Question

Animals that live in colonies with alarm calls usually live in large family groups. Animals that use alarm calls put themselves at risk but increase the chances of their relatives’ survival. Alarm calls put the caller at increased risk of predation by drawing attention to its location. Why might this behavior be favored by evolution?

Answer 1

This kind of behavior is called altruism. Altruism can be defined as a behavior that benefits other organisms at an individuals own cost. Now, intuitively it might seem as a disadvantage as the individual's own survival is put at risk. But here you have to understand that evolution focuses at survival of a gene pool or a population and not a single individual. Ultimately the more of your genes are spread in subsequent generations, the more evolutionarily successful you are.

To get a clearer picture we will try to discern the concept of inclusive fitness. This concept was developed by William Hamilton and Robert Trivers. Inclusive fitness is a combination of direct fitness and indirect fitness. Direct fitness is an individuals' chances of reproducing and passing on genes directly. Indirect fitness is capability to promote gene pool by promoting reproduction of relatives or kins. In this case if the individual organism is not able to reproduce to its' maximal possible extent, by ensuring the survival of its kins (which will carry many common genes) it increases its overall
fitness.

Whether altruism is favored or not can be estimated by Hamilton's Rule. It states that if rB > c (or rB - c > 0) then altruism is favored. Here r stands for coefficient of relatedness (the probability that an allele is shared by the two individuals in question); B for benefit to the recepient and c for cost to the donor.

Example - Q. Is it favorable for a bird to risk its life to save three of its siblings?
Ans. r = 0.5 (see below); B = 3; c = 1
Therefore, rB > c (1.5 > 1)
Yes, this is altruistic behavior.

r values for various relationships...

Parent - Offspring = 0.5 Siblings = 0.5 Half siblings = 0.25 Cousins = 0.125

The concept which is mentioned here is a classical model and many more interersting interpretations and models have branched off from here. For instance, one theory suggests that alarm calls might increase direct fitness of the organism too, that is by attracting other predators also (groups or species) and allowing the organism to escape in the confusion and panic created.