Lecture 22. Misleading signals of crypsis Warning signals of aposematism S ignals as isolating mechanisms Sexual selection Parental social selection Membracid bizzare may be sensory crypsis Neuron stereotypy and the omega neuron. Male stalked eye fly Papua New Guinea (PNG)
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Misleading signals of crypsis
Warning signals of aposematism
Signals as isolating mechanisms
Parental social selection
Membracid bizzare may be sensory crypsis
Neuron stereotypy and the omega neuron
Papua New Guinea (PNG)
Exhibit sexual dimorphism
Males gain access to mates
by defending resources
Stalked eye lengths are important in rivalry
Some morphology just seems bizarre.
is part of a bizarre
display. Why does it seem bizarre? Because natural selection should improve flight, not detract from it. How could natural selection favour survival and reproduction in individuals that fly badly?
bird of paradise
Beautiful but bizarre
Signals are transmitted information. The bodies of animals are used to communicate with others of their species. And body form is selected as the basis of a signal or display; stereotyped movements of specially modified body parts become a display: a signal between a sender and receiver. Sometimes an animal\'s body is selected to NOT send signals or more precisely, to \'misinform\': cryptic patterning and coloration is really the absence of signals. Many animal bodies involve crypsis combined with an absence of movement.
Both eggs and hatchlings blend into the substratum and are difficult to detect visually.
Smaller majority website
The body may also be selected to signal a warning: an honest warning in a coral snake that this creature is truly dangerous and should be left alone; a dishonest warning in a mimic of the venomous (noxious) model.
Red and yellow kill a fellow
Red and black venom lack
An oedipodine crackling locust on an abandoned asphalt road surface, near Lake Superior. This coloration is certainly not bizarre, it is expected, i.e., is produced by natural selection favouring beneficial markings.
Cryptic and aposematic body forms are reasonably seen as the product of natural, not sexual or social selection. Long tails are bizarre and seem unlikely to contribute aerodynamically to flight.
Why do these birds have long tails?
Males and females engaged in mating behaviour exchanged signals using structures, either on the body itself (genitalia), or removed from the body itself, i.e., sound and vibration signals, chemical signals, light waves.
There is another way of explaining the evolution of species-distinctive body parts: not by natural but by sexual selection. This is selection arising from premating choices that occur in two possible ways: either as one sex fights for the other or as one sex chooses the other. Fighting, rivalry, threat, ways of settling the issue of mate access occurs usually between males: it involves morphological features such as horns serving as threat signals and weapons; but it can involve things like stalked eye width.
Choice on the other hand is usually imposed by females and leads males to compete intensely to gain the female\'s attention. Male bodies become altered by adornment that female’s notice. The sensory capacities of the female channel the sort of display that evolves (sensory drive). If she sees red better than blue then the feathers of a male\'s display will do better affecting her choice if they are red-pigmented. Body surface features become elaborately altered to promote being chosen. These changes often will be at odds with body functions shaped by natural selection. The tail of a widow bird male increases his success in mating at a cost to his ability to fly (Balmford et al. 1993). Other male birds fly better because they are closer to the optimum dictated by natural selection, but if the females pick the longer tailed bird, he is NET better in the competition for mates and his kind of geneholder increases in the population.
There is an arbitrariness in signals. What evolves under the reproductive isolation hypothesis doesn\'t matter: it just has to be different to keep the species separate and encode the right information. The same arbitrariness is present in sexually selected mate choice by females: it doesn\'t matter if one female prefers red and yellow plumage marks on the carpal (elbows) of the male (red-wing blackbird) or tail length (widow birds). Distinctiveness must be achieved for species isolation. Distinctiveness is a byproduct of Fisherian runaway selection in the case of female imposed sexual selection.
The prevailing view now as to why there are diagnostic genitalia, and diagnostic acoustic and visual displays: has to do with arbitrary features being seized upon and rapidly elaborated by female preferences operating within a species (just as indicated in the argument advanced by Eberhard). The selection occurs independently and the features rapidly diverge along separate paths. So because females arbitrarily respond positively, i.e., \'choose\' a long tail rather than carpal elbow plumage this is what is elaborated as a signal mediating mating pairing. Females with the genetic basis for choosing long tails have sons with longer tails who in turn are more preferred within the population: this is runaway sexual selection. It may run counter to natural selection: tail length compromises naturally selected features of tails: "Tails provide lift during take-off and flight and act as a rudder for aerial turns. Elongation of the tail for display purposes increases the drag on the flying bird and greatly reduces agility, maneuverability, and flight speed." (Bradbury & Vehrencamp, p. 552).
signals selected to achieve parental attention as chicks compete for food. They are produced by the choices being made by the parents.
Parental choice can impose selection too and create signals that may seem bizarre though perhaps carrying less cost than display plumage.
Lyon B.E., Eadie J.M., Hamilton L.D. 1994. Parental choice selects for ornamental plumage in Amercian coot chicks. Nature 371: 240-243.
A bizarre pronotum
Crypsis and sensory
diversity. There is no sexual dimorphism
involved in these bizarre pronotal differences. So they seem not to be the product of sexual selection.
Could they actually be cryptic but it is not apparent to us because we have different sensory capacities than the predators whose eyes were the basis of this selection?
B Prud’homme et al. Nature473, 83-86 (2011) doi:10.1038/nature09977