Presentation on theme: "Lecture 21 Morphology of signals Communication and fireflies Cryptic signals: misinformation Aposematic signals Membracid ‘helmets’"— Presentation transcript:
Lecture 21 Morphology of signals Communication and fireflies Cryptic signals: misinformation Aposematic signals Membracid ‘helmets’
Animal morphology as a signal generator or receiver Communication involves signals (packets of information) passing between animals. One animal’s behaviour (receiver) changes as a result of a signal received from some other animal (sender). A male firefly approaches a flashing female on the ground below. There are structures whose purpose is to generate and receive signals and they operate in different modalities : vocal chords and ears in the acoustic modality; light organs and eyes in the visual modality. An obvious morphological feature of this male firefly are its highly developed eyes and at the rear an elaborate light organ. www.firefly.org Species-specific flash and flight patterns enable these beetles to signal to conspecific females on the ground below and to pair with mates of their own species. Understanding firefly morphology requires understanding its mating behaviour and the signals sent and received by potential mates. Front and back this insect’s form testifies to the importance of light signals.
James Lloyd: firefly man luring males down to a pen-light flashlight by appropriate latencies Fireflies also use their lights to land on leaves in the dark. Different species flashing distinctively in the same place at the same time, some in the canopy, some in the understorey. See www.firefly.org
Dita Klimas Cryptic Tettigoniidae, leaf mimic katydid Morphology is sometimes devoted to passing along the wrong information, predator-utilized cues like leg angles obscured and looking like a dead leaf.
Costa Rica, acanthodine katydid mimicking lichen. A katydid mimicking tree bark. G.K. Morris
See the ‘smaller majority’ website. glass katydid Phlugis sp. Nymph of Phlugis Transparency can make you look like you’re not there at all: a glass katydid by Piotr Naskrecki; the adult (green) is shown to the left. Crypsis explains much of this insect’s morphology.
Red-wattled lapwing Wikkipedia Another example of an animal's body selected to 'misinform': cryptic patterning is adapted to the sensory capacities of the would-be predator and the features of the substratum.
Trimerotropis verruculata: males like open spaces where they signal in flight to attract females with hind-wing colousr [visual] and sound [crepitation] displays. When the highway gets straightened ‘side roads’ of abandoned asphalt are left. The picture below shows the way individuals of this species had adapted to match their asphalt habitat.
Integument colour: aposematic [warning] coloration Red and yellow kill a fellow Red and black venom lack The body may also be selected to signal a warning: honest in this coral snake species which is truly dangerous; a dishonest warning is given by a mimic of the noxious model. Shockingly obvious colour patterns suggest some sort of noxious ability.
Backyard bird lover Selection produces genetic-based differences in form between males and females. Reproductive system differences relating to the production of sperm vs eggs (testes vs ovaries), structures for the intromission of sperm and its reception, behaviour for the effective feeding and care of the young: these are forms (and behaviours) that are produced, just as wings for better flight, or bills for better seed-handling -- by natural selection. Selection for a better working ovary or testis, that contributes to an overall better fitness for the animal. Signal morphology differs between sexes contributing to sexual dimorphism But the sexually dimorphic red colour of the male cardinal shows his plumage is under different selection than hers: a special selection of which the female is the agent. Her preference for red feathers affected his reproductive success. While being naturally selected for feather effectiveness in insulation, flight, etc, his success reproductively also depended on what colour she ‘liked’.
Euplectes long-tailed widow bird A shorter tail would give much better aerodynamics; but sexual selection overrides natural; finding a mate is more important than optimizing aerodynamics. Females make choices between potential male mates and males compete to attract.
Mate choice is typically imposed by females upon males (in some spp. it works the other way). Male structures, internal and external, evolve to better effect a female’s preference, e.g., damselfly aedeagi. The female’s sensory capacities may channel the display that evolves (sensory drive). Female cardinals see red very well. Since she sees red better than blue, red feathers will do better affecting her discrimination in favour of a male's display. Over generations male body features become elaborately altered to promote being chosen and special motor patterns arise to move wings and tails in evocative ways (signals are refined). These changes are often at odds with the living 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). Shorter-tail birds fly better because they are closer to the optimum dictated by natural selection, but because females favour mating the longer tailed bird, such a male is better off in the competition for mates: his long-tail genes increase in the population.
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...” The tail structure of the barn swallow suggests no role for it in mate choice: it is thus ‘free’ to be aerodynamically more ideal.
The classic extreme of elaborate male plumage: the peacock.
Orchelimum males engage in grappling fights as they defend territories using broadcast sound. The sound signals attract females and could well be sexually selected by female mate choice. But alternatively the fighting could establish access to a female. By maintaining control over a resource location in the habitat, e.g., a plant used by females for oviposition, a male could gain access to a female that comes in search of that plant. So some animals undergo another form of sexual selection: males competing in rivalry to gain access to females. Sexual selection can involve choice by females or rivalry by males. And male rivalry can lead to remarkable structural elaboration of weapons such as horns.
Male rhinoceros beetles fight by manoevering to lift an opponent (normal to his body long axis, grasping the elytra) within cephalic and prothoracic horns. Lifted clear of the substratum (e.g., sugar cane) he has lost the fight and can be dropped the distance to the ground – which should discourage him.
Sexual selection that involves male rivalry for access to mates can result in remarkable morphological features, not necessarily weapons, serving as important signals in establishing dominance. Some structural features have to do with ritualized threat and recourse to overt aggression is rare.
Male stalked-eye fly (Papua New Guinea, PNG); a bizzare sexual dimorphism gives males access to females via defended resources. Stalked eye lengths are used to establish dominance; there is no fighting. Natural selection for effective vision is compromised. Eyes of female are normal.
Parental choice is another form of selection, distinguished from natural selection, that can create aremarkable signal morphology between parent and offspring. Gouldian finch chick begging: gape marks are signals selected to achieve parental attention as chicks compete for food. They are produced by the choices being made by the parents. S.R. Pryke It is critical for young birds being fed by parents to gain their attention; gape markings are signals evolved to compete with siblings.
Wikki Coot Evolution of chick morphology by parental choice reaches a very extreme state in the waterfowl called a 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. The chicks are so distinctive in the context of competing to be fed their plumage would seem to present a warning coloration.
Enchenopa binotata, a treehopper species on black locust that exhibits parental care and elaborate vibrational substratum species-specific signalling. Membracidae: their astonishingly bizarre pronota are not sexually selected; both males and females exhibit these same morphologies. 3200 spp., ingesting sap and apparently no cibarial pump, so perhaps phloem- feeders like aphids? See also Nov 1 membracid entry of >http://thesmallermajority.com/
Enchenopa binotata seen where they live Testable hypothesis: if they are mimicking thorns one would suppose they always face in a direction consistent with the nearby thorns on the tree.
So if the diversity of membracid pronota (helmets) is not the product of sexual selection – (mate choice or rivalry) what is a functional explanation? [what was the adaptive consequence that produced them?] Natural selection for crypsis is the accepted answer. And if this seems implausible, given the strikingly odd form diversity of the ‘helmets’, keep in mind you are not viewing these species against their natural environment with its vagaries of weather, or in natural lighting and that in any case your eye is not the same as the eye of the predator against which such crypsis will have evolved.
Sculpture by Alfred Keller Berlin Nature Museum A bizarre pronotum