Perch color preference in juvenile green tree pythons,Chondropython viridis

Green tree pythons, Chondropython viridis, are polymorphic for color as juveniles, commonly being primarily yellow or brown until becoming mostly green at about 1 year of age. We tested the hypothesis that the different morphs arose as a result of selection for differential background matching, yellow morphs selecting light-colored backgrounds, and brown morphs selecting dark-colored backgrounds. Twelve yellow and eight brown morphs were placed repeatedly in individual testing enclosures and allowed to choose between black and white or yellow and brown halves of a t-perch. Trials showed that both color morphs preferred dark over light perches. We tentatively suggest that individuals chose dark-colored perches for purposes of concealment. © 1994 Wiley-Liss, Inc.     

Differential visual predation on morphs of Timema cristinae (Phasmatodeae:Timemidae) and its consequences for host range

Timema cristinae is a herbivorous insect that exhibits polymorphism for body coloration (green, red and grey morphs) and for pattern (striped, expressed only in the green morph, and unstriped). The striped green morph is associated with ceanothus ( Ceanothus spinosus ) and the unstriped green morph is associated with chamise ( Adenostoma fasciculatum ). This study examines the relative vulnerabilities to predation of the different pattern and colour morphs on their natural backgrounds. The vulnerabilities of the striped and unstriped morphs on their two food plants were tested using uncaged wild birds (Scrub Jays) and captive western fence lizards. Strong differential predation was observed suggesting that each morph is most cryptic on the food plant on which it is most common. Furthermore, in a mark-recapture experiment in a patch of ceanothus the unstriped and red morphs were recaptured in higher proportion than the other morphs. The vulnerabilities of the grey and green morphs on the ground and foliage were tested using lizards. The grey morph was more vulnerable on the plants than the green morph, but the inverse was observed on the ground (where they drop after a disturbance). This may be why the grey morph is not associated with specific food plants. The striped and colour polymorphisms in T. cristinae appear to be an evolutionary consequence of differential predation on different backgrounds. The implications of differential predation to food-plant utilization are discussed.     

Camouflage that is spot on! Optimization of spot size in prey-background matching

Camouflage is a key predation prevention mechanism. The most intuitive camouflage strategy is background matching, where a prey resembles the colour and pattern of its background. Spots (e.g. geometric ‘elements’ that constitute a pattern) are a common feature on the body of prey, but the effect of their size in relation to elements in the background is rarely examined in camouflage studies. Here, we test the survivorship of computer generated prey possessing patterns of elements grouped into eight size classes on backgrounds composed of elements of the same eight size classes. All 64 possible combinations were presented to human volunteers (n = 10) acting as ‘predators’ in a computer simulation. As hypothesised, prey survivorship was high when the prey-background element size difference was low. We found significant asymmetry, however, in survivorship pattern: prey morphs with elements larger than those of the backgrounds were harder to detect than morphs with elements smaller than those of the backgrounds. When testing for potential trade-offs we determined that, in a habitat consisting of an equal proportion of two backgrounds with a large element size difference between them, net survivorship of specialist prey morphs (same element size as one of the backgrounds) was similar to that of generalist morphs (intermediate element size). When the two backgrounds were less distinctive, i.e. reduced element size difference between the backgrounds, generalist prey morphs achieved significantly better net survivorship than specialists.     

Tests of hypotheses on predation as a factor maintaining polymorphic melanism in coastalplain fox squirrels (Sciurus niger L.)

Fox squirrels ( Sciurus niger ) in the south-eastern U.S. coastal plain differ from those in the rest of the species' range by having black heads with white nose and ears. Postcranially on the dorsum, they also show interindividual colour variation, ranging from all-light agouti to all-black non-agouti. I present results of experiments undertaken to test whether the evolution of these pigmentary features can be attributed to interactions with predators. Comparative static crypsis (when the squirrels are still) was tested by determining how well specimens of the squirrel morphs matched their backgrounds in terms of intensity (brightness) distributions and patch size. Comparative dynamic effects (when the squirrels are moving) were tested using captive red-tailed hawks ( Buteo jamaicensis ) and models of the fox squirrel morphs. Results indicated that morphs of coastal-plain fox squirrels with all-light backs are better static matches to unburned backgrounds than are the darker morphs with which they coexist. Against fire-blackened backgrounds, fox squirrel morphs with intermediate and all-black backs are better intensity matches than those with all-light backs, but the dark morphs are not better patch-length matches than light morphs. The superiority of dark morphs in intensity matching for a short time after a fire does not seem sufficient to account for their long-term maintenance. Hawks responded more slowly to moving fox squirrel models with intermediate amounts of black on the back than to all-light or all-dark morphs; this result suggests a possible factor that would favour retention of genes for dorsal blackness in the coastal-plain population. Patchy black and white heads appear to promote static crypsis of south-eastern fox squirrels, and hawks reacted more slowly to moving squirrel models with such colouration than to those with plain heads.     

Polymorphism and speciation in Arctic charr

The Arctic charr Salvelinus alpinus exhibits 1–4 sympatric morphs in postglacial lakes, of which one or two are epibenthic zoobenthos feeders, one is a limnetic planktivore and one is a piscivorous form. In addition, northern rivers support partly migratory populations with anadromous and freshwater resident fish. The morphs vary in their coloration, morphology, life history, behaviour and genetic characteristics. The morphs usually differentiate according to their ontogenetic stage at maturity, which parallels paedomorphism in amphibians. The young usually start as epibenthic zoobenthivores, but may become pelagic at a certain size according to the predation risk experienced at that time. From a length of > 20–25 cm, charr start to become piscivorous. The proportion of piscivorous fish increases with increasing body size. In partly anadromous populations, fish that mature before smolting become freshwater resident, the others anadromous. In some rivers, the morphs occupy separate niches (epibenthic and limnetic), from emergence onwards. The morphs exhibit different degrees of reproductive isolation that vary from a high degree of interbreeding to complete isolation. Usually, they spawn within morph (assortative mating), but alternative male mating behaviour (sneaking, fighting) may occur in stream‐spawning populations and at great depths in lakes. Morphologically specialized morphs appear to feed more effectively than intermediate forms, and selection according to feeding mode, site fidelity and associated assortative mating are prerequisites for the evolution of the different morphs. Charr morphs develop into stable feeding niches under conditions of intense intraspecific competition when there is little competition with other species. Sympatric morphs exhibit different degrees of speciation, but similar morphs in different systems are not individual species because of (1) their polyphyletic origin, (2) the supporting systems are often young, transient environments making the future situation for the populations uncertain, and (3) the genetic differentiation among morphs is low. Sympatric morphs may interbreed and produce fertile hybrids. Nevertheless, sympatric charr morphs should be managed as separate species. Changes in the natural conditions or human impacts to which the morphs are adapted will have a strong influence on the persistence and survival of each different morph.     

Variation in the Visual Habitat May Mediate the Maintenance of Color Polymorphism in a Poeciliid Fish

The conspicuousness of animal signals is influenced by their contrast against the background. As such, signal conspicuousness will tend to vary in nature because habitats are composed of a mosaic of backgrounds. Variation in attractiveness could result in variation in conspecific mate choice and risk of predation, which, in turn, may create opportunities for balancing selection to maintain distinct polymorphisms. We quantified male coloration, the absorbance spectrum of visual pigments and the photic environment of Poecilia parae, a fish species with five distinct male color morphs: a drab (i.e., grey), a striped, and three colorful (i.e., blue, red and yellow) morphs. Then, using physiological models, we assessed how male color patterns can be perceived in their natural visual habitats by conspecific females and a common cichlid predator, Aequidens tetramerus. Our estimates of chromatic and luminance contrasts suggest that the three most colorful morphs were consistently the most conspicuous across all habitats. However, variation in the visual background resulted in variation in which morph was the most conspicuous to females at each locality. Likewise, the most colorful morphs were the most conspicuous morphs to cichlid predators. If females are able to discriminate between conspicuous prospective mates and those preferred males are also more vulnerable to predation, variable visual habitats could influence the direction and strength of natural and sexual selection, thereby allowing for the persistence of color polymorphisms in natural environments.     

Polymorphism in a mangrove snail in Papua New Guinea

Snails in the genus Littorina (Gastropoda, Prosobranchia) were examined in an area of mangroves in Papua New Guinea. It is shown that these may be separated into three species belonging to the L. scabra complex. Two of the species live on the roots only while the third is restricted to the leaves. The root species are monomorphic and cryptic and co-exist with three other monomorphic cryptic species of snails. The leaf species is polymorphic. This is a particularly clear-cut case of the tendency seen in gastropods for species of dark, uniform habitats to be monomorphic, while those on bright, patterned foliage backgrounds are polymorphic. It is suggested that the polymorphism is maintained by resemblance of the morphs to different elements in the mangrove foliage background, which are present at rather constant frequencies over a wide area.     

The interrelationship between crypsis and colour polymorphism

The mechanisms behind the evolution and maintenance of conspicuous visible polymorphisms comprising tens of morphs present a challenge to evolutionary theory. However, for cryptic forms Endler (Evol. Biol., 11, 1978, 319) conjectured that complex backgrounds facilitate polymorphism because in such habitats there are several ways to resemble the resting surface. We use computer simulation to explore the evolution of cryptic morphs on increasingly complex backgrounds under regimes that include selection for crypsis, apostatic predation and dietary wariness. We show that there is a monotonic increase in the number of morphs evolving in a population as the potential number of cryptic morphs increases. The relationship is very weak with selection for crypsis alone, but much stronger with the addition of apostatic selection. In contrast, when dietary wariness is added to the model the plot of number of morphs maintained, as a function of the potential number of cryptic forms available, is minimized at an intermediate number of cryptic forms, i.e. is V-shaped. These counter-intuitive patterns are robust to varying strengths of apostatic selection and different implementations of dietary wariness, and are more pronounced when predator and prey generation lengths are similar.     

Colour polymorphism and thermal capacities in Theba pisana (O.F. Müller 1774)

Several land snail species are highly polymorphic regarding their shell colouration. This polymorphism has been related to predatory effects as well as climatic reasons, assuming that dark morphs benefit from being more cryptic and therefore less prone to predation, whereas pale morphs are at an advantage under solar radiation, as they are suspected to heat up less. However, the assumption of different thermal capacities of these morphs is based on experiments with little standardisation or little environmental relevance. In this study, we aimed at measuring thermal capacities of two different morphs (pale versus dark-brown banded) of the Mediterranean land snail Theba pisana, applying a standardised and environmentally relevant test set-up, in order to prove whether darker morphs indeed do heat up more than lighter coloured morphs. We did not find any differences in the thermal capacity of the different morphs and conclude that thermal capacity of the shell is predominantly defined by its material rather than its coloration. These results are discussed with regard to previous studies on thermal characteristics of different land snail morphs and correlations between climate and morph distribution.     

Natural selection and the distribution of shell colour morphs in three species of Littoraria (Gastropoda: Littorinidae) in Moreton Bay, Queensland

The distribution of yellow, brown and red morphs of sympatric species of Littoraria were recorded on mangrove trees of the genus Avicennia within Moreton Bay, Queensland. The roles of background mimicry (leaf vs. bark, dark vs. light), niche selection and thermal tolerance (sunny vs. shaded positions and height above ground) were examined. The yellow advantage found previously in the area was tested. Total yellow morph frequency adjusted to a reduction in leaf background on pruned trees. Morph frequencies in Littoraria species reflect differences in habitat use. L. filosa (high yellow frequency) was more frequently found on leaves at the highest tree levels, while L. luteola (high brown frequency) was more frequently found on branches at lower levels. It is therefore argued that morphs mimic background elements. Previously reported niche selection by yellow and brown morphs of leaf and bark backgrounds is shown to be a result of the distribution of L. luteola on branches and L. filosa on leaves. At warmer times of the year, yellow L. filosa were more common in sunny positions; this is thought to be a result of thermal tolerance. There appears to be some advantage to particular morphs on particular tree types, but this relationship needs to be examined further. Mangrove-dwelling Littoraria are a promising model to investigate molluscan polymorphism. In the past, erroneous identification of sympatric species may have influenced the accuracy of reported patterns. We used allozyme electrophoretic markers as a precise identification technique. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 219–232.     

Differential Habitat Selection by Pygmy Grasshopper Color Morphs; Interactive Effects of Temperature and Predator Avoidance

Habitat selection behavior is affected by complex interplays between competing requirements. Here we combine field observations with laboratory experiments to examine how thermal benefits and predator avoidance influences habitat selection by different color morphs of the pygmy grasshopper Tetrix undulata. The composition of substrate types and surface temperatures in areas selected by free-ranging individuals did not reflect relative availability, and varied among morphs and sexes. Surface temperatures of selected habitats deviated less from the range of preferred body temperatures than would result from a random utilization of surface temperatures, suggesting that grasshoppers selected habitats with thermal properties which were suitable for maintaining preferred body temperatures. The thermal property of habitats occupied by different color morphs suggests that darker morphs (which absorb more solar radiation) selected cooler habitats to avoid overheating. Dissimilarities in substrate use among color morphs in the field and laboratory emphasize a role also of predator avoidance by background matching for habitat choice. The degree of habitat selectivity was lowest in the striped morph, supporting the notion that a disruptive color pattern may constitute a solution to the trade-off between relative crypsis in different visual backgrounds. Finally, individuals modified their habitat use when subjected to elevated risk of predation, showing that habitat choice is governed by conflicting priorities. Collectively, our findings suggest that, as a result of direct and indirect effects of coloration on performance, alternative color morphs use different solutions to the trade-off between competing requirements. Our results also lend support to the notion that relative fitness of alternative color morphs and sexes may be dependent on microhabitat selection, as predicted by the theory of multiple niche polymorphisms.     

Migratory differences between ecotypes of the snail Littorina saxatilis on Galician rocky shores

Local migration patterns may be crucial to gene flow in species of marine gastropods which do not broadcast pelagic larvae. In some species, dispersal over distances of a few metres may influence micro-scale population structures. We investigated the migration pattern in Galician populations of the snail Littorina saxatilis in which populations of contrasting morphologies occupy different tidal levels of the same rocky shore. Two distinct morphs, one at the upper and one at the lower shore, overlap in distribution in a small mid-shore region where hybrids are produced. We documented the distances and directions of migration of both parental morphs and hybrids 1 month after they had been marked and released at different shore levels. When placed at their native shore level, snails migrated less than about 2m and usually in independent directions. This supports the suggestion of a low local gene flow. At an alien shore level, however, the morphs often moved further and more directionally compared with native morphs. These differences may help to keep the two morphs separated at different shore levels. As fitness of an individual is highest in its native habitat, this seems to be an adaptive strategy.     

Substrate colour-matching cues in the cryptic grasshopper Circotettix rabula rabula (Rehn & Hebard)

Experiments with two sympatric colour morphs of the cryptic grasshopper Circotettix rabula rabula suggest that substrate colour-matching may be achieved by a visual comparison between body colour and those backgrounds which provide crypsis. The significance of this mechanism of background matching to polymorphic species in heterogeneous environments is discussed.     

Lake Erie water snakes revisited: Morph- and age-specific variation in relative crypsis

Summary Populations of water snakes (Nerodia sipedon insularum) on islands in western Lake Erie are variable in colour pattern, consisting of unbanded, intermediate, and banded morphs. In contrast, mainland populations (N. s. sipedon) consist solely of banded morphs. Previous investigators hypothesized that natural selection favoured unbanded morphs on exposed island shorelines and banded morphs in overgrown mainland habitats and that gene flow from mainland populations was responsible for the persistence of banded morphs on islands. To clarify the potential role of natural selection, I quantified relative crypsis among morphs and age classes of water snakes by comparing the size of patches making up their colour patterns with the size of patches in island and mainland backgrounds. This analysis reveals that if unbanded morphs are more cryptic than intermediate and banded morphs on islands, it is only in the young-of-the-year age class. For older snakes on islands and for all snakes on the mainland, unbanded morphs are consistently less cryptic than intermediate and banded morphs. Given these results, the net direction of selection in island populations should depend on the intensity of predation on different age classes of snakes. Overall, selection may favour unbanded morphs (e.g. if predation occurs primarily on young-of-the-year), intermediate and banded morphs (e.g. if predation occurs primarily on older snakes), or be weak or absent (e.g. certain combinations of predation on young-of-the-year and older snakes). Using estimates of relative crypsis to guide reanalysis of morph frequency data, I find support for the hypothesis that unbanded morphs are favoured by natural selection in island populations.     

The role of selection in the colour polymorphism of Littorina rudis Maton and Littorina arcana Hannaford-Ellis (Prosobranchia: Littorinidae)

Forty-one mixed samples of winkles containing the closely related species, Littorina rudis and L. arcana , were collected from different parts of the British Isles. Littorina rudis was the more ubiquitous species, with L. arcana being more-or-less confined to vertical cliffs and rocks. The frequencies of different shell colour patterns were determined for both species in each sample. Several colour morphs were diagnostic of one or other of the species over large areas, which confirms that these are separate species. The frequencies, in the two species, of the two commonest morphs, brown and fawn, were strongly correlated; in several other morphs their presence and absence in the two species was significantly associated; the levels of phenotypic diversity in the two species were also correlated. These patterns could not have arisen if the variation between shores was the result of genetic drift or founder effects, so some form of selection is implicated. In the brown morph there is some evidence for frequency-dependent selection, and it is suggested that the polymorphism may be maintained by visual predators through apostatic selection.     

Activity and weight loss in relation to solar radiation in the polymorphic land snail Cepaea nemoralis

Shell spectral reflectance, activity and weight loss in the land snail Cepaea nemoralis were examined to test whether solar radiation has different effects on the behavioural and physiological responses of different morphs. A uniform area from the empty shells of different morphs was cut and a spectrophotometer was used to measure the spectral reflectance at wavelengths from 220 nm to 800 nm. Snails were put into cages made of mesh hardware cloth, and their activities were observed from dawn onwards on two hot, dry days. Weight loss of snails under shade and under direct sunlight was also measured.
The results show that unbanded morphs reflect more light than five-banded ones, but the yellow morphs reflect only a little more than pink morphs. Five-banded morphs became inactive more quickly than unbanded ones from dawn onwards, but the differences in activity patterns were not significant between pink and yellow morphs. There are significant differences in percent weight loss between snails placed under direct sunlight and under shade. The weight loss of different morphs under shade was not statistically different. But when exposed to sunlight, five-banded morphs lost more weight than pink and yellow unbanded morphs.
Because of their lower reflectance of solar radiation, the five-banded morphs are, in a hot, arid climate, at a selective disadvantage compared to unbanded morphs. However, they can avoid such selective disadvantage by actively seeking a shaded area, as all other morphs do in the field. Such behavioural habitat selection could help the maintenance of genetic polymorphisms in natural populations.     

Defensive Behavior in Primary- and Secondary-Host Generations of the Soldier-Producing Aphid, Pemphigus bursarius (Hemiptera: Aphididae)

The genus Pemphigus comprises several species that produce soldiers (defensive morphs) in galls on the primary host. At the moment, it is unclear if host-alternating species also produce defenders on their secondary host. We therefore examined how P. bursarius morphs of the secondary host generations respond to predators to test whether they show defensive behavior. We further examined how this response compares with the antipredator behavior of soldiers in P. bursarius and P. spyrothecae occurring on the primary host. We performed two manipulative experiments using two predatory species to quantify the behavior of the different morphs in response to predators. In both experiments, secondary host morphs of P. bursarius showed no attacking behavior and antipredator behavior in these morphs was limited to escaping natural enemies by walking away. In contrast, the first instars of the primary host generations in both Pemphigus species showed attacking behavior and were capable of killing the predators.     

Difference in habitat selection by different morphs of the land snail Arianta arbustorum

The behaviour of morphs of Arianta arbustorum has been investigated using a new technique involving a paint which fades in daylight. The technique was used to study the response of different morphs to exposure to sunlight at a homogeneous woodland habitat and a heterogeneous open habitat. The yellow morph showed a significantly higher rate of fading than the brown morph. A mark, release and recapture programme was used to analyse the association between frequencies of different morphs recaptured and not recaptured. In both habitats, there was a general tendency for the frequency of recapture to increase with increasing age in both morphs. Habitat selection resulting from different behaviour of different morphs of A. arbustorum is discussed. Morph frequencies can be related to habitat, the brown morphs of A. arbustorum are significantly less frequent in the open habitat. The behavioural responses of Cepaea hortensis , present with A. arbustorum in two other populations, to the exposure of sunshine were also studied. Climatic selection by solar energy associated with different morphs, species and habitat are also discussed.     

A potential role for parasites in the maintenance of color polymorphism in urban birds

Urbanization is a major challenge for biodiversity conservation, yet the evolutionary processes taking place in urbanized areas remain poorly known. Human activities in cities set new selective forces in motion which need to be investigated to predict the evolutionary responses of animal species living in urban areas. In this study, we investigated the role of urbanization and parasites in the maintenance of melanin-based color polymorphism in the feral pigeon Columba livia. Using a correlative approach, we tested whether differently colored genotypes displayed alternative phenotypic responses to urbanization, by comparing body condition, blood parasite prevalence and parasite load between colored morphs along an urbanization gradient. Body condition did not vary with urbanization, but paler individuals had a higher body condition than darker individuals. Moreover, paler morphs were less often parasitized than darker morphs in moderately urbanized habitats, but their parasite prevalence increased with urbanization. In contrast, darker morphs had similar parasite prevalence along the urbanization gradient. This suggests that paler morphs did better than darker morphs in moderately urbanized environments but were negatively affected by increasing urbanization, while darker morphs performed equally in all environments. Thus, differently colored individuals were distributed non-randomly across the urban habitat and suffered different parasite risk according to their location (a gene-by-environment interaction). This suggests that melanin-based coloration might reflect alternative strategies to cope with urbanization via different exposure or susceptibility to parasites. Spatial variability of parasite pressures linked with urbanization may, thus, play a central role in the maintenance of plumage color polymorphism in this urban species.     

How fish-helminth associations arise: an example from Arctic charr in Loch Rannoch

Three sympatric morphs of Arctic charr Salvelinus alpinus occur in Loch Rannoch, Scotland, and are identified by their differing head morphology and diet. These are small-headed benthic, large-headed benthic and pelagic morphs. Six species of endoparasitic helminth were found in the fish, but the morphs had different patterns of infection. Overall infections in pelagic charr were heavier than in large-headed benthics, which were in turn heavier than in small-headed benthics, even though benthic charr live longer than pelagics. Pelagic fish had high prevalences and intensities of pseudophyllidean tapeworms, the intermediate hosts of which are copepods. The prevalence and intensity of metacercariae of Diplostomum sp. (the intermediate hosts of which are snails) were high in the benthic morphs. The results are discussed in terms of the effects of ecological factors on transmission of helminth parasites to their hosts and the evolution of host-parasite associations.