Question

We are analyzing a species of bird in which males either stay behind and help care for siblings and delay having offspring for a year (helper males). Alternatively, males may also delay having offspring for a year, but provide no help in caring for siblings (delayer males). The helper male bird cares for 2 full sisters (r=.5) in the first year. However, by caring for his sisters the male uses a lot of energy and only produces 1 offspring in the second year(r=.5). The delayer male bird had no offspring in the first year, but then has two offspring in his second year (r=.5).

What was the benefit to the male in units of indirect fitness for caring for his siblings? What is the calculation for direct fitness for the helper male? Add these numbers together for the helper male. For the delayer male, what is the calculation for direct fitness for the offspring produced in the second year? Compare your calculations. In this species is it better for a male to help with siblings or delay?

Answer 1

Direct fitness is given by the number of offspring an individual produces, while indirect fitness is given by the number of relatives an individual helps rear. Both direct and indirect fitness is calculated by the number of offspring/relative multiplied by the degree of relatedness (given here by r).

Inclusive fitness is given by the sum of both direct and indirect fitness.

So, for the helper male bird:

Indirect fitness in the first year = 2 sisters x 0.5 degree of relatedness to each of them = 1

Direct fitness in the second year = 1 offspring x 0.5 degree of relatedness = 0.5

Therefore, inclusive fitness = Indirect + direct fitness = 1 + 0.5 = 1.5

And, for the delayer bird,

Indirect fitness in the first year = 0, as it does not help to rear any siblings.

Direct fitness in the second year = 2 x 0.5 degree of relatedness for each of them = 1

Therefore, Inclusive fitness = 1

In other words, since the degree of relatedness between full siblings and offspring of an individual is same, the helper bird over two years helps raise 3 offspring equivalents in total. However, the delayer bird on the other hand successfully raises only 2 offsprings.

Therefore, overall inclusive fitness is greater for helper male birds than delayers. So, we can say that for this species it is better for males to help with their siblings than delay, as it would increase the bird's inclusive fitness. The two full sisters of the helper bird would further reproduce and his genes will be carried on through his nephews and nieces as well as grandchildren, which ultimately increases the bird's fitness even if it is not personally reproducing.