Three models that can explain the maintenance of altruism are:2. In some bird species, individuals born the previous year (or even earlier) remain at the nest to help their parents raise this year's batch of young. Provide some possible explanations for this cooperative behavior in which offspring initially forego attempts at their own reproduction to help raise their siblings. Try to express your answers in the context of a cost-benefit analysis.
a) kin selection- altruists can indirectly increase their fitness by helping their relatives. Because altruists and their relatives share genes, helping a relative leave more offspring in the next generation can increase the altruist's genetic contribution to the next generation.b) reciprocity- unrelated individuals should help each other if they can expect to get help in the future (“tit-for-tat” strategy). This requires that the individuals can recognize each other, and are likely to interact more than once (reciprocity will not work between strangers). Individuals that cheat (don’t reciprocate) should not be helped in the future.
c) group selection- unrelated individuals can help each other if you consider subdivided populations (structured demes). Suppose a population of birds consists of 25 altruists that make alarm calls, and 25 selfish birds that do not call. They forage in groups of 10 birds, where the groups have different numbers of altruists and selfish birds. Within each group, altruists have a higher risk of predation because they are conspicuous when they call. But groups that consist of more altruists suffer less mortality overall because the whole group is more likely to detect and escape predators. When the groups rejoin during the breeding season, there will be more surviving altruists than selfish birds.
Possible benefits of helping raise siblings: 1) a helper increases its inclusive fitness (it benefits indirectly) by helping raise its relatives. Because a helper shares 0.5 of its genes with its sibs (assuming full sibs), it gains the same amount of genetic contribution to future generations by helping raise its siblings as it would by raising its own offspring (see link below for schematic explanation of this). 2) In addition, a young, inexperienced bird can benefit directly by staying at the nest, helping raise its younger siblings and learning how to raise offspring. This will allow the helper to be more successful in raising its own offspring in the future (novice breeders do not usually have as high offspring survivorship as do older, more experienced breeders-this is thought to be due to inexperience).3. Why might ritualized conflict behaviors, like growls and aggressive/submissive posture have evolved?
Possible costs of helping raise siblings (and being in a larger group at the nest): 1) a helper puts off its own reproductive efforts (although this isn’t a big cost since a helper is equally related to its siblings as it is to its own offspring), 2) possibly competition for resources, 3) possible increased risk of disease or parasite transmission, and 4) possible attraction of predators (due to larger group size).
By ritualizing conflict behaviors, animals can assess (i.e., “size each other up”) and determine who is stronger (more dominant) and more likely to win a fight. Thus, weaker, subordinate individuals can avoid serious injury or even death.4. What are some of the possible costs and benefits to living in a group?
Costs of living in a group: large groups are more conspicuous and can attract predators more easily than solitary individuals; large groups may experience increased competition for resources and increased risk of disease transmission. In addition, if outcrossing is limited (individuals don’t mate with individuals outside their group), inbreeding can result in reduced offspring viability (in extreme cases).5. For the following hypothetical organisms, please indicate whether you'd expect the male or female to exhibit more mate choice (i.e., which should be "choosier"), and whether you'd expect the male or female to be more brightly colored or adorned with "advertisements". Please also explain your rationale.Benefits of living in a group: possible benefits from altruism (reciprocity); added protection from predators (via mobbing behavior, scanning, alarm calls, swamping or dilutioneffects); easier to find mates; groups may capture or find patchily distributed food more easily.
b. A male builds a nest and guards the eggs from predators. The nest is small and can hold only a single clutch. Males cannot raise more than one brood, although females can lay several clutches of eggs in a single season.
c. Fertilization is external. Females require great quantities of energy to make eggs than males require to make sperm.
d. Fertilization is internal and development is viviparous (within the female). Brood size is very small (one or two young) and gestation time is long.
a. male reproductive success is limited by access to females, therefore males should advertise and females should be choosy.
b. female reproductive success is limited by males, so females should advertise and males should be choosy .
c. females make a greater contribution than males (costs are greater for females), so males should advertise and females should be choosy.
d. again, females make a greater contribution than males (costs are greater for females), so males should advertise and females should be choosy (females can only have a few young over a long period of time, but males can try to inseminate as many females as possible during that same amount of time).
Behavioral ecologists use optimization models to evaluate and find solutions to complex problems that often involve trade-offs, costs and benefits that would be difficult to intuit otherwise. Since animals have limited time and energy to devote to a range of activities, the underlying assumption of optimization models is that animals try to choose the "best strategy", which will maximizes their fitnesses. These strategies are compromises among devoting time to different activities such as scanning for predators, looking for food, reproducing, etc.7. In optimal foraging models, a number of trade-offs and costs and benefits are assumed. Give some examples.
Scientists do not believe animals are perfect machines. For instance, animals makes mistakes, and there could be several different strategies that result in similar fitness. Or, the environment is complex and variable, so the "best" strategy is always changing. Or, it may not be possible to attain the "best " strategy. In general, behavioral ecologists often consider optimization models as "yardsticks" by which to judge the actual behavior of animals. They also use these models as a way to formulate hypotheses and generate tests of these hypotheses as was seen in the Werner and Hall experiments and Mittelbach experiments.
Assumptions of optimal foraging models:8. Imagine four different types of prey that are eaten by a species of bird.
Benefits: gain energy (calories) from food and gain essential nutrients.Costs: risk of predation while foraging and time taken away from other vital activities such as reproducing.
Trade-offs: the best feeding locations may also be the riskiest (open water for small bluegill fish), the most energetically rich foods may also be harder to capture or require more handling time, handling and searching time aren’t simultaneous-the more time you spend handling prey, the less time you have to search for prey, and finally, because time is limited, you have a limited amount of time to reproduce and avoid predators.
| PREY TYPE | ENERGETIC VALUE | HANDLING TIME |
| pill bug | 3 joules | 10 sec |
| earthworm | 20 joules | 20 sec |
| beetle | 15 joules | 30 sec |
| millet seed | 2 joules | 1 sec |
a) If all prey are equally abundant, and their densities are very high (so that search time is minimal), please use optimal foraging theory (as discussed in lecture for the bluegill-daphnia system) to predict what the bird will feed on (i.e., which prey will be included in its diet)?b) How will the diet change as the total prey density declines? Or in other words, which prey will the bird attack and consume as prey density gets lower and lower?
First, you must figure out the profitability (e/h) of each prey type. To do this, divide each prey type's energtic value by its handling time. What you get is e/h for:Figures a and b
pill bug = 0.3
earthworm = 1.0
beetle = 0.5
millet seed = 2.0a) Remember, if prey is really abundant, then you can afford to be really selective and eat prey with the highest profitability (e/h-energy per prey item per handling time). So, birds should select millet seeds.
b) Since prey is usually not super abundant in reality (organisms have to consider searching time), remember there is some optimum profitability, C*, where the density of prey determines how selectively you should forage (see link below for graphical representation). In other words, for a given density of prey, any prey item that is above C*, the bird should eat, and any prey item below C*, the bird should not waste its time eating (figure a). Therefore, as prey density declines, the bird should forage not only on millet seed, but also earthworms. As the overall density of prey continues to decline, the bird should feed on millet seed, earthworms and beetles (C* is depressed or moves to the right along the x axis of figure b as prey density declines). If prey densities become low enough, then the bird should not forage selectively and eat any prey item it finds.