Behavioral Ecology

Part I: Keep Growing or Start Reproducing?
Female green swordtails, Xiphophorus helleri, grow throughout their lives (indeterminate growth)
although the growth rate slows down after they become sexually mature. Swordtails have internal
fertilization and give birth to live offspring. Body size is positively correlated with brood size: larger
female fish produce more offspring per pregnancy. Predation on females takes place almost exclusively
in the non-breeding season. You can assume for the purpose of this assignment that females live up to
six years and can have a maximum of five breeding seasons. Suppose that studies have shown that there
are three distinct life history strategies in a population of these fish.
Strategy 1: Early maturation – if females mature after their first year of life at 25 mm, they will produce
on average:

Breeding season 1:
Breeding season 2:
Breeding season 3:
Breeding season 4:
Breeding season 5:
9 offspring at 25 mm
12 offspring at 30 mm
18 offspring at 35 mm
31 offspring at 40 mm
43 offspring at 45 mm
Strategy 2: Delayed maturation – if females remain juveniles for one more year, they will be 35 mm at
maturity and produce
on average:
Breeding season 1:
Breeding season 2:
Breeding season 3:
Breeding season 4:
Breeding season 5:
0 offspring – immature
18 offspring at 35 mm
31 offspring at 40 mm
43 offspring at 45 mm
60 offspring at 50 mm
Strategy 3: Late maturation – if females remain juveniles for two more years, they will be 45 mm at
maturity and produce
on average:
Breeding season 1:
Breeding season 2:
Breeding season 3:
Breeding season 4:
Breeding season 5:
0 offspring – immature
0 offspring – immature
43 offspring at 45 mm
60 offspring at 50 mm
70 offspring at 52 mm

Question 1 (3 pts): If female survival is high, which of the three patterns above maximizes lifetime
reproductive success? Explain your reasoning.
Question 2 (4 pts): Let’s see how survival of females figures into this. Predators target females that are
47 mm or larger and, on average, 50% of the females that are 47 mm or larger survive to breed to the
next season. For simplicity, assume that growth happens right after the breeding season is over, before
predators begin to attack. For example, females of strategy 2 grow to 47 mm big right after completing
breeding season 4 and they are targeted by predators before starting breeding season 5.
How do you predict females in this population and under these circumstances to evolve? Which strategy
maximizes their lifetime reproductive success? Explain your reasoning and show your work, i.e. include
your calculations.
Hint: figure out what the average number of offspring is for each life history strategy.
Another question on the next page!
Note: Females don’t make a conscious decision
to do one or the other but rather they have alleles
that encode proteins that somehow make them an
early, delayed or late breeder.


Part II: Size and number of eggs (has nothing to do with the scenario above)
Question 3 (3 pts): Evaluate the graphs below (familiar from our virtual class discussion on Tue 9/29).
Each graph shows the body size and the number of eggs produced over the lifetime of an average
individual for species 1 and species 2. Suppose that egg size is
not correlated with body size. Based on
your understanding of life history, what can you predict about the egg size of these two species? Do you
expect one species to have larger eggs? Or should they be similar – or perhaps you cannot make any
predictions? Whatever your answer, explain your reasoning.

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