- Define behavior and know what types of organisms exhibit behavior.
- Recognize that behaviors are encoded by genes and can evolve by natural selection.
- Define and differentiate between proximate and ultimate behaviors.
- Learn how behavior generates evolution of life history strategies through an evolutionary cost-benefit analysis.
- Connect behaviors with individual and inclusive fitness, and the potential for altruism; inclusive fitness is dictated by the relatedness between individuals.
Heritable response to stimuli are genetically encoded
Behaviors are actions in response to stimuli, and almost all organisms exhibit some form of behavior. Humans are pretty biased toward animal behavior, but even single-celled organisms react to their environmental surroundings. For instance, some bacteria emit small chemical signals into their environment. When they sense a critical mass, or quorum, of those signals, they change their own behavior, perhaps by releasing a toxin or by swimming away. This process is called quorum sensing, and it’s how the bacteria Vibrio cholerae causes the diarrheal disease cholera. In the slime mold genus Dictyostelium, individuals live as single cells until conditions become adverse. On an environmental cue such as low food availability, Dictyostelium swarm together into a multicellular fruiting body, where only a few individuals will survive to reproduce. Plants also respond to stimuli such as growing or turning toward the light, growing away from the pull of gravity, and some even respond to touch. And, of course, animals behave.
All behavior are encoded by genes at some level, and are therefore subject to evolutionary processes like genetic drift and natural selection. As a result, every behavior can be considered for how the action occurs (proximate cause) and why the action occurs (ultimate cause). Three-spined sticklebacks are small, spiny fresh or salt-water fish that have elaborate mating behaviors that include nest building and defense. Male sticklebacks in mating season have bright red bellies that are attractive to female mates. Males build nests in the rocky or sandy substrate near shore and then protect the nest from other red things, on the assumption that red things are males hoping to eat their eggs. Experiments have shown that males do not react to the presence of other sticklebacks, or stickleback-shaped things. Instead, they react exclusively to color, attacking any shape that is red (Tinbergen 1951). So, the proximate cause of the stickleback attack behavior is a red visual cue. The ultimate cause of the behavior is to protect their offspring, which will increase their reproduction, a key component of evolutionary fitness that we learned about in the evolution module. Here’s a short video re-enactment of his experiments, just for fun (the video is silent for the first few frames).
Think back to the Dictyostelium and Vibrio examples from above. What are the proximate and ultimate behaviors in those two examples?
Cost versus Benefit: When is a trait favored evolutionarily?
So, behaviors are encoded by genes, making them subject to selection. When an organism behaves in a way that is costly, natural selection should act to remove those organisms from the population. However, some behaviors are costly in one way but beneficial in another way. Many males have exaggerated physical traits like long and showy tails that might slow down escape from a predator but have an unexpected advantage in attracting mates. If females choose to mate with the males that have longer tails, or brighter colouration, or larger horns, or another seemingly costly trait, then those males have more offspring than males with shorter tails, duller colors, or smaller horns. So, the cost according to natural selection is outweighed by the benefit of sexual selection.
Cost-benefit analysis can also help us understand how behaviors that don’t seem immediately advantageous to the individual can be sustained over time in a population. How, for instance, can we explain the behaviors of the individual Dictyostelium who becomes one of the non-reproductive cells in the fruiting body? Or what about a yellow jacket worker, who spends her adult life helping the colony queen reproduce, but does not have the opportunity to have her own offspring? These seemingly altruistic acts can be explained evolutionarily using a cost benefit formula that includes a surprising element–the relatedness between the altruist and the recipient of the altruistic act.
Say that you have a clan of meerkats, social mongooses from southern Africa. Some clan members act as lookouts, giving a warning call when a predator is sighted. Calling is a risky behavior that can get a young meerkat killed, yet meerkats will still call, protecting the clan. A young male meerkat (focal) is brother to the alpha male in the clan. The focal meerkat is killed by a predator after giving an alarm cry that saves the other clan members, including the focal meerkat’s brother and his 6 babies. The focal male has an individual fitness of 0 offspring. However, he shares 50% of his genes with his brother, who has 6 offspring. By inclusive fitness, the focal has the equivalent of 3 offspring through his relatedness to his reproductively successful brother. Mathematically, inclusive fitness = (individual fitness) + (relatedness) x (fitness of relative). The table below shows common relatednesses:
Hamilton (1963) showed that altruism can occur via natural selection if r B > C (called Hamilton’s Rule), where
- r = fraction of shared genes between altruist and recipient (coefficient of relatedness)
- B = average number extra offspring beneficiary produces (benefit)
- C = how many fewer offspring altruist produces (cost)
Hank Green summarizes these ideas well in his Crash Course video on Animal Behavior: