Life history tradeoffs and nutrition
Abstract
Students will learn how nutrient requirements vary at different stages of an organism’s life history. Also,
the concept of life history tradeoffs will be introduced and students will learn how animals allocate
energy to different needs (ex: juvenile: growth vs. adult: reproduction).
Background
The way animals allocate nutrients depend on many factors: age, sex, season, availability of resources
and type of resources required. Acquisition and allocation of these nutrients dictate if individuals and
species can survive and reproduce.
One of the most basic considerations in nutrient allocation is dietary requirements. All
organisms are defined by whether they can create their own energy (autotrophs) or must obtain energy
from other organisms (heterotrophs). Plants are the prime example of autotrophs as they use
photosynthesis to create energy from the sun. Animals, however, must obtain their energy by
consuming other organisms. They can be further broken down into categories based on which
organisms they consume to obtain their energy. Herbivores consume plants while carnivores consume
other animals. Many animals fall into a third category, which obtain nutrients from both plants and
animals (omnivores). Humans and dogs both fall into this category.
Based on which nutrients animals consume, they must make energy tradeoffs. Herbivores can
find nutrients more easily. Plants can be found almost everywhere, and they can’t run away from their
predators. However, plants are made largely of complex carbohydrates, which are difficult for animals to
digest. Thus, many herbivores have special digestive adaptations that allow them to obtain their
nutrients. For example, many herbivores, like cows, have four different stomachs! Carnivores, on the
other hand, can get a lot of nutrients from one meal. Animals are made of protein, which is very easy to
digest. However, it’s hard to catch other animals, and carnivores must use a lot of energy to obtain their
food. Carnivores use a couple of strategies to hunt: chasing prey and hiding (ambush predators).
Depending on their life stage and energy allocation, predators may be able to live a few days on one
meal where herbivores must eat almost constantly.
Once animals consume their nutrients, they need to allocate them for their basic life
requirements. One significant energy requirement is heat. Endotherms, which generate heat internally,
expend a lot of energy maintaining body temperatures. Ectotherms require much less energy because
their body temperature fluctuates with the environment. Depending on the season, it may be much
more difficult for animals to keep warm. Endotherms might use a lot of energy storing extra fat or fur.
In addition, animals may expend energy on travel and hunting. Many animals must travel long
distances to find nutrients. This could also depend on the season. Birds expend a great deal of energy
flying, for example, and must eat almost constantly while awake.
Animals may have varying nutrient requirements during life stages as well. Juveniles will exhaust
a lot of energy growing. Once they have reached full size, they then have to decide how much energy to
expend on reproduction. This is obviously very costly for females, who must store enough nutrients to
lay eggs or give birth. After the birth, many animals invest resources in parental care. There are two
basic parental care strategies: r and K strategists. R strategists focus on producing as many offspring as
possible. This is often caused by an unstable environment. The advantage of the r strategy is that plants
or animals produce many offspring that may carry on their genes. However, they invest low or no
parental care, and offspring have a low chance of survival. Many types of plants, particularly weeds, are
a great example of this. In the animal world, mice are a great example. K strategists, on the other hand,
invest a great deal of parental care, often providing resources for offspring until they are more than a
year old. These plants and animals (such as trees and elephants) tend to grow larger and live in more
stable environments. However, because they invest so much parental care, they usually must wait until
they are older to produce offspring and produce much fewer. Thus, if they lose an offspring, it is a huge
loss and a threat that their genes may not get passed along.
Males often also invest a lot of resources in reproduction. In some species, such as birds or
seahorses, males invest in raising the children. But they also must invest more in finding a mate. Bird are
a particularly good example of the cost of reproduction, often expending significant energy in bright
colors or elaborate mating dances that also put them in danger by making them more obvious to
predators.
Materials
Beans
Colored objects
Character cards
Four or five colors of play dough
Special r vs K dice
One regular dice
Engage
Begin with videos of different feeding styles
10 minutes
 Video of mothers feeding: http://animal.discovery.com/videos/mothers-dayclips-2007-feeding.html
Follow up by asking the students some basic questions:
 What are these animals doing?
 Why do we need to eat? What do we need energy to do?
Explore
Give each student a card with a character and limitations of that character. Send them
15 minutes
out to the playground to collect as many rocks as they can in two minutes (or until the
beans have all been collected).
 Male finch of reproductive age (must also collect colorful items)
 Adult python
 Pregnant python (must carry full backpack on front of body)
 Pregnant mouse (must carry full backpack on front of body)
 Adult cheetah
 Mother cheetah of three (must carry full backpack on back)
 Juvenile cheetah
 Adult cow
 Juvenile calf (can only walk)
 Mother calf of two (must carry full backpack on back)
 Mother finch of six (must carry full backpack on back)
 Senior cheetah (can only hop on one leg)
 Senior cow (can only hop on one leg)
 Can repeat or add as necessary
Explain
Bring students back inside and have them count their beans. Write a list of
15 minutes
requirements on the board that must be met (or a PowerPoint has been created to
address these concepts if you prefer):
 Endotherms: 10 rocks
 Herbivores: 20 rocks for food
 Carnivores: 10 rocks for food and 20 rocks for hunting

Expand
30 minutes
Reproductive males: discuss how more colorful items made them more
attractive to females and see who had most
 Pregnant females: r can have one baby for each 5 rocks; K needs 15 rocks per
child
 Mothers: need 20 extra rocks for each child to survive (only 10 for birds)
 Juveniles: 25 rocks to grow
Discuss what happens if the students don’t meet the requirements. Define r and K
strategists, discuss the differences between herbivores and carnivores and discuss
different life cycles and investment of parental care.
Have students flip a coin with r strategist on one side and K on the other. Based on
this, have them create their own organism. Decide what it will eat and where it lives.
Must have a food source to provide it energy and must decide how much it requires
daily depending on its size and energy requirements. How will it get its energy and
how will it expend it? Where will its energy source be found? Is it easy to find?
Give them each a small blob of play dough. Have them create their organism. Each 30
seconds, give everyone another blob, alternating color. This is for their organism to
grow. The r strategists will receive three blobs while the K strategists will get five.
Once they have received the play dough, they must now use their resources to
reproduce. So now, each 30 seconds, the students must use half a blob to create an
offspring (r strategists will start this before K strategists). When the r strategists run
out of play dough, the game ends.
1. First 30 seconds: all students get one blob of play dough.
2. All students get a second blob of play dough.
3. All students get a third blob of play dough.
4. K strategists get a fourth blob of play dough. R strategists must begin making
offspring with their play dough, so they donate a half blob to offspring.
5. K strategists get a fifth blob. Rs donate another half blob for another
offspring.
6. Both donate a half blob to offspring. Rs need only a half blob per offspring. K
need a full blob, so producing each offspring will take twice as long.
7. Continue until r strategists have produced all offspring.
Evaluate
20 minutes
Discuss with students what happened. Why do the K strategists still have an organism
while all r strategists have are babies? What are the pros and cons of being an r vs K
strategist? Roll dice again where 1-4 is a bad year and 5-6 is a good year. In a good
year, everyone survives, but in a bad year, r lose one offspring. If there is three bad
years in a row, Ks lose one as well. Roll 12-15 times and discuss how many organisms
students have left.
Have students research their favorite animals to find one that is a K strategist and one
that is an r strategist. Have them write a page discussing characteristics of this animal
such as:
 How old must it be to produce first offspring?
 How many offspring per litter?
 How often can it produce a litter?
 What does it eat?
 Is this resource readily available or difficult to obtain?
Then, have them discuss why they believe this animal is an r/K strategist rather than
the other. Our students loved presenting, so allow them to present to the class if they
would like.