Lecture 15: Evolution of Communication


APS209 Animal Behaviour

The Evolution of Communication


1. Present examples of honest communication in animals.
2. Describe the logic in how natural selection can favour honest communication.
3. Present a detailed account of the logic behind begging as an honest signal of need in chicks.


1. Learn specific examples.
2. Understand why honest communication is an evolutionary puzzle.
3. Understand the logic behind how cost or common interest can make a signal honest.

We humans communicate all the time. Is this communication honest? Consider two situations. In the first, one person is selling a car to another and is telling the potential buyer about it. In the second, a pilot and navigator are flying an aircraft. In the first situation there is considerable incentive for dishonesty in communication. The vendor may not tell the truth about the car and the buyer may disguise his interest. But in the second situation there is no incentive for dishonesty concerning communication needed to keep the aircraft operating properly. Honest communication is also favoured in the waggle dance communication between nestmate honeybees, or between cells and organs in your body. In both cases there is nothing to benefit from making a dishonest signal. The communicating parties benefit equally from any advantage to plane, colony or body. Communication can still be honest or reliable, however, even if the interests of the different communicating parties are not identical. Consider the car sale. If the vendor tells obvious lies about the car the potential buyer will be put off, so there is incentive in being reasonably honest. (Society may also impose laws preventing sellers from dishonestly describing their goods, but that is another matter.)
            Ravens who have found an animal carcass in the Maine woods sometimes call others to the feast by yelling. Yelling is a deliberate and honest signal that food is available. How can natural selection favour one animal making a signal that attracts a competitor to food? It seems that yelling is not always favoured. If a resident raven finds a carcass on its territory it does not yell and may be able to have it all to itself, or to share it with its mate. But if a non-resident bird finds a carcass it yells to attract other non-residents. The attracted birds are too many to be driven off by the territory owners.
            Nestling birds often beg loudly. Parents often give more food to chicks that beg more. Why do they do this? If chicks that beg the loudest are also the hungriest then it makes sense for the parents to feed them more because a piece of food given to a hungry chick will normally increase its survival more that when given to a well-fed chick. By doing this, parents have more surviving offspring. But if parents give more food to chicks that beg more, what stops well-fed chicks from also begging as loudly as hungry chicks? There should be no physiological reason why they could not make as much noise. The puzzle can be resolved when we realise that begging also has a cost. Not only the energy of making the calls but also the attraction of predators. (And remember inclusive fitness. It is not just your own death, but that of your relatives, which is costly to cause.) The same amount of noise made by two chicks, one hungry and one well fed, would presumably be equally harmful in attracting predators. But the benefit of the begging would be greater to the hungry chick. Thus the optimum begging intensity would be greater for a hungrier chick. Paradoxically, it is the cost of making the signal that can keep begging an honest signal of need. (Question—if begging had zero cost what would be the evolutionary consequences of this for chicks and parents in terms of begging intensity and parental response?)
            By using simple figures, the cost-benefit logic underlying this argument can easily be seen. We can also see that if the cost of begging is reduced then the optimum begging intensity rises. One way that the cost can be reduced is via extra-pair parentage. This lowers the relatedness among chicks and so lowers the cost of nest predation to a begging chick, which would value the lives of the other chicks in the nest less. Data support this prediction. Chicks beg more loudly in species in which extra-pair parentage is higher.
            A similar cost-benefit argument can be made for male displays to attract females. Some displays, such as the peacock’s tail, are so elaborate that they actually handicap the male. If the handicap is more costly to make for a low-quality male than for a high-quality male, then the optimum size of the display (handicap) will correlate with male quality. In other words, male display can be a revealing signal of male quality when the display is costly.
            Some signals are honest because they are uncheatable. This is referred to as an index by Maynard-Smith and Harper (2003). For example, larger males may be able to make louder or deeper calls than small males. In many animals, from red deer to toads, deep male calls can deter rivals because a deep call will normally mean that a large male is making them. It is difficult to cheat on body size, although animals can cheat to some extent by inflating their abdomen, puffing out their chest, or making their hair stand on end. In doing this they are trying to look as large as possible, because large size is correlated with fighting ability in males and fecundity in females. In some spiders contests are decided by body size. A resident spider in a web can gauge the weight of an intruder by its effect on the web. Body weight is a reliable indicator of size.

            The book Animal Signals (Maynard Smith & Harper 2003) mainly addresses the question of honesty in communication. It categorises three basic ways that a signal can be honest or reliable.

common interest       e.g. communication within your body, bee dance, pilot & navigator
handicap/cost            e.g. costly chick begging, male displays that are handicaps.
index of quality         e.g. uncheatable indices of size such as the roar of a male red deer

Finally, it is worth remembering that communication can be subject to deception and conflict. The bolas spider attracts male moths by producing the pheromone made by female moths to attract males. Some predatory female fireflies mimic the flashing signal used by females of other species to attract males. These are known as “fire fly femme fatales”. Why do the males allow themselves to be attracted to a predator? Presumably, the predators are rather rare in relation to the legitimate signallers. On average, a male increases his fitness by responding to the signal, even if a small proportion of the signals are deceptive and lead the amorous male to his doom, rather than to a receptive female.

See Chapter 9 in Alcock’s Animal Behavior (2009).

See also: Maynard Smith, J., Harper, D. 2003. Animal Signals. Oxford University Press