Lecture 22

Misleading signals of crypsisWarning signals of aposematismSignals as isolating mechanisms

Sexual selectionParental social selection

Membracid bizzare may be sensory crypsisNeuron stereotypy and the omega neuron

Some morphology just seems bizarre.Male stalked eye flyPapua New Guinea (PNG)Exhibit sexual dimorphismMales gain access to matesby defending resourcesStalked eye lengths are important in rivalry

This tailis part of a bizarredisplay. 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?

Euplecteslong-tailedwidow bird

NationalGeographic

Lecture 22PNGbird of paradise

Beautiful but bizarre

Both eggs and hatchlings blend into the substratum and are difficult to detect visually.

red-wattled lapwing

Wikkipedia

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.

Dita Klimas

Dita Klimas

Smaller majority websitePiotr Naskrecki

glasskatydid

Phlugis sp.

Aposematic warning coloration

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 fellowRed and black venom lack

Pardalota reimeri

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?

Mot mot

Widow bird

And surely this tail is bizarre for a flying animal.

Signals as isolating mechanisms

• The other day I talked about the evolution of diagnostic genitalia. Why should such body parts be distinctive, so distinctive that by genitalia alone one can diagnose species? Taxonomists use such distinctions to tell species – perhaps the animals tell too? Perhaps the distinctively unique male and female claspers, manipulators of sperm etc. in damselflies evolved to make sure that mating occurs only within the species, i.e., the genitalia isolate species. Isolation might be adaptive because there are costs to mating with the wrong species: the resulting hybrid offspring would receive an unusual mix of genes from the parents and be less likely to survive and reproduce. Offspring of interspecific matings would be at a disadvantage against both parental species. A male and female mating outside their species waste resources. For the longest time, thinking about such differences was focused on isolating mechanisms between species: features that were species distinctive were seen as evolving as isolating mechanisms by natural selection.

• 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.

• Signals exchanged between mates: songs/calls of birds, frogs and insects, flashes of fireflies are all remarkably species diagnostic. These aspects of communication were seen as evolving to promote correct species to species matings. A firefly female in the grass only responded to the correct species-specific flash pattern of the male flying above. A cricket male calling from a burrow attracted only females of his species to localize his position in the dark and mate. Cricket, katydid, grasshoppers, frogs, birds were all making diagnostic calls because of species isolation. The function of species-distinctive forms, their adaptive basis, was maintenance of species integrity.

• 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.

Rivalry between males for access to mates: frilled lizards

• 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.

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• 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).

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Source reading: Balmford, A. et al. 1993. Aerodynamics and the evolution of long tails in birds. Nature 361: 628-631.

• It is the male who must bear the cost of altering his flight ability to please the female. Sexual dimorphism is a part of sexual selection by female choice: males (typically) the sex least in demand, wind up having to compromise their naturally selected features.

• from their abstract: Regarding “...Darwin’s suggestion that female choice for ornate males may account for the evolution of long tails in birds.” “We have integrated aerodynamics theory with comparative data on sexual dimorphism in tail length to evaluate the flight costs of different forms of tail elongation [in various species of bird]. We report here that long tails with shallow forks are aerodynamically optimal, exhibit correspondingly low sexual dimorphism and may therefore have evolved under natural selection. Other long-tail types impair flight and show greater sexual dimorphism ...”

Parental choice can impose selection too and create signals that may seem bizarre though

perhaps carrying less cost than display plumage.

Gouldian finch chick begging: gape marks aresignals selected to achieve parental attention as chicks compete for food. They are produced by the choices being made by the parents.

S.R. Pryke

Wikki

Coot

Lyon B.E., Eadie J.M., Hamilton L.D. 1994. Parental choice selects for ornamental plumage in Amercian coot chicks. Nature 371: 240-243.

Another example of parental choice creating a body form but this one perhaps with more obvious costs.

• Enchenopa binotata, a treehopper species on black locust that exhibits parental care and elaborate vibrational substratum species-specific signalling.

The bizarre Membracidae

Do they mimic thorns of the black locust?

Sculpture Alfred KellerBerlin Nature Museum

A bizarre pronotum

B Prud’homme et al. Nature 473, 83-86 (2011) doi:10.1038/nature09977

Morphological diversity in treehopper pronota

Crypsis and sensorydiversity. There is no sexual dimorphisminvolved 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?