Evolution of Colouration in Bird Eggs - Essay Example

Evolution of colouration in bird eggs The theory of evolution posits the idea of natural selection, through which certain species are able to survive because of the advantages accorded to them by one or more particular traits. Charles Darwin in his germinal publication The Origin of Species (1859) describes this: "Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure" (153). Contingent on this idea is a further one of divergence of species' characteristics, which grants them slight but distinct variations that led Darwin to call each form an "incipient species." These differences may occur for more than one reason, and are evident in more than one stage of any creature's lifecycle. One key fact states that the more diversified a population becomes, the more likely it is to survive as a species. This increases its ability to occupy more territory and gain access to more sustenance, or to spread itself out and become more inconspicuous among predators. Several species of birds demonstrate this type of variation at the embryonic stage. The eggs they lay are coloured or spotted in particular ways, the complete reasons for which have been somewhat elusive to researchers for some time. In fact, birds are the only species that produce pigmented egg shells (Gosler, et al. 2005, p. 1105), and this leads scientists to believe that the pigmentation serves a discoverable purpose. Such reasons as crypsis and the prevention of parasites have been hypothesised. Other hypotheses have been based on sexual selection or on the chemical structure of the eggshell and its influence on eggshell fragility and vulnerability. Hybridization has also been suggested as a factor that influences egg colour. These hypotheses, though varied, have important implications on the evolution of the bird shells and can give insight into the reasons for the various pigmentations that birds' eggs carry. Parasitism Safety is a very important issue for birds when it is noted that their eggs are prone to different forms of predation and parasitism. Two types of brood parasitism exist. Conspecific brood parasitism occurs when birds of similar species place their foreign eggs into the nest of a host. Interspecific brood parasitism occurs when birds of other species infiltrate the nest and place their own eggs in the clutch. This can be very dangerous to the existence of a particular avian species, since the some parasites are known to be vicious and ruthless. The most notorious of these parasites are cuckoos, and their parasitism is dangerous as they often hatch before the genuine brood and expel authentic eggs from the nest, terminating that attempt of the species to reproduce. This is truer of some species than of other, depending on the type of predation suffered by each. The great tit (Parus major), for example, does not expel parasites from its nest, and this appears to be contingent on the fact that it is not a host to the European cuckoo parasite (Gosler, Higham and Reynolds, 2005. p. 1105-6). Village weavers, on the other hand, do remove foreign eggs from their nests, and have therefore to learn the appearance of their eggs (Collias, 1993, p. 684). The implication of this fact is that the weaver eggs must have progressively developed a distinctive appearance in order to facilitate recognition. It has been observed that West African village weavers' spotted eggs have noticeable intraspecific differences, and hypotheses have been formed concerning the reasons for this. The first spots on eggs might have been achieve by one incidence of genetic mutation in the species, but this kind of spotting in weavers is now commonplace. Relying on the reasoning of the previous paragraph, it was predicted that within the weavers' clutches the diversity of colouration would be minimised in the absence of interspecific parasites and maximised in their presence. Researcher David Lahti found opportunity for an experiment involving these West African weavers in the fact that the species had been introduced into some Caribbean countries without an accompanying introduction of its natural parasite, the diederik cuckoo (Chrysococcyx caprius). The idea behind the experiment was to scrutinise the eggs of the weavers from the two regions in order to ascertain the level of speckle-diversity accorded to eggs within and among clutches of the same species. As Lahti writes, "species introductions can reveal the operation of natural selection as a mechanism of evolution when environments of source and introduced populations differ in ways that lead to evolutionary predictions" (2005, p. 18057). The results of the study were telling. The study revealed that in some cases colouration of eggshells does have much to do with parasitism. "The more unusual a weaver egg is in its population, the more likely a parasitic egg laid in its nest will differ from it sufficiently for the host to detect and remove it; whereas a weaver egg of common appearance will be more likely to be matched"(Lahti, 2005, p. 18057). When conspecific parasitism occurred in the population of weavers, it exerted only a minimal drive toward the selection of eggshell colours than that created by the diederik cuckoo. The reason for this was that other weaver eggs developed alongside that of their host, and the hosts' eggs did not necessarily suffer because of their presence. The implications of this for eggshell colouration is that eggs that are conspecifically parasited are more likely to have variations within the brood, as there is no very urgent drive toward the uniformity of eggs in a brood. In the case of interspecific parasitism, where infiltration of the weaver nests by cuckoos is a real threat, it is necessary that the cuckoos strive for a more recognisable pattern of colouration on the eggshells of their own clutch. Because of this widespread effort on the part of each clutch to resemble its members (or more specifically, because of the detriment of the ones that do not), the eggs have developed a marked differentiation among the clutches of different broods within the species, but also a marked uniformity within each clutch. Furthermore, the diederik cuckoo also evolves alongside the weavers, making it necessary for the weavers to be continually able to distinguish their own eggs from that of the diederik. Therefore, the colouration of the cuckoo's eggs stems from its desire to mimic the eggshell patterns of other birds, and those birds must continually produce patterns that are distinctive and unusual in order to escape parasitism and prolong the existence of its species (Lahti, 2005). These findings can be explained in an evolutionary context. Of the birds that are prone to this kind of parasitism, those who have managed to survive are most likely the ones who have been able to recognise their own eggs. This ability is most acute in the species of birds that have developed the most sophisticated and unique speckle patterns. As the survivors have been the only ones able to pass on their genes, the logical result of the evolution is that more of the parasitically hunted birds that recognise their eggs (that is, birds with distinctive colourations and patterns) are the ones in existence today. Comparison of the weaver eggs in the Caribbean with those of the West African weavers produced information that matched the details of the foregoing hypothesis. The bird populations that were introduced and that have no natural parasitic predators tend to have fewer incidences of spotting on eggs, or at least more spot variations on the shells (Lahti, 2005). Mineralization Birds lay what are known as amniotic eggs, which are distinguished by their calcified outer layer that covers the chorion membrane. Below this membrane lies another, the amnion, in which is held the nutrient-containing fluid that nourishes the chick (Benton, 2005, p. 1). The evolution of the amniote (creature who lays amniotic eggs) has interested many a researcher, and it is widely believed that their eggs have been around for the past 310 million years. They evolved, it is thought, as a step toward adapting to life on land. Evidence suggests that the shells of ancestral amniotes were not rigid, but pliable. Rigid eggshells are synapomorphic, evidently coming into being much later via Reptilia, the last ancestor shared by turtles the diapsid subclass, which contains all crocodiles, snakes, lizard, and birds. Dinosaur eggs were not coloured; neither are the eggs of other oviparous species, but what they do have in common are the highly calcified nature of the shells. This mineralization of eggs an important factor in the formation of shells, and those species that failed to achieve this had a very low rate of survival due to the reduced ability of the egg to preserve itself from predators or other natural occurrences (Laurin, Reisz and Girondot, 2000, p. 313-14). The main ingredients of egg shells are calcium carbonate (which forms 95% of its contents), calcium phosphate, and magnesium carbonate (Grellet-Tinner, 2005, p. 99). The comparatively durable aspect of the hard shell is achieved by the presence of such minerals, and this lends a greater level of protection to the embryos as they develop. The shell structure as it regards the concentration and distribution of these minerals is also important in the nutrition of the embryo, as its porous quality allows for the infiltration of oxygen while excluding many microorganisms, and also reducing the likelihood of fluid loss. Benton offers a terse definition of the function of the shell: "the shell acts as a semipermeable membrane, allowing oxygen to pass in, and carbon dioxide out, but limiting the loss of water vapour" (2005, p. 1). Shell structure, while providing safety and resistance from outside, also facilitates actions that allow the bird to free itself once it has attained the correct developmental stage. Research carried out by Gosler et al. (2005) suggests that the amount of calcium present in a bird's diet has a bearing on the colouration of its eggs. It was discovered that the spots on the eggshells of the birds studied, the great tit, consist mainly of protoporphyrins, which form an integral part of the constitution of eggshells (Labisky and Jackson, 1966, p.385; Thear, 2005). Their function is believed to be very important for the survival of the egg. Protoporphyrins are believed to act as shock absorbers within the shell, as they are structured in a way that resembles some engineering lubricants. They might also function as producers of cold spots during incubation and aid water-loss reduction, as they are very conspicuous in the infra red (p. 1106). The thickness of the eggshell determines the shell's strength, rate of water loss, and the amount of calcium available to the hen during the incubation period. Calcium must be sought daily by birds in order to facilitate the formation of eggs, which each have an incubation period about twenty hours. It turns out that pigments might be a part of the eggs inherent structure, and if they are Gosler hypothesises that there should also show up a relationship between pigmentation and the thickness of the eggshell. There should also be demonstrated a correlation between calcium availability (which affects thickness) and pigmentation. Gosler even cited a third possibility of a correlation between pigmentation and egg mass-if the mass has anything to do with the shape of the egg, and that, in turn, affects eggshell thickness (Gosler, et al., 2005, p. 1106; UCLA, p. 2; "The psittacine egg"). Gosler and his team studied 30 clutches of great tit eggs in a given area, carrying out statistical evaluations of the correlations of the thickness, mass, and pigmentation of the eggshells. They also analysed the surrounding soil to assess the level of calcium available to the birds. The data proved in conjunction with the hypothesis, as thinner eggs proved to be more darkly pigmented. The calcium correlated positively with the thickness of the eggs, which is consistent with the prediction that calcium is a resource that limits the breeding capacity of passerines. Therefore, pigmentation showed itself to be associated with eggshell thinness, and the thinner the shell the darker the pigmentation. Because the protoporphyrins are more visible in the absence of the whitening calcium, darkly coloured speckles or patches appeared on the eggshells (Gosler, et al., 2005, p. 1110-1112). Sexual Selection What have been missing from many discussions on egg colouration are hints about the reason for the vibrancy of the colours associated with many bird eggs. In many types of eggs, such as those species that demonstrate colour dimorphism, the correlation between colour and predation is weak, as are several theories regarding the filtering of solar radiation. Therefore, the reason for blue and green colour of eggs has remained shrouded in mystery. It is believed by Moreno and Osorno that "biliverdin could be advertising antioxidant capabilities during a particularly stressful phase" (2003, p. 805, Butcher and Miles, 2003, p. 2). Other research too has revealed the possibility of a negative relation between well-being of birds and the quality of the shells of the eggs they lay (Roberts, 2002, p. 5). Biliverdin is the contributor of the blue-green pigmentation on bird eggs, and might be a signal of stress in the female bird (McGraw, 2005, p. 759). The shell gland deposits the pigment, and biliverdin is known to be an antioxidant inhibitor of free radicals. Therefore, the deposition of this substance on the egg can weaken the female as it signifies the removal of antioxidants from her body. As hinted at in the previous section, protoporphyrin is what contributes to the brown colouration in eggshells (Univ. of Washington, 2004). It is derived from haem, which is that part of the blood that contains iron. The use of this as a pigment deprives the hen's body of haem, which is also debilitating to the bird, but its accumulation in the liver leads also to "oxidative stress". They go on to state that "The deposition of increasing amounts of protoporphyrin in eggshells may indicate the capacity to sustain elevated levels of these pro-oxidants in the blood and uterus, and thus a high capacity of the antioxidation system" (Moreno and Osorno, 2003, p. 804). These observations led them to the conclusion that the connection between these two pigments with cell damage and free radicals should not be dismissed as coincidence. Another aspect of the colouration problem is found in the fact that mutation occurred in regard to eggshell colour in some Japanese quail, whose patterns characterise the mother (Woodard, et. al, 1973, p. 2). The result was that the amount of protoporphyrin and biliverdin were reduced in their eggshells. It was also observe that pigment distribution among the Stumidae family was random, some having both of the pigments and some having only one of the two. The phylogenetical variability of sexual traits gives the idea that the colouration of birds might be sexually selected (Moreno and Osorno, 2003, p. 804). Male birds that are polygynous are known to compare the eggs of different clutches in order to decide whether to offer paternal care to the hen and her nest. It has been supposed that reaction to egg colour might be as a result of the males' "researching" the clutches of the hens he has mated with, developing more acute sensitivity to the egg colours as he ages. It has also been suggested that males' reaction to egg colours might be a built-in natural mechanism, as colourants have been thought to signify a "superior health state" (McGraw, 2004, p. 760). It is possible, therefore, that males tend to take better care of clutches depending on the colour of the eggs they contain. It was found that egg colouration signals "female condition or genetic quality to mates" and eggs with vibrant colours garner more biparental care than those with duller colours, as were eggs with long periods of nestling (Moreno and Osorno, 2003, p. 804-5; Soler, et al. 2005, p. 641). There are several implications of this. For clutches that have duller colour and therefore (possibly) no male parental care, the natural selection of colour that might occur would angle in favour crypsis, as it would be necessary for eggs to remain hidden from predators in the absence of constant protection. On the other hand, in a clutch that has male parental care, it might be more beneficial to the females to lay brightly coloured eggs in order to retain the assistance of the male birds. Therefore one finds the possibility of eggshell colour evolution happening in two directions (Moreno and Osorno, 2003, p. 805). Hybridization In other experiments with the subspecies interbreeding of domestic birds, it was suggested that pigmentation can also be affected by hybridization. The white eggshell colour is noted to reflect an absence of brown pigmentation (Sellers). In an experiment done on hens in Spain, the Prat Lleonada was crossed with the Empordanesa Roja to create the RP group and the Penedesenca Negra was crossed with the Prat Lleonada to create the NO group. The experiment revealed that heterosis "was present in RP crossbred at 25 weeks, and in both at 39 weeks, being higher in RP than in NP." (Franesch, Casanovas, and Fontgibell, p. 4) The genes were improved for egg colouration, depending on how old hens were when they laid them. The older the hybrid (RP or NP) hen at the time of laying, the greater the likelihood of egg colouration. This hybridization of birds does occur in nature, and there is evidence that suggests correlation between eggshell colour and interbreeding of species (Francesch, A., J. Estany, L. Alfonso, and M. Iglesias, 1997, p. 1630). According to research done on some passerines, the evolution of its pigmentation has much to do with these functions of the eggshell. It is observed that the incidence of pigmented eggs occurs no more abundantly in those birds that nest in the open. Maculated or speckled eggs can be found in the populations of almost all passerines, and research has shown that the availability of calcium-more specifically, the lack of it is responsible for the dark speckling of bird eggs. Other research has offered as an explanation of bird eggshell colour the evolutionary device of natural selection that stems from conspecific and interspecific nest parasitism. Another posited reason for the evolution of colouration for bird eggs is the theory of sexual selection, in which hens reveal either their state of wellbeing or a response to the paternal reactions to eggshell colour. In addition, evidence exists for and against crypsis and other types of signalling; and lastly, it was found that hybridisation too has some bearing on the colouration of eggshells and its evolution through the ages. References Benton, Michael J. (2005). "Reptilia." Encyclopedia of Life Sciences. Chichester: John Wiley and Sons Ltd. http://www.els.net/ [doi: 10.1038/npg.els.0004126] Butcher, G. D. and R. D. Miles. (2003). "Factors causing poor pigmentation of brown- shelled eggs." IFAS. Gainesville: U. of Florida. Collias, E. C. (1993). "Inheritance of egg-color polymorphism in the village weaver."The Auk. vol. 110.4. 683-692. Darwin, Charles. (1859). The Origin of Species. New York: Random House. Francesch, A., P. Casanovas, and A. Fontgibell. "Heterosis in Catalan poultrygenetic stocks under egg production traits selection." IRTA: Unitat de Genetica Avicola. Francesch, A., J. Estany, L. Alfonso, and M. Iglesias. (1997). Breeding and Genetics: genetic parameters for egg number, egg weight, and eggshell color in three Catalan poultry breeds." Poultry Science. 76: 1627-1631. Gosler, Andrew G., James P. Higham, and S. James Reynolds. (2005). "Why are birds' eggs speckled" Ecology Letters. vol. 8. 1105-1113. Grellet-Tinner, Gerald. (2005). "Membrana testacea of titanosaurid dinosaur eggs from auca mahuevo (argentina): implications for exceptional preservation of soft tissue in lagersttten." Journal of Vertebrate Paleontology. vol. 25.1. 99-106. Labisky, R.F. and G. L. Jackson. (1966). Characteristics of egg-laying and eggs of yearling pheasants." The Wilson Bulletin. U. of New Mexico. vol. 78.4. 379-399. Lahti, David C. (2005). "Evolution of bird eggs in the absence of cuckoo parasitism." The Natural Academy of Sciences of the USA. vol. 102. 50. 18057-18062. Laurin, M., R. R. Reisz, and Marc Girondot. 2000. "Caecilian viviparity and amniote origins: a reply to Wilkinson and Nussbaum." Journal of Natural History. 34. 311-315. http://www.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_JNH 2000.pdf#search='amniotic%20eggshell' McGraw, K. J. "The antioxidant function of many animal pigments: are there consistent health benefits of sexually selected colourants" Animal Behaviour. 69. 757-764. Moreno, Juan and Jos Luis Osorno. "Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality" Ecology Letters. vol. 6. 803-806. "Psittacine Egg, The." Hybridization http://users.accesscomm.ca/parrot/SB/Egg.htm Roberts, Juliet R. (2002). "Travel Report: 9th European symposium on the quality of eggs and egg products." Rural Industries Research and Development Corporation. Sellers. "Genetics of Eggshell Color." Poutry Genetics for the non-professional. http://marsa_sellers.tripod.com/geneticspages/page0.html Soler, J., J. Moreno, J. M. Aviles, A. P. Mller. (2005). "Blue and green egg-color intensity is associated with parental effort and mating system in passerines: support for the sexual selection hypothesis." Evolution. vol. 59.3. 636-644. Thear, Kate. "Eggshell Colour." Starting with ducks. Broad Leys Publishing. http://www.blpbooks.co.uk/broad_leys_books/starting_with_ducks.php UCLA Marine Science Center. "The Brown Pelican." Cruising Classroom. University of Washington. (2004). "Penguins ingest mollusc shells to obtain calcium for thicker eggshells." Science Daily. http://www.sciencedaily.com/releases/2004/05/ 040511041810.htm Woodard, A. E., H. Abplanalp, W. O. Wilson, and P. Vohra. (1973). "Japanese quail husbandry in the laboratory." Department of Avian Sciences. Davis: U. of California. Read More