Hold on for your life! Part II

In Part I the data showed that, after the hurricanes, anole lizards had on average larger bodies, shorter legs, and larger toe pads. The patterns were clear and consistent across the two islands, indicating that these traits are adaptations shaped by natural selection from hurricanes. At this point, however, Colin was still not convinced because he was unable to directly observe the lizards during the hurricane.

Still shot of lizard clinging to an experimental perch in hurricane-force winds. Wind speed meter is displaying in miles per hour

The activities are as follows:

Colin was unable to stay on Pine Cay and Water Cay during the hurricanes and directly observe the lizards. To be more confident in his explanation, Colin needed to find out how lizards behave in hurricane-force winds. He thought there were two options for what they might do. First, he thought they might get down from the branch and hide in tree roots and cracks. Alternatively, they might hold onto branches and ride out the storm. If they tried to hold on in high winds, it would make sense that traits like the length of their limbs or the size of their toepads would be important for their survival. However, if they hid in roots or cracks, these traits might not be adaptations after all.

To see how the lizards behaved, Colin needed to design a safe experiment that would simulate hurricane-force winds. He bought the strongest leaf blower he could find, set it up in his hotel room on Pine Cay, and videotaped 40 lizards as they were hit with high winds. Colin first set up this experiment to observe behavior, but he ended up learning not only that, but a lot about how the traits of the lizards interacted with high winds.

To begin the experiment, Colin placed the anoles on a perch. He slowly ramped up the wind speed on the leaf blower until the lizards climbed down or they were blown, unharmed, into a safety net. He recorded videos of each trial and took pictures. 

Featured scientist: Colin Donihue from Harvard University

Written with: Bob Kuhn and Elizabeth Schultheis

Flesch–Kincaid Reading Grade Level = 8.4

Additional teacher resources related to this Data Nugget:

To engage students in this activity, show the following video in class. This video gives some information on the experiment and Colin’s research.

Hold on for your life! Part I

Anolis scriptus, the Turks and Caicos anole, on Pine Cay.

The activities are as follows:

On the Caribbean islands of Turks and Caicos, there lives a small brown anole lizard named Anolis scriptus. The populations on two small islands, called Pine Cay and Water Cay, have been studied by researchers from Harvard University and the Paris Natural History Museum for many years. In 2017, Colin, one of the scientists, went to these islands to set up a long-term study on the effect of rats on anoles and other lizards on the islands. Unbeknownst to him, though, a storm was brewing to the south of the islands, and it was about to change the entire trajectory of his research.

While he was collecting data, Hurricane Irma was developing into a massive category 5 hurricane. Eventually it became clear that it would travel straight over these small islands. Colin knew that this might be the last time he would see the two small populations of lizards ever again because they could get wiped out in the storm. It dawned on him that this might be a serendipitous moment. After the storm, he could evaluate whether lizards could possibly survive a severe hurricane. He was also interested in whether certain traits could increase survival. Colin and his colleagues measured the lizards and vowed to come back after the hurricane to see if they were still there. They measured both male and female lizards and recorded trait values including their body size, femur length, and the toepad area on their forelimbs and hindlimbs.

Colin was not sure whether the lizards would survive. If they did, Colin formed two alternative hypotheses about what he might see. First, he thought lizards that survived would just be a random subset of the population and simply those that got lucky and survived by chance. Alternatively, he thought that survival might not be random, and some lizards might be better suited to hanging on for their lives in high winds. There might be traits that help lizards survive hurricanes, called adaptations. He made predictions off this second hypothesis and expected that survivors would be those individuals with large adhesive pads on their fingers and toes and extra-long legs – both traits that would help them grab tight to a branch and make it through the storm. This would mean the hurricanes could be agents of natural selection.

Not only did Hurricane Irma ravage the islands that year, but weeks later Hurricane Maria also paid a visit. Upon his return to Pine Cay and Water Cay after the hurricanes, Colin was shocked to see there were still anoles on the islands! He took the measurements a second time. He then compared his two datasets from before and after the hurricanes to see if the average trait values changed.

Featured scientist: Colin Donihue from Harvard University

Written with: Bob Kuhn and Elizabeth Schultheis

Flesch–Kincaid Reading Grade Level = 9.9

Additional teacher resources related to this Data Nugget:

To engage students in this activity, show the following video in class. This video gives some information on the experiment and Colin’s research. For Part I stop the video at minute 1:30.

All washed up? The effect of floods on cutthroat trout

The activities are as follows:

Mack Creek, a healthy stream located within the old growth forests in Oregon. It has a diversity of habitats because of various rocks and logs. This creates diverse habitats for juvenile and adult trout.

Streams are tough places to live. Fish living in streams have to survive droughts, floods, debris flows, falling trees, and cold and warm temperatures. In Oregon, cutthroat trout make streams their home. Cutthroat trout are sensitive to disturbances in the stream, such as pollution and sediment. This means that when trout are present it is a good sign that the stream is healthy.

Floods are very common disturbances in streams. During floods, water in the stream flows very fast. This extra movement picks up sediment from the bottom of the stream and suspends it in the water. When sediment is floating in the water it makes it harder for fish to see and breathe, limiting how much food they can find. Floods may also affect fish reproduction. If floods happen right after fish breed and eggs hatch, young fish that cannot swim strongly may not survive. Although floods can be dangerous for fish, they are also very important for creating new habitat. Floods expand the stream, making it wider and adding more space. Moving water also adds large boulders, small rocks, and logs into the stream. These items add to the different types of habitat available. 

A cutthroat trout. It is momentarily unhappy, because it is not in its natural, cold Pacific Northwest stream habitat.

Ivan and Stan are two scientists who are interested in whether floods have a large impact on the survival of young cutthroat trout. They were worried because cutthroat trout reproduce during the spring, towards the end of the winter flood season. During this time juvenile trout,less than one year old, are not good swimmers. The fast water from floods makes it harder for them to survive. After a year, juvenile trout become mature adults.These two age groups live in different habitats. Adult trout live in pools near the center of streams. Juvenile trout prefer habitats at the edges of streams that have things like rocks and logs where they can hide from predators. Also, water at the edges moves more slowly, making it easier to swim. In addition, by staying near the stream edge they can avoid getting eaten by the adults in stream pools.

Ivan and Stan work at the H.J. Andrews Long Term Ecological Research site. They wanted to know what happens to cutthroat trout after winter floods. Major floods occur every 35-50 years, meaning that Ivan and Stan would need a lot of data. Fortunately for their research they were able to find what they needed since scientists have been collecting data at the site since 1987!

To study how floods affect trout populations, Ivan and Stan used data from Mack Creek, one of the streams within their site. They decided to look at the population size of both juvenile and adult trout since they occupy such different parts of the stream. For each year of data they had, Ivan and Stan compared the juvenile and adult trout population data, measured as the number of trout, with stream discharge, or a measure of how fast water is flowing in the stream. Stream discharge is higher after flooding events. Stream discharge data for Mack Creek is collected during the winter when floods are most likely to occur. Fish population size is measured during the following summer each year. Since flooding can make life difficult for trout, they expected trout populations to decrease after major flooding events.

Featured scientists: Ivan Arismendi and Stan Gregory from Oregon State University

Written by: Leilagh Boyle

Flesch–Kincaid Reading Grade Level = 7.5

Additional teacher resource related to this Data Nugget:

Beetle, it’s cold outside!

Frozen lady beetles.

The activities are as follows:

Walking across a snowy field or mountain, you might not notice many living things. But if you dig into the snow, you’ll find a lot of life!

Until recently, climate change scientists thought warming in winter would be good for most species. Warmer winters would mean that species could avoid the cold and would not need to deal with freezing temperatures as often or for as long. Caroline is a scientist who is thinking about winter climate change in a whole new way. Snow covers the soil, acting like an insulating blanket. Many species rely on the snow for protection from the winter’s cold. When temperatures climb in the winter, snow melts and leaves the soil uncovered for longer periods of time. This leads to the shocking pattern that warmer temperatures actually means the soil gets colder!

Caroline is interested in how species that rely on the snow will respond to climate change. She studies a species of insect called lady beetles. Lady beetles are ectotherms, meaning their body temperature matches that of their environment. Because climate change is reducing the amount of snow in the lady beetle habitat, Caroline wanted to know how they would respond to these changes.

Caroline and her team, Andre and Nikki, decided to investigate what happens to lady beetles when they are exposed to longer periods of time in cold temperatures. When soil temperatures drop below freezing (0℃), lady beetles go into a chill coma, or a temporary, reversible paralysis. When temperatures are below freezing, it is so cold that they are unable to move. When temperatures rise back above freezing, they wake from their chill comas. When lady beetles are in chill comas, they are easier for predators to catch because they can’t escape. They are also unable to find food or mates. Scientists can measure how fast it takes lady beetles to recover from chill coma, called chill coma recovery time, and use this as a measure of their performance.

Beetles in their pre-testing habitat are on the right; tubes with beetles about to be immersed in a cooler filled with crushed ice are on the left.

They designed an experiment to test whether the amount of time lady beetles spend in freezing temperatures affects how long it takes them to wake up from a chill coma. Caroline thought that lady beetles exposed to lengthy freezing temperatures would be harmed because freezing causes tissue damage and the insect must use more energy to survive. She predicted that the longer the lady beetles had been exposed to the cold, the longer it would take them to wake up from their chill comas.

To begin the experiment, Andre and Nikki placed groups of lady beetles in tubes. They then placed the tubes in an ice bath, bringing the temperature down to 0℃, the point when lady beetles enter chill coma. They varied the amount of time each tube was in the ice baths and tested chill coma recovery times after 3, 24, 48, 72, or 96 hours. After removing the tubes from the ice baths, they put the lady beetles on their backs with their legs in the air and left them at room temperature, 20℃. Andre and Nikki timed how long it took each beetle to wake up and turn itself over.

In the experiment, they used two different populations of lady beetles. Population 1 had been living in the lab for several weeks before the experiment began. They were not in great health and some had started to die. In order to make sure they had enough beetles for the experiment, Caroline purchased more lady beetles, which she called Population 2. Population 2 only spent a few days living in the lab before testing and were in much better health. Caroline noted the differences in these populations and thought their age, health, and background might affect how they respond to the experiment. She decided to track which population the lady beetles were from so she could analyze the data separately and see if the health differences between Population 1 and 2 changed the results.

Featured scientists: Caroline Williams & Andre Szejner Sigal, University of California, Berkeley, & Nikki Chambers, Biology Teacher, West High School, Torrance, CA

Flesch–Kincaid Reading Grade Level = 9.8

Clique wars: social conflict in daffodil cichlids

A male and female daffodil cichlid

The activities are as follows:

Have you ever thought about what it would be like to live completely alone, without contact with other people? Nowadays, humans are constantly connected by phones, texting, and social media. Our social interactions affect us in many unexpected ways. Strong social relationships can increase human lifespan, and lower the risk of cancer, cardiovascular disease, and depression. Social relationships are so important that they are actually a stronger predictor of premature death than smoking, obesity, or physical inactivity! Like humans, social interactions are important for other animals as well.

Jennifer is a behavioral ecologist who is interested in daffodil cichlids, a social species of fish from Lake Tanganyika, a Great Lake in Africa. Daffodil cichlids live in social groups of several small fish and one breeding pair. Each group defends its own rock cluster in the lake. The breeding male and female are the largest fist in the group, and the smaller fish help defend territory against predators and help care for newly hatched baby fish. About 200 social groups together make up a colony.

Social groups of daffodil cichlids in Lake Tanganyika

Behavior within a social group may be influenced by the presence of other groups in the colony. For example, neighboring groups can be a threat because they may try to take away territory or resources. After reading about previous research on social interactions in species that live in groups, Jennifer noticed there were very few studies that looked at how neighboring groups affected behavior within the group. Jennifer thought that the presence of neighboring groups may force the breeding pair to be less aggressive towards each other and work together to protect their group’s resources against the outside threat.

To test her idea, Jennifer formed breeding pairs of daffodil cichlids in an aquarium laboratory. She first observed the breeding pairs for any aggressive behaviors when they were isolated and could not see other groups. She observed each group for 30 minutes a day for 10 days. Next, Jennifer set up a clear barrier between the breeding pair and a neighboring group. The fish could see each other but not physically interact. Jennifer again watched the breeding pair and documented any aggressive behaviors to see how the presence of a neighboring group affected conflict within the pair. She again observed each group with neighbors for 30 minutes a day for 10 days.

During these behavioral tests, Jennifer counted the total number of behaviors done by the breeding pair. She measured several behaviors. Physical attacks were counted every time contact between the fish was made (biting or ramming each other). Aggressive displays were counted when fish give signals of aggression without making physical contact (raising their fins or swimming rapidly at another fish). Submissive behaviors, or actions used to prevent aggression between the breeding pair, were also counted. Finally, behaviors used to encourage social bonding were counted and are called affiliative behaviors. Jennifer predicted that the breeding pair would perform fewer physical attacks and aggressive displays when a neighboring group was present compared to when the breeding pair was alone. She also thought the breeding pair would perform more submissive and affiliative behaviors when the neighboring group was present. In this way, the presence of an outside group would impact the behaviors within a group.

Featured scientist: Jennifer Hellmann from The Ohio State University

Flesch–Kincaid Reading Grade Level = 11.3

Tree-killing beetles

A Colorado forest impacted by a mountain pine beetle outbreak. Notice the dead trees mixed with live trees. Forests like this with dead trees from mountain pine beetle outbreaks cover millions of acres across western North America.

The activities are as follows:

A beetle the size of a grain of rice seems insignificant compared to a vast forest. However, during outbreaks the number of mountain pine beetles can skyrocket, leading to the death of many trees. The beetles bore their way through tree bark and introduce blue stain fungi. The blue stain fungi kills the tree by blocking water movement. Recent outbreaks of mountain pine beetles killed millions of acres of lodgepole pine trees across western North America. Widespread tree death caused by mountain pine beetles can impact human safety, wildfires, nearby streamflow, and habitat for wildlife.

Mountain pine beetles are native to western North America and outbreak cycles are a natural process in these forests. However, the climate and forest conditions have been more favorable for mountain pine beetles during recent outbreaks than in the past. These conditions caused more severe outbreaks than those seen before.

Logs from mountain pine beetle killed lodgepole pine trees. The blue stain fungi is visible around the edge of each log. Mountain pine beetles introduce this fungus to the tree.

When Tony moved to Colorado, he drove through the mountains eager to see beautiful forests. The forest he saw was not the green forest he expected. Many of the trees were dead! Upon closer examination he realized that some forests had fewer dead trees than others. This caused him to wonder why certain areas were greatly impacted by the mountain pine beetles while others had fewer dead trees. Tony later got a job as a field technician for Colorado State University. During this job he measured trees in mountain forests. He carefully observed the forest and looked for patterns of where trees seemed to be dead and where they were alive.

Tony thought that the size of the trees in the forest might be related to whether they were attacked and killed by beetles. A larger tree might be easier for a beetle to find and might be a better source of food.To test this idea, Tony and a team of scientists visited many forests in northern Colorado. At each site they recorded the diameter of each tree’s trunk, which is a measure of the size of the tree. They also recorded the tree species and whether it was alive or dead. They then used these values to calculate the average tree size and the percent of trees killed for each site.

Featured scientist: Tony Vorster from Colorado State University

Flesch–Kincaid Reading Grade Level = 8.3

There is one scientific paper associated with the data in this Data Nugget. The citation and PDF of the paper is below:

Bringing back the Trumpeter Swan

Joe with a Trumpeter Swan.

The activities are as follows:

The Kellogg Bird Sanctuary was created in 1927 to provide safe nesting areas for waterfowl such as ducks, geese, and swans. During that time many waterfowl species were in trouble due to overhunting and the loss of wetland habitats. One species whose populations had declined a lot was the Trumpeter Swan. Trumpeter swans are the biggest native waterfowl species in North America. At one time they were found across North America, but by 1935 there were only 69 known individuals in the continental U.S.! The swans were no longer found in Michigan.

The reintroduction, or release of a species into an area where they no longer occur, is an important tool in helping them recover. In the 1980s, many biologists came together to create a Trumpeter Swan reintroduction plan. Trumpeter Swans in North America can be broken up into three populations – Pacific Coast, Rocky Mountain, and Interior. The Interior is further broken down into Mississippi/Atlantic and High Plains subpopulations. Joe, the Kellogg Bird Sanctuary manager and chief biologist, wrote and carried out a reintroduction plan for Michigan. Michigan is part of the Mississippi/Atlantic subpopulation. Joe and a team of biologists flew to Alaska in 1989 to collect swan eggs to be reared at the sanctuary. After two years the swans were released throughout Michigan.

The North American Trumpeter Swan survey has been conducted approximately every 5 years since 1968 as a way to estimate the number of swans throughout their breeding range. The survey is conducted in late summer when young swans can’t yet fly but are large enough to count. Although the surveys are conducted across North America, the data provided focuses on just the Interior Population, which includes swans in the High Plains and Mississippi/Atlantic Flyways.

Featured scientist: Wilbur C. “Joe” Johnson from the W.K. Kellogg Bird SanctuaryWritten by: Lisa Vormwald and Susan Magnoli from Michigan State University.

Flesch–Kincaid Reading Grade Level = 11.5

Additional teacher resource related to this Data Nugget:

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What big teeth you have! Sexual selection in rhesus macaques

Cayo Santiago rhesus macaques. Photo by Raisa Hernández Pacheco.

The activities are as follows:

It is easy to identify a deer as male when you see his huge antlers, or a peacock as male by his stunning set of colorful tail feathers. But you may wonder, how do these traits come about, and why don’t both males and females have them? These extravagant traits are thought to be the result of sexual selection. This process happens when females mate with males that they think have the sexiest traits. These traits get passed on to future male offspring, leading to a change in the selected traits over time. Because females are only choosing these traits in males, sexual selection often leads to sexual dimorphism between males and females. This means that the sexes do not look the same. Often males will be larger and have more elaborate traits than females.

Craniums of an adult male (left) and an adult female (right) rhesus macaque. Photo by Raisa Hernández Pacheco and Damián A. Concepción Pérez.

One species that shows strong sexual dimorphism is rhesus macaques. In this species of monkey, males are much larger than females. Cayo Santiago is a small island off the shore of Puerto Rico. On this island lives one of the oldest free-ranging rhesus macaque colonies in the world. This population has no predators and food is plentiful. Scientists at Cayo Santiago have gathered data on these monkeys and their habitat for over 70 years. Every year when new monkeys are born they are captured, marked with a unique tattoo ID, and released. This program allows scientists to monitor individual monkeys over their entire lives and record the sex, date of birth, and date of death. Once a monkey dies and its body is recovered in the field, skeletal specimens are stored in a museum for further research.

Damián measuring canine length in a rhesus macaque skeletal specimen. Photo by Raisa Hernández Pacheco.

These skeletal specimens can be used by scientists today to ask new and exciting questions. Raisa and Damián are both interested in studying sexual dimorphism in rhesus macaques. They want to find out what causes the differences between the sexes. They chose to focus on the length of the very large canine teeth in male and female macaques. They expected that canine teeth may be under sexual selection in males for two reasons. First, rhesus macaques are mostly vegetarians, so they don’t need long canines for the same purpose as other meat-eating species that use them to catch prey. Second, male rhesus macaques often bare their teeth at other males when they are competing for mates. Females could see the long canines as a sign of good genes and may prefer to mate with that trait. Excited by these ideas, Raisa and Damián set out to investigate the museum’s skeletal specimens to check whether there is sexual dimorphism in canine length. This is the first step in collecting evidence to see whether male canines are under sexual selection by females.

They measured canine length of four male and four female rhesus skeletal specimens dating back to the 1970s. Measurements were only taken from individuals that died as adults to make sure canines were fully developed and that differences in length could not be attributed to age. Raisa and Damián predicted that males would have significantly longer canines compared to those of females. If so, this would be the first step to determine whether sexual selection was operating in the population.

Featured scientists: Raisa Hernández-Pacheco from University of Richmond and Damián A. Concepción Pérez from Wilder Middle School. Research conducted at the Laboratory of Primate Morphology at the University of Puerto Rico Medical Sciences Campus. Skeletal specimens came from the population of rhesus macaques on Cayo Santiago.

Flesch–Kincaid Reading Grade Level = 9.8

Damián and Raisa created a teaching module, called Unknown Bones. It is an inquiry-based educational activity for high school students in which they apply data analysis and statistics to understand sexual selection and illustrate sexual dimorphism in Cayo Santiago rhesus macaques.


About Raisa: I am interested in understanding the drivers shaping population dynamics, and have dedicated my studies to modeling the effects of biotic and abiotic factors on populations of invertebrates and vertebrates. In 2013, I obtained my PhD from the University of Puerto Rico after assessing the effects of mass bleaching on Caribbean coral populations. Right after, I joined the Caribbean Primate Research Center and the Max-Planck Odense Center to study the long-term dynamics of the Cayo Santiago rhesus macaque population. At the Grayson lab, I am studying the population of red-backed salamanders in Richmond; its density, spatial arrangement, and space use.


About Damián: I am a middle and high school Science and Math teacher. I have always been searching for innovative ways to get my students engaged in the science classroom and to connect their new knowledge with the real-world. In thinking of ways to help my students learn, I engaged my self with the scientific community collaborating in scientific projects and creating hands-on, interactive, and inspiring teaching lessons. It is my main interest to develop ideas that could positively contribute to any student’s STEM education.

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Which would a woodlouse prefer?

Woodlice are small crustaceans that live on land. They look like bugs, but are actually more closely related to crabs and lobsters! Photo credit Liz Henwood.

The activities are as follows:

Woodlice are small crustaceans that live on land. They look like bugs, but are actually more closely related to crabs and lobsters. To escape predators they hide in dark places. They spend most of their time underground and have very poor eyesight.

One day, when digging around in the dark dirt of her compost pile, Nora noticed that there were many, many woodlice hiding together. This made her wonder how woodlice decide where to live. Because woodlice have very simple eyesight, Nora thought that maybe they use dark and light colors to decide where to go. They might choose to move towards darker colors and away from lighter colors to prevent ending up above ground where predators can easily find them.

Nora collecting woodlice from the compost pile.

Nora, along with classmates in her ecology class at Michigan State University, decided to run an experiment to study woodlice behavior. She collected 10 woodlice from her compost pile and placed them in a jar. She brought the jar into the lab. Then she chose a set of trays to work with from what she had in the lab – white, with tall sides. The sides of the tray were tall and smooth so the woodlice were not able to climb out. On one end of the tray Nora put some dark soil, and on the other side she put lighter leaves. If her hypothesis was correct, Nora predicted that woodlice would more often choose to move towards the dark soil habitat, compared to the lighter leaves habitat.

For each trial, Nora gently picked up a single woodlouse with forceps. She then placed it in the center of the tray. All the woodlice were positioned so they started facing the top of the tray, not at either habitat type. The woodlice then chose to move towards one end of the tray or the other. When they reached one of the piles the students recorded which habitat they chose. It was then picked up with forceps. Nora and her classmates recorded its length and placed it in a new jar so it could be released back into the compost pile once the experiment was done.

The tray where the preference trials were conducted. To the right of the tray is the soil pile, and to the left is the leaf pile. The center was purposefully left empty and wiped down before each run.

After running this experiment and looking at the data, Nora realized it did not work. The small sample size of only 10 individuals was not enough to see a pattern. Also, she realized that after one woodlouse went a certain way, all the others would follow it, maybe because they were following a scent trail. She decided she had to do the experiment again, this time with more woodlice and in a way that would prevent them following each other’s scent trails.

For her second try, Nora collected 51 woodlice from a different compost pile. Just like the first experiment, Nora placed lighter leaves on one end of a white tray and dark soil on the other. All the methods were the same, except for a few important changes. To get rid of scent trails, this time Nora wiped down the middle of the tray with a clean wet paper towel between trials. She also added equal amounts of water to both habitats to control for humidity. This ensured that if woodlice did show a preference for either habitat it would be due to habitat color, not humidity. This time Nora used a stopwatch and recorded how long it took for an individual to choose one of the two habitats.

Featured scientist: Nora Straquadine from Michigan State University

 Flesch–Kincaid Reading Grade Level = 7.7

Additional teacher resource related to this Data Nugget:

  • PowerPoint slideshow of images of woodlice and Nora’s experiment.
  • A great video to show before the Data Nugget to engage students with the activity – gives background on woodlice and describes the role that water plays for these crustaceans that live on land:

  • A video of woodlice on a fallen tree. This video has no audio, but can be useful for students to observe woodlice behavior:


About Nora: Nora is currently an undergraduate getting her B.S. in Zoology with a concentration in Zoo and Aquarium as well as a minor in Marine Ecosystem Management from Michigan State University. Although aquatic life is her main interest, she think it’s important to appreciate other animal groups and take a break to play and explore the nature around you. That curiosity was how she was able to volunteer in labs on campus from entomology to genetics, and how she came to spend a summer at the Kellogg Biological Station in Michigan.

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Is it better to be bigger?

An anole lizard on the island, about to be captured by Aaron.

The activities are as follows:

When Charles Darwin talked about the “struggle for existence” he was making the observation that many individuals in the wild don’t survive long enough to reach adulthood. Many die before they have the chance to reproduce and pass on their genes to the next generation. Darwin also noted that in every species there is variation in physical traits such as size, color, and shape. Is it simply that those who survive to reproduce are lucky, or do these traits affect which individuals have a greater or lesser chance of surviving? Evolutionary biologists often work to see how differences in traits, such as body size, relate to differences in survival among individuals. When differences in traits are related to chances of survival, they are said to be under natural selection.

Brown anole lizards are useful for studies of natural selection because they are abundant in Florida and the Caribbean, easy to catch, and have a short life span. Brown anoles are very small when they hatch out of the egg. Because of their small size, these anole hatchlings are eaten by many different animals, including birds, crabs, other species of anole lizards, and even adult brown anoles! Predators could be a significant force of natural selection on brown anole hatchlings. Juvenile anoles that get eaten by predators will not survive to reproduce. Traits that help young brown anoles avoid predation and reproduce will get passed on to future generations.

Aaron with a baby anole lizard.

Aaron and Robert are scientists who study brown anoles on islands in Northeastern Florida. Along with their colleagues, they visit these islands every 6 to 10 weeks during the summer to survey the populations and measure natural selection in action. Aaron and Robert selected a small island that had a large brown anole population because they were able to find and measure all of the individuals on the island. Aaron observed that in the late summer there were thousands of hatchling lizards on the island, but by the middle of the summer the following year, only a few hundred of those lizards remained alive. He also observed that hatchlings varied greatly in body size and wondered if those differences in size affected the chances that an individual would survive to adulthood. He predicted that smaller hatchlings are more likely to die than larger ones because they are not as fast, and therefore not as likely to escape from predators and face a higher risk of being eaten.

To test this, Aaron and Robert captured hatchlings in July, assigned a unique identification number to each anole, measured their body length, and then released them back onto the island. In October of the same year, they returned to the island to capture and measure all surviving lizards. They calculated the average percent survival for each size category. Aaron predicted longer individuals would have higher survival. This would indicate that there was natural selection for larger body size in hatchlings.

Featured scientists: Aaron Reedy and Robert Cox from the University of Virginia. Co-written by undergraduate researcher Matt Kustra.

Flesch–Kincaid Reading Grade Level = 11.7

Additional teacher resource related to this Data Nugget:

  • For additional images of Robert and Aaron’s research with anoles in Florida, we have created PowerPoint slides that can be shown in class.
  • Aaron conducted this research as a graduate student in Robert Cox’s lab. To learn more about anole research, visit the lab’s website. To learn more about Aaron, visit his website.

Once your students have completed this Data Nugget, check out this video on anole size and natural selection from hurricanes!

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