How the cricket lost its song, Part II

In Part 1 you determined that the Kauai flatwing mutation led to a decrease in parasitism rates for male crickets. Today, most of the male crickets on Kauai have evolved flat wings and can no longer produce songs that were previously used to attract female crickets. Without their songs, how do males attract females?

Robin collecting data on satellite behavior in normal and flatwing mutation males.

Robin collecting data on satellite behavior in normal and flatwing mutation males.

The activities are as follows:

Without their song, how are flatwing crickets able to attract females? In some other animal species, like birds, males use an alternative to singing, called satellite behavior. Satellite males hang out near a singing male and attempt to mate with females who have been attracted by the song. This helps satellite males in two ways: they don’t use energy to make a song, and they avoid attracting enemies like the fly. Perhaps the satellite behavior gives flatwing males the opportunity to mate with females who were attracted to the few singing males left on Kauai.

Collecting crickets at the speaker.

Collecting crickets at the speaker.

To test this idea, Robin set up a speaker playing cricket songs in the fields where the crickets live on Kauai, Oahu, and the Island of Hawaii. The speaker tricks male and female crickets into thinking there is a male cricket in the area making songs. Before the start of the experiment, Robin removed all the males found within a 2-meter circle around the speaker. She then broadcast cricket songs from the speaker for 20 minutes. She then returned and counted the number of males in the 2-meter circle, measured the distance from male to the speaker, and noted whether each male was normal or flatwing. Robin expected that flatwing males would be more likely to use satellite behavior and, therefore, would be on average closer to the speaker than normal males.

Featured scientist: Robin Tinghitella from the University of Denver

Flesch–Kincaid Reading Grade Level = 10.0

Additional teacher resources related to this Data Nugget include:

  • A video introducing the study system and describing how, in fewer than 20 generations, crickets on the island of Kuai went from singing to silent!

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CSI: Crime Solving Insects

Scientist Paula catching blow flies in the field using an insect net.

Scientist Paula catching blow flies in the field using an insect net.

The activities are as follows:

Most people think that maggots (larvae of blow flies) are gross, but they are important decomposers in many ecosystems. Without maggots, we would all trip over dead bodies every time we went outside! Not only do maggots break down dead animal bodies in nature, but they decompose human bodies as well!

Forensic entomology uses these amazing insects to help the criminal justice system. Remember the next time you swat away a fly, these little insects help police solve crimes! Adult blow flies are usually the first to arrive at a crime scene with a dead body. The blow flies oviposit, or lay their eggs, shortly after their arrival. These eggs hatch and become maggots that feed on the body. Scientists can use the age of the maggot to help estimate how long someone has been dead. The longer a body has been dead, the longer ago the eggs hatched and the older the maggot larvae will be.

Kristi and Paula, two forensic entomologists, were in the field on day, documenting the timing of blow fly oviposition. They noticed something unexpected! There were wasps stuck in the traps they were using to catch blow flies. The scientists wondered if these wasps would affect a blow fly’s decision to oviposit. Wasps will attack adult blow flies and will also eat their eggs. Blow flies have an incredible sense of smell and sight. Blow flies may be able to use their senses to detect if a wasp is near a body. If a wasp is detected, blow flies could choose to avoid the area or delay laying their eggs. The scientists predicted that blow flies would delay their oviposition when wasps were present near a body. If wasps cause blow flies to delay oviposition, this could change how scientist’s use maggot age to determine how long ago a body died.

Control bait cup with a large number of blow flies on the chicken liver

Control bait cup with a large number of blow flies on the chicken liver

To test their hypothesis, the scientists did 10 trials in the field. They used bait cups that contained chicken liver to simulate a dead human body. A total of 9 bait cups were used in each of the 10 trials, for a total of 90 cups. Of the 9 cups used in each trial, three contained only chicken liver, to represent a body with no wasps present. These cups were used as controls. Three cups contained chicken liver and a wasp pinned to the side of the bait cup so that there was a visual cue of the wasp. The final three cups had a crushed wasp sprinkled over chicken liver to see if blow flies could use a smell cue to tell that a wasp was present without seeing them. Researchers checked the cups every half hour for the presence of blow fly eggs. If they saw any eggs, they recorded the time of oviposition in hours after sunrise. They then brought the maggots to the lab and raised them to the third larval stage and identified them to species.

Featured scientists: Kristi Bugajski and Paula Stoller from Valparaiso University

Flesch–Kincaid Reading Grade Level = 7.6

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How the cricket lost its song, Part I

Screen Shot 2015-06-22 at 12.41.05 PMThe activities are as follows:

Some of the most vibrant and elaborate traits in the animal kingdom are signals used to attract mates. These mating signals include the bright feathers and loud calls of birds or the swimming dances performed by fish. Most of the time the males of the species perform the mating signals, and females use those signals to choose a mate. While mating signals help attract females, they may also attract unwanted attention from other species, like predators.
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Robin is a scientist who studies the mating signals of Pacific field crickets. These crickets live on several of the Hawaiian Islands. Male field crickets make a loud, long-distance song to help females find them and then switch to a quiet courtship song once a female comes in close. Males use specialized structures on the wings to produce songs.

One summer, Robin noticed that the crickets on one of the islands, Kauai, were unusually quiet. Only a couple of years before, Kauai had been a very loud place to work; however, that year Robin heard no males singing! After taking the crickets back to the lab, she noticed that there was something different about the males’ wings on Kauai. Most (95%) of males were missing all of the structures that are used to produce the calling and courtship songs—they had completely lost the ability to produce song! She decided to call this new type of male a flatwing male. But why did these males have flat wings?

Screen Shot 2015-06-22 at 12.29.38 PMOn Kauai, songs of the male crickets attract female crickets, but they are also overheard by a deadly parasitoid fly. The fly sprays its larvae on the backs of the crickets. The larvae then burrow into the crickets’ body cavity and eat them from the inside out! Because flatwing males cannot produce songs, flat wings may help male crickets remain unnoticed by the parasitoid flies. To test this idea, Robin dissected the males to look for fly larvae. She compared infection levels for 67 normal males—collected before the flatwing mutation appeared in the population—to 122 flatwing males that she collected after the flatwing mutation appeared. She expected fewer males to be infected by the parasitoid fly after the appearance of the flatwing mutation in the cricket population.

Featured scientist: Robin Tinghitella from the University of Denver

Flesch–Kincaid Reading Grade Level = 9.1

Additional teacher resources related to this Data Nugget include:

  • A video introducing the study system and describing how, in fewer than 20 generations, crickets on the island of Kuai went from singing to silent!

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Cheaters in nature – when is a mutualism not a mutualism?

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The activities are as follows:

Mutualisms are a special type of relationship in nature where two species work together and both benefit. Each partner trades with the other species, giving a resource and getting one in return. This cooperation leads to partner species doing better when the other is around, and without their partner, each species would have a harder time getting resources.

One important mutualism is between clover, a type of plant, and rhizobia, a type of bacteria. Rhizobia live in small bumps on the clovers’ roots, called nodules, and receive protection and sugar food from the plant. In return, the rhizobia trade nitrogen to the plant, which plants need to photosynthesize and make new DNA. This mutualism works well when soil nitrogen is rare, because it is hard for the plant to collect enough nitrogen on its own, and the plant must rely on rhizobia to get all the nitrogen it needs. But what happens when humans change the game by fertilizing the soil? When nitrogen is no longer rare, will one partner begin to cheat and no longer act as a mutualist?

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Worldwide, the nitrogen cycle is off. Not that long ago, before farmers used industrial fertilizers and people burned fossil fuels, nitrogen was rare in the soil. Today, humans are adding large amounts of nitrogen to soils. The nitrogen that we apply to agricultural fields doesn’t stay on those fields, and nitrogen added to the atmosphere when we burn fossil fuels doesn’t stay by the power plant that generates it. The result is that today, more and more plants have all the nitrogen they need. With high nitrogen, plants may no longer depend on rhizobia to help them get nitrogen. This may cause the plant to trade less with the rhizobia in high nitrogen areas. In response, rhizobia from high nitrogen areas may evolve to try harder to get food from the plant, and may even cheat and become parasitic to plants. If this happens, both species will no longer be acting as mutualists.

When Iniyan was a college student, he wanted to study human impacts on the clover-rhizobia mutualism. To find out more, he contacted Jen Lau’s lab at the Kellogg Biological Station one summer, and joined a team of scientists asking these questions. For his own experiment, Iniyan chose two common species of clover: hybrid clover (Trifolium hybridum) and white clover (Trifolium pretense). He chose these two species because they are often planted by farmers. Iniyan then went out and collected rhizobia from farms where nitrogen had been added in large amounts for many years, and other farms where no nitrogen had been added.

Iniyan completed this research as an REU at KBS.

Iniyan completed this research as an REU at KBS.

To make sure that there were no rhizobia already in the soil, Iniyan set up his experiment in a field where no clover had grown before. He then planted 45 individuals of each species in the field. He randomly assigned each plant to one of three treatments. For each species, he grew 15 individuals with rhizobia from high nitrogen farms, and 15 with rhizobia from low nitrogen farms. To serve as a control, he grew the remaining 15 individuals without any rhizobia. To add rhizobia to the plants he made two different mixtures. The first was a mix of rhizobia from high nitrogen farms and water, and the second was a mix of rhizobia from low nitrogen farms and water. He then poured one of these mixtures over each of the plants, depending on which rhizobia treatment they were in. The control plants just got water. No nitrogen was added to the plants.

After the plants grew all summer, Iniyan counted the number of leaves and measured the shoot height (cm) for each individual plant. He did not collect biomass because he wanted to let the plants continue to grow. He then averaged the data from each set of 15 individuals. Plants with fewer leaves and shorter shoots are considered less healthy. He predicted rhizobia that evolved in high nitrogen soils would be worse mutualists to plants, while rhizobia that evolved in low nitrogen soils would be good mutualists.

Featured scientist: REU (NSF Research Experience for Undergraduates) Iniyan Ganesan from the Kellogg Biological Station

Flesch–Kincaid Reading Grade Level = 9.5

For more information on the evolution of cheating rhizobia, check out these popular science articles:

If you are interested in performing your own classroom experiment using the plant-rhizobium mutualism, check out this paper published in the American Biology Teacher describing methods and a proposed experimental design: Suwa and Williamson 2014

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