Question

The Tasmanian native hen (Gallinula mortierii) is a flightless bird in which males often outnumber females. In this system, either a pair (male and female) raises offspring, or a trio (a pair helped by a subordinate male) raises offspring. The subordinate male in a trio is usually the brother of the male of the pair. Given the following data: a. First year breeding pairs with no helpers average 1.1 offspring produced during that year. b. First year breeding trios (Male and Female + subordinate male) produce 4.1 offspring on average. c. Experienced pairs produce about 5.5 offspring per year without helpers d. Experienced pairs with helpers (i.e. a trio) produce about 6.5 offspring per year. If a first year male is faced with three choices -- breed on its own, help a brother in his first year, or help an older, experienced brother -- which should it choose and why? Show your work and calculations, and state which of the three options is best, second best, and worst. (Remember, helpers are helping raise nephews/nieces, not siblings).

Answer 1

The above situation can be explained under the concept of inclusive fitness and altruism. Inclusive fitness is a theory which states that the genetic success of an organism depends upon the cooperation, altruistic behavior (behavior of an organism that benefits others at its own expense), and social interactions. Altruism might evolve as a trait as long as its benefits are seen in individuals that are genetically related to the donor. Altruism will evolve when fitness cost to the donor is less than the fitness benefit of the recipient. W.D. Hamilton demonstrated this mathematically which became Hamilton's Rule. He devised a formula, r X B > C where,

B is the benefit gained by the recipient of altruism. It is expressed as the number of offspring equivalents.

C is the cost suffered by the donor while performing altruistic behavior. It is expressed as the number of offspring equivalents.

r is the coefficient of relatedness i.e., the genetic relatedness of altruist to the beneficiary. r is 0.5 between parent-offspring or between full siblings, 0.25 for nephews and nieces, 0.125 for first cousins and so on.

The mathematical expression can also be written as (rB X B) > (rC X C)

According to Hamilton's rule,

Relatedness of the donor to own offspring, rC = 0.5

Relatedness of the donor to brother's offspring, rB = 0.25

C = cost in terms of direct fitness i.e., producing its own offspring = 1.1 offspring (as given in the question)

B when helping the first-year brother = 4.1 - 1.1 = 3 offspring (net benefit to the first-year altruist)

B when helping experienced brother = 6.5 - 5.5 = 1 offspring (net benefit to the experienced altruist)

So, substituting the above values,

rC X C (cost in terms of producing own offspring, measured as direct fitness) = 0.5 X 1.1 = 0.55

rB X B when helping the first-year brother (benefit of helping the first-year brother, measured as indirect fitness) = 0.25 X 3 = 0.75

rB X B when helping the experienced brother (benefit of helping experienced brother, measured as indirect fitness) = 0.25 X 1 = 0.25

As we can see from the above results, helping the first-year brother would be most beneficial (0.75) followed by breeding alone or producing own offspring (0.55) with the least beneficial being helping the experienced brother (0.25).