When different species live together in close contact, they can interact with each other in a number of different ways.
In this lesson you will investigate the following:
• What are the types of symbiosis?
• What is parasitism?
• What are the different types of parasite-host relationships?
• What’s it like to be a parasite?
So let’s get stuck in and start sucking the life out of this lesson!
This is a print version of an interactive online lesson.
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Cuckoos are known for not building their own nests. Instead these birds let someone else do all the work to build a nest, then lay their eggs there. They don’t even wait around to bring up their chicks when they hatch – they let the other birds do that too.
Scientists have just discovered how they have been getting away with this for so long. While having an extra mouth to feed is a burden, it seems the cuckoos do provide something useful in exchange.
While studying crows’ breeding habits in Spain, scientists saw lots of cuckoos laying eggs in crows’ nests and magpies’ nests. The magpies fought back and threw out the cuckoo eggs (if they noticed them). But the crows allowed the eggs to stay, letting them hatch and then feeding the cuckoo chicks as they grew.
Curious, the scientists investigated, taking note of how well the crows with cuckoos did compared to crows without a cuckoo tenant.
They discovered that when they handled the cuckoo chicks, the little birds released a foul-smelling substance that predators wouldn’t eat. So by having the baby cuckoos in their nests, the crow parents could leave their own chicks alone knowing that the cuckoos would protect them. It’s a great example of two species helping each other out.
Read or listen to the full Cosmos magazine article here .

Left: Cuckoo chicks release a foul-smelling slime, which seem to drive some predators away. Right: Sea anemones sometimes hitchhike on the back of hermit crabs. Credit: Daniela Canestrari and National Geographic / Getty Images.
Identify: Describe at least five interactions with other organisms that you frequently have in your life – these do not necessarily have to be interactions with other humans and they do not have to all be positive interactions. Place a positive [+] sign next to the individuals that are benefiting from the relationship, a negative [–] sign next to the individuals that are being harmed by the relationship, and a neutral [0] sign next to the individuals that are unaffected by the relationship.
For example: I [+] like to interact with my dog [+]. He helps me to feel happy and I teach him new things.
Hint: To share your answer with your class, ask your teacher to open the discussion board at the bottom of this page and write your answer there as well.

All organisms are dependent on other organisms in some way. For example, during its lifetime, an animal such as a kangaroo is likely to interact with millions of other organisms including microbes, parasites, mates, predators and food.
In contrast with predator–prey relationships, where the predator attacks the prey with the intention of killing and eating it, symbiotic relationships occur where different species interact and live side by side with each other.
Symbiosis refers to a long term ecological interaction between different species that live close together, where one or more of the species benefit. Think back to the answers you gave to the question in the Introduction section of this lesson. You may now recognise that each of the interactions you described can be classified as symbiotic.
There are, in fact, three different categories of symbiotic relationships.
1. Parasitism
2. Commensalism
3. Mutualism

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Credit: Untamed Science / YouTube.
Match: With the help of the information in the media clip above, draw lines on the sketchpad to match the types of symbiosis with the appropriate description and photo.

Complete: Select one of the examples above (cleaner fish with a shark, barnacles on a whale or ticks on a dog) and assign a number to each species to complete the following table.
Example selected
Species 1
Species 2
Describe what you think would happen to the population of species
1 if species 2 was no longer present
Describe what you think would happen to the population of species 2 if species 1 was no longer present
Below are three sets of graphs, each representing one of mutualism, commensalism or parasitism. The graphs show the populations of two species over time, living separately and together. Let's call the species represented by the red line species A and the species represented by the blue line species B.

Describe: Analyse each set of graphs to infer the type of symbiotic relationship represented and briefly explain your choices.
Set
A
B
C
Type of symbiotic relationship Explanation

Left: Fleas live symbiotically with cats. What kind of symbiotic relationship is it? Right: Scanning electron micrograph of a cat flea (Ctenocephalides felis). Credit: ViviSuArt / Getty Images and Biophoto Associates / Getty Images.
If your pet has ever had fleas then you have already witnessed parasitism in action. Parasitism refers to a symbiotic relationship, where the parasite benefits at the expense of the host.
Parasites generally benefit from the relationship with their host by obtaining nutrients from them, but other potential benefits include gaining protection from predators and geographical transportation. While parasites are adapted to living inside
(endoparasite) or on the outside (ectoparasite) of their hosts, they can only survive by reproducing offspring that are capable of finding new hosts.
Although the host is harmed by its interaction with a parasite, ideally the relationship does not end with the death of the host, otherwise the parasite must then find another host. However, some parasites are more destructive than others and can actually change the behaviour of their hosts.

Ant
Cricket
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Credit: Howcast / YouTube.
Summarise: Watch the media clip above and explain how each parasite can change the behaviour of its host so that it can grow and spread to even more hosts.
Image Description of how the parasite can change the behaviour of its host
Explanation of how this helps the parasite spread and survive
Snail
Host organism
Many parasites have complex "life histories" that involve one or more host species for the different stages of the its life cycle. The following media clip shows the life cycle of a tapeworm called Dipylidium caninum that uses not one, but two host species.

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Credit: Sally Cantle / YouTube.
Complete: Sequence the items in the following list into their correct locations on the life cycle diagram below.
immature tapeworms flea larvae sac of tapeworm eggs tapeworm eggs multi-segmented tapeworm excreted from anus of cat or dog eaten by reproduces digested by cat or dog to release grows inside small intestine of cat or dog

Suggest: The dog and cat tapeworm is much smaller, and develops much faster, than its host. This is also true of the cuckoo brood parasite, which is smaller and develops much faster than the crow chicks it shares a nest with. Suggest what advantages this might give a parasite.

These nasty little parasitic protozoa are called "sleeping sickness". They enter the blood stream through the bite of a tsetse fly, causing the host fevers, headaches and sleeping difficulty, among other symptoms. This coloured scanning electron micrograph shows them amongst red blood cells. Credit: Eye of Science / SPL Creative / Getty Images.
Imagine that you are scientist at the World Health Organisation (WHO). Your main role is to research parasitic diseases and document novel parasite-host interactions. Sometimes you find yourself thinking about what life might be like as a parasite....
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Credit: The Fuse School / YouTube.

Parasites have adapted to survive best in their host's specific environment.
These adaptations might be:
Structural (e.g. a mouth with hooks to attach to host intestines)
Physiological (e.g. ability to digest blood to obtain energy)
Behavioural (e.g. making a cricket jump into water)
Create: Your challenge in this activity is to design an original, never before discovered, parasite that has adapted to survive in a specific host.
The following questions will help guide you through the design process of your parasite and its host. Type your answers to the questions in the Project Space below.
1. What type of organism will the host of your parasite be?
2. What environment will the host of your parasite live in?
3. What will the body size and shape of your parasite be?
4. What methods of travel will your parasite use to reach the host?
5. What part of the host body will your parasite infect?
6. What strategies will your parasite use to obtain its food?
7. What damage will your parasite do to its host?
8. What evasive or defensive strategies will your parasite use to avoid being killed by the host?
Now create a visual representation of your parasite and its host with labels that describe how each of its features help it survive in or on its host. Identify these adaptations as structural, physiological or behavioural. You may wish to design your parasite and host on poster paper, photograph it and upload the image below.
Give scientific names to your parasite and its host.

Sly tricksters, singing divas, and musical drummers are just a few of the many interesting characters Naomi Langmore
meets through her job. But she doesn't work in Hollywood – Naomi is a zoologist from the Australian National University who studies the complex and often amusing world of birds.
Naomi spent the first eight years of her life in Papua New
Guinea. Although she left the lush rainforests behind, Naomi’s love for animals followed her all the way back to Canberra, where she moved to attend school. Growing up with lots of different pets, Naomi learnt she could make a career out of doing what she loved most – studying animal behaviour as a behavioural ecologist.
She pursued her dream by studying biology and psychology in high school. But Naomi credits her English classes with polishing her writing ability, a skill crucial for a successful career in science.
Throughout her career, Naomi has looked at many curious aspects of bird behaviour. She has studied how cuckoos trick other birds into raising their babies, why female birds sing, and even how cockatoos drum out a beat using tree branches. Being able to answer these fascinating questions in biology is incredibly exciting, she says. For Naomi, it’s the best job in the world.
Much like the birds she studies, Naomi’s lifestyle changes drastically with the seasons. In the spring and summer, she gets to watch her birds out in the field. The life of a bird is a busy one, and Naomi usually has her hands full keeping up to date with the many social dramas in a flock. When autumn and winter roll around, Naomi returns to university to spend the cold months indoors, teaching and writing.
Even when she isn’t working, Naomi enjoys the wilderness. She loves going camping, bird-watching and bush-walking with her family and her dog, Panda.

Research: Naomi's career as a behavioural ecologist developed out of a love for animals, which she has had since childhood. Think back to when you were a child - which animals did you love? Were you fascinated by worms or beetles or lizards; or perhaps even daddy-long-legs spiders?
Using the internet, research a parasite-host relationship that your favourite childhood animal is involved in. Identify whether it is playing the role of parasite or host and describe how it benefits from this close ecological relationship. As part of your research, geographically locate a country and institution (e.g university) where research about this parasite-host relationship is taking place.
Cosmos Lessons team
Education Director: Daniel Pikler
Education Editor: Bill Condie
Art director: Robyn Adderly
Profile author: Yi-Di Ng
Lesson authors: Hayley Bridgwood and Paula Mills