organism of Study: I work with the red-eared slider turtle (Trachemys scripta). 

A picture of a T. scripta hatchling half emerged from its shell. The head, left leg, and part of the carapace are visible. The eggshell appears torn and there is a number written on the top of the egg in pencil.
Image of T. scripta hatchling emerging from its shell. 

Field of Study: Animal Behavior & Physiology

What is Animal Behavior & Physiology? Animal behavior studies the way animals interact with themselves, others, and their environment. Their behavior can be affected by both internal and external conditions. Animal physiology studies the internal conditions like hormone regulation, temperature, or metabolic function. External conditions may be access to food and water or the presence of predators. Basically, we look at WHAT animals do and WHY they may be doing it. 

Check Out My Original Paper: “Red-eared slider hatchlings (Trachemys scripta) show a seasonal shift in behavioral types” 

A QR code that links to the original publication.
QR Code to the original publication

Citation: H. Nichols, A.W. Carter, R.T. Paitz, R.M Bowden, Red-eared slider hatchlings (Trachemys scripta) show a seasonal shift in behavioral types. J Exp. Zool. Part. A. 331.9: 485-493 (2019). 

Research at a Glance: Behavioral types can be thought of as an animal’s personality. They are patterns of behavior that repeat. The behavioral type that best supports survival may differ across environments, like seasons. We hypothesized that mothers could use their own behavior and physiology to help their hatchlings survive in the conditions that they are born in. That is, they can help them by making sure that the hatchling behavioral type matches the environment. To test this hypothesis, we measured righting response of juvenile red-eared slider turtles across the nesting season. The righting response is the ability of the turtle to turn itself over onto its abdomen (plastron) after it has been placed on its back (carapace). It is a signal for behavioral type, or “personality”. We studied turtles hatching from early and late season clutches to understand if their personalities change based on nesting season. We found that the nesting season has a significant effect on righting response, with early season hatchlings righting more quickly than late season hatchlings. 

To figure out why this happens, we explored two potential causes: maternal estrogen and maternal investment. From prior research, we know that eggs from late season clutches have more estrogen than eggs from early season clutches. To see if maternal estrogen was causing the difference in righting response, we coated early season eggs with an estrogen hormone to mimic the amount of estrogen found in late season eggs. We found that this did not affect hatchling behavior. To see if maternal investment was causing the difference in righting response, we looked to see if mothers were giving extra energy resources to her eggs when she lays them. We measured the mass of the egg, the hatchling mass, and the mass of the residual yolk on the hatchling’s body. We found that early season eggs have more yolk than late season eggs. We also found that early season hatchlings were larger; they used a higher percentage of their yolk to grow new tissue, rather than just keeping existing tissues healthy. Interestingly, in both seasons, hatchlings that had less yolk also used a higher percentage of the yolk to make tissue, but we found no direct relationship to righting response. Overall, our research shows that behavioral types vary across the nesting season, but it appears that neither maternal estrogen nor maternal investment directly leads to this change. 

Highlights: We we were able to show seasonal differences in the red-eared slider turtle (T. scripta) righting response between seasons, but were not able to determine its cause. We showed that early season turtles are given more yolk and use more of this yolk to grow tissues than late season turtles. We were able to determine the amount of yolk given to each hatchling by measuring the yolk from one egg of each clutch shortly after they were laid. After hatching, we measured the mass of each turtle and the mass of their remaining yolk. We then used statistical tests to look at the differences between these groups. 

Figure 1 shows the results of our righting response behavioral test. It shows that turtles that came from eggs laid late in the nesting season righted themselves more slowly than those laid early in the nesting season

A data figure that shows the time that it takes a hatchling to right itself, relative to the season in which it was born. It shows that turtles born in the early season took about 450 seconds to right themselves. It shows that turtles born in the late season took about 550 seconds to right themselves.
Figure 1. Hatchling righting response trial. The x-axis shows whether the turtle was laid in the early or late season. The y-axis shows the amount of time, in seconds, that it took for the hatchling to turn itself over. Adapted from Nichols et al. 2019. 

What My Science Looks Like: The photo below shows the arena that we used during the righting response trial. The turtles in the top left corner cell and center cell have completed the desired behavior: they were able to turn from the carapace (back) to the plastron (abdomen). Each turtle is given a unique number based on their egg and clutch. We took photos of the plastron of each individual to verify the identity of the hatchling. 

A picture of the righting response trial arena. It is an aerial view of a 3 by 3 grid of small cardboard boxes, each with an open top. Each box has a hatchling inside it. The top left and center box have turtles that are on their feet. All other boxes have turtles that are on their backs.
T. scripta in the trial arena. 

The Big Picture: My research is important because it helps us understand how animal behaviors and physical responses form. We used a species that is sensitive to the environment. Knowing how the environment shapes the body and behavior can help us predict how animals will respond to these changes. 

Decoding the Language: 

Behavioral types: A pattern of behavior in an animal. 

Carapace: The top part of a turtle’s shell. 

Clutch: A group of eggs that are laid together at the same time by a single female. 

Estrogen: One of the primary female hormones; previous studies show it increases in eggs over the nesting season, causing more females in later clutches (Carter et. al 2017, see below).

Maternal investment: The services a mother provides her young, such as, provision of food, care, and protection.

Nesting season: The time of the year eggs are laid. For my species there is an early nesting season (late May to mid-June) and a late nesting season (mid- June to late June). 

Residual yolk: The unused yolk remaining after turtle hatching. 

Righting response: The behavior of turning oneself over from carapace to plastron (the “right” position for walking etc.).

Plastron: The bottom part of a turtle’s shell.

Physiology: A branch of biology that studies how different parts of the body carry out chemical and physical functions.

Trachemys scripta (T. scripta): The scientific name for the red-eared slider turtle.

Yolk: The yellow-orange, nutrient-rich portion of the egg that supplies food to the developing embryo. 

Learn More: 

Physiology, Neuroscience, and Behavior sequence at ISU

Animal physiology:

Animal behavior

Information about the red-eared slider turtle

A.W. Carter, R.M. Bowden, R. T. Paitz, Seasonal shifts in sex ratios are mediated by maternal effects and fluctuating incubation temperatures. Funct. Ecol. 31: 876–884 (2017). 

Synopsis edited by: Rosario Marroquin-Flores (she/her/hers), PhD (Anticipated Spring 2022), School of Biological Sciences, Illinois State University

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