Many species of insects and fish follow a semelparous life history, as well as all annual plants. The strategy is simple. Grow and develop, and then at some point stop growth and invest all remaining energy in reproduction. The organism invariably dies after it expends all of its reproductive effort. The literature often refers to this strategy as 'Big Bang Reproduction'. Semelparity contrasts with iteroparity in which reproduction occurs repeatedly throughout the life course (as with humans).
Make the following changes in the Life Table:
| Age: | 0-9 | 10-19 | 20-29 | 30-39 | 40-49 | 50-59 | 60-69 | 70-79 | 80-89 | 90-99 | 100+ |
| Death Rate: | 0.45 | 0.24 | 0.03 | 0.03 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Fertility: | 0 | 0 | 0 | 0 | 13 | 0 | 0 | 0 | 0 | 0 | 0 |
Notes on simulation values for semelparity:
For the purposes of the simulation, infant and juvenile mortality set to three times the baseline, because of no parental care. Death occurs 21.47 years before the average baseline lifetime, saving 21.47 years of somatic investment. Say one extra child equals equals three years of somatic investment. Total reproductive output equals 6.207 children plus 7.2 extra children from saved somatic investment for a total of 13 children.
An organism following this strategy invests time in learning about the world. This costs a delay in reproduction, but results in a slight decrease in mortality and an increase in later fertility. Researchers study variations in this strategy extensively because this process models growth and development. For example, longer growth phase (meaning delayed reproduction by definition) results in a bigger animal that acquires resources at a higher rate than smaller competitors, therefore enjoying higher fertility over a shorter reproductive span.
Make the following changes to the life table:
| Age: | 0-9 | 10-19 | 20-29 | 30-39 | 40-49 | 50-59 | 60-69 | 70-79 | 80-89 | 90-99 | 100+ |
| Death Rate: | 0.075 | 0.064 | 0.024 | 0.024 | 0.045 | 0.09 | 0.27 | 0.405 | 0.495 | .675 | 1 |
| Fertility: | 0 | 0 | 2 | 4 | 2 | 0 | 0 | 0 | 0 | 0 | 0 |
Notes on simulation values for education/growth:
For the purposes of the simulation, infant mortality reduced by 50% because of better parenting. Young mortality reduced by 20% due to later onset of competition for mates. Ten percent lower mortality after age 40 due to greater body size and health. Fertility delayed, and greater in later reproductive age classes.
Now consider an extremely common life history among primates and some sea mammals. In this case males compete for access to females in the population. This results in fierce aggressive behavior among males. Although risky, this strategy can result in high payoffs in reproductive success. To model this relationship, you will quintuple adult mortality and double adult fertility.
Make the following changes to the life table:
| Age: | 0-9 | 10-19 | 20-29 | 30-39 | 40-49 | 50-59 | 60-69 | 70-79 | 80-89 | 90-99 | 100+ |
| Death Rate: | 0.15 | 0.4 | 0.15 | 0.15 | 0.25 | 0.1 | 0.3 | 0.45 | 0.55 | 0.75 | 1 |
| Fertility: | 0 | 0 | 6 | 6 | 2 | 0 | 0 | 0 | 0 | 0 | 0 |
Notes on simulation values for aggression:
For the purposes of the simulation, mortality during reproductive ages increased five times. First reproduction delayed until full adult. Fertility doubled at all ages.
This strategy particularly interests evolutionary epidemiologists. In the face of a high risk of infection from a Sexually Transmitted Disease, why should anyone have unsafe sex? When considering lethal STDs such as HIV this question becomes very important. Unprotected sex greatly increases the chance of conception, but does this benefit offset the cost of acquiring infection? In this model you will increase the first years of fertility and increase the mortality force at later ages by an extreme amount. This simulates the progression of AIDS in humans.
Make the following changes to the life table:
| Age: | 0-9 | 10-19 | 20-29 | 30-39 | 40-49 | 50-59 | 60-69 | 70-79 | 80-89 | 90-99 | 100+ |
| Death Rate: | 0.15 | 0.08 | 0.06 | 0.6 | 0.6 | 0.8 | 0.8 | 1 | 1 | 1 | 1 |
| Fertility: | 0 | 2 | 3 | 3 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
Notes on simulation values for unsafe sex:
For the purposes of the simulation, double mortality for 20-29 age class, then extreme mortality afterwards. One extra child added in age class 10-19. Note that its very hard for this not to be the best strategy, regardless of mortality levels.
Quite common in nature, this strategy competes with the aggressive strategy. Given variation in adult body size, sneakers avoid competition altogether and "sneak" in some matings behind the backs of aggressive males.
Make the following changes to the life table:
| Age: | 0-9 | 10-19 | 20-29 | 30-39 | 40-49 | 50-59 | 60-69 | 70-79 | 80-89 | 90-99 | 100+ |
| Death Rate: | 0.15 | 0.008 | 0.003 | 0.003 | 0.005 | 0.1 | 0.3 | 0.45 | 0.55 | 0.75 | 1 |
| Fertility: | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
Notes on simulation values for sneaky:
For the purposes of the simulation, all mortality reduced to 10% of baseline. Fertility reduced to minimum for each age class, but onset of post-reproductive sterility delayed.
Create a life course according to some behavioral strategy that interests you. Make it as realistic as possible (for example IV drug use, smoking, high-risk sports, teen sex, etc.). Check with me before continuing.
Part II
Now you will create a population growth simulator in order to see how these strategies play off against one another. This kind of model assumes some heritable component to each behavioral strategy.
Go to the 'Generations' worksheet.
In the Reproductive Value column enter the reproductive values for each appropriate strategy.
Assume that each strategy starts with 100 individuals. Each individual generates offspring equal in number to its reproductive value. If an individual's strategy dictates 2 offspring in her lifetime, and 100 individuals follow that strategy, then (all else equal) there will be 200 individuals following that strategy in the next generation. This results in EXPONENTIAL POPULATION GROWTH.
Under generation 'g2' enter the following equation: '=D6*$C6'. Recall that the $ tells Excel to anchor column C as an absolute reference. This formula calculates that the number of individuals following a particular behavioral strategy in the next generation equals the number of individuals in the previous generation following that strategy multiplied by the number of offspring each produces.
Copy this formula to all of the behavioral strategies.
In the 'Total Population' cell use the 'Sum' icon (next to the Function Wizard) to sum the number of people in generation 1.
Copy cells E6 through E12 to all of the remaining generations.
You now see how each of these behavioral variants grow in the population. Unfortunately you can't really compare them to one another given the large values. To aid comparison, turn each frequency into a proportion of the total population:
Go down to the next table (cell D24). Enter the following formula: '=D6/D$12'. This formula calculates the proportion of the total population following the first strategy in generation 1.
Copy this formula to the rest of the behavioral variants.
Copy the formula to all generations.
Create a line graph showing the change in the proportion of the total population that each variant represents as the generations progress. To get started, block off cells C23 to M29. Choose the Chart Wizard and follow the directions.
When finished, your graph should look something like this:
Now go to the "ASSIGN #3 Answer Sheet" and answer the questions that you can. Print the answer sheet and your graph and you're finished with this exercise.