The differences between handicaps and indices are usually distinguishable in formal mathematical models or in unambiguous real-world cases. Often though, classifying a trait as a handicap, an index, or even a signal at all, can be quite a difficult task.
For the purposes of illustration I will use Fluctuating Asymmetry (FA for short) as an example. Fluctuating asymmetry is the term used to refer to deviation from symmetry in paired morphological structures (ranging from birds’ tails to human faces) that should be, all being well, bilaterally symmetric. Deviations from the ideal symmetrical phenotype are caused by inherent genetic perturbations and exposure to environmental disturbances occurring in early development.
Is FA a signal?
In their 2005 book Animal Signals, Maynard-Smith and Harper define a signal as:
‘Any act or structure which alters the behaviour of other organisms, which evolved because of that effect, and which is effective because the receiver’s response has also evolved’
They then argue that FA is unlikely to function as a signal because it is difficult to discern whether receivers respond directly to FA and because there appear to be few examples of displays in which signallers actively advertise their symmetry to receivers.
More elephant based news!
A new study from the Proceedings of the Royal Society B, published today, has found that elephants pay attention to the oldest female elephant in their group when a predator is approaching.
The research, carried out in Kenya, used recordings of roars from both male and female lions and monitored the reactions of groups of African Elephants. It has been known for a long time that elephants social groups are formed around a matriarchy. The experiment found that groups of elephants with matriarchs quickly organised themselves into defensive bunch formations after appearing to stop and pay attention to their female leader. These groups were also much more likely to approach the loud speaker producing the roar in an aggressive manner.
Male lions present a greater threat to groups of elephants as they are much more likely to attack elephants when alone and are usually much more successful than females who will only attack when part of a group. The elephants showed an ability to differentiate between male and female lions. The study also showed that matriarchs who were much older were much more likely to react in the appropriate way to roars made by male lions which is thought to be the result of experience.
The signals which allow the Matriarch to elicit this co-ordination among her group are still largely unknown due to the lack of loud vocalisations and Karen McComb and Graeme Shannon, who lead the initial study, are now looking into finding quieter, less obvious vocalisations and posture cues.
The study provides the first empirical evidence that within a social group, individuals may gain benefits from paying attention to an older leader because of their abilities in making decisions when under threat. This generates insights into selection for longevity in cognitively advanced social mammals.
One of the most important concepts in animal signalling theory, proposed by Amotz Zahavi in a seminal 1975 paper and in later works (Zahavi 1977; Zahavi & Zahavi 1997), is the handicap principle. A general definition is that females have evolved mating preferences for males who display exaggerated ornaments or behaviours that are costly to maintain and develop, and that this cost ensures an ‘honest’ signal of male genetic quality.
As a student I found it quite difficult to identify a working definition for this important type of signal mainly due to the apparent ‘coining fest’ that has taken place over the years since Zahavi outlined his original idea in 1975. For this reason, I have decided to provide a brief outline of the terminological and conceptual differences that exist in relation to the handicap principle in an attempt to help anyone who might be struggling to navigate the literature.
As Zahavi did not define the handicap principle mathematically, a number of interpretations can be found in the key literature due to scholars disagreeing as to the true nature of his original idea. Until John Maynard Smith and Harper simplified and clarified things wonderfully in their 2003 publication Animal Signals, to my knowledge at least four different interpretations of the handicap were being used and explored empirically and through mathematical modelling, each with distinct differences that aren’t all that obvious to grasp without delving into the maths.