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preface
The molecular structure of porphyrins, the front cursor of protoporphyrins, resembles phthalocyanines used in solid-state engineering, suggesting that protoporphyrin pigmentation can increase the strength of the shell by acting as a shock absorber.
This leads to the structural-functional hypothesis, that is, when exogenous Ca is scarce, protoporphyrin is deposited on the eggshell for structural strengthening, and pigmented spots have greater fracture toughness than uncolored shells, compensating for the decrease in eggshell thickness and the increase in shell strength.
In the great, protoporphyrin pigment spots divide areas with thinner shells, deeper areas cover thinner areas, eggshell calcium content is positively correlated with local soil calcium content, similar results have been found in Eurasian sparrowhawks, which breed in areas contaminated with DDT, a pollutant that prevents shell glands from absorbing calcium.
Shell thinning was positively correlated with an increase in DDT concentration and, to a greater extent, with internalized pigment spots.
Alternative hypothesis
Other hypotheses include the extortion hypothesis of Hanley et al., which proposes that if a female lays an obvious egg, a sexual conflict load may be imposed on the male, forcing the male to increase his care for his father.
Since protoporphyrins come from the blood, an increase in anemia leads to a decrease in protoporphyrin pigmentation in eggshells, in addition, it has been further suggested that pigment spots may act as a defensive bacteria.
Protoporphyrins, if activated by light, reduce bacterial survival on the eggshell, and large amounts of protoporphyrins increase the risk of death in the developing embryo.
Although these hypotheses have received a lot of attention in the past, surprisingly little is known about eggshell formation, especially in species unrelated to the poultry industry.
In particular, the process of how eggshell pigments are synthesized, mobilized, and deposited, and how the presence of these pigments directly and/or indirectly affects eggshell calcium and other eggshell traits is largely unknown.
Hypothesis explaining ovosis in the great and blue
Parrot-like eggs are representative of all 22 passerine families throughout the Arctic, with large and blue whose eggs are predominantly white and covered with protoporphyrin pigment spots.
Throughout the phylum , cave-nesting species tend to lay unpigmented eggs than open-nesting species.
Cave-nesting species are unlikely to evolve eggshell color for concealment or caution because these species are less likely to prey, although eggs from both species may have been found due to their evolutionary history.
While the eggs of cave nesting species are not susceptible to overheating caused by direct sunlight, they are sensitive to changes in heat and humidity within the nest cavity.
In cross-cultured large eggs, increased spots lead to an increase in water loss, while in blue, increased spots lead to shorter latency, suggesting that pigmentation affects the thermoregulatory properties of the eggshell.
Whether this is an evolutionary effect or a secondary effect caused by a decrease in pigment shell thickness is unclear, and conspecific hatching parasitism, i.e. females laying eggs in the nests of other conspecifics, occurs in more than 200 bird species.
The eggs of the two types of vary widely between litters in spots and spotting patterns, but not within litters, and there is evidence that neither intraspecific parasitism nor the ability to reject parasitic eggs occurs in blue or great.
However, data showed that conspecificic hatchery parasitism occurred in blue in populations with high reproductive densities, and the latter study refuted the hypothesis that eggs of different colors were laid by different females without the use of other methods.
There are no systematic differences in size and amount of yolk testosterone in parasitic eggs compared to host eggs, suggesting that the lack of suitable nesting sites has led some females to use monozygotic parasitism as the best working strategy.
The eggs hatch and hatch successfully, and the chicks fly from the nests of females of closely related species, suggesting that it is possible for monozygotic parasitism to evolve in offspring.
Eggshell spotting, that is, the relationship between protoporphyrin content and female health and physical condition, in blue, females who lay more spotted eggs are in a lower physical state, with higher stress protein concentrations and lower immunoglobulin concentrations.
Eggs with larger spots and less evenly distributed contained higher concentrations of antibodies, suggesting that eggshell pigmentation may reflect the mother's investment in immune compounds entering the yolk.
In the great, heavier females lay fewer spotted eggs, but neither the supply of males nor the growth rate of chicks were independent of eggshell spots.
In addition, larger females and females with better innate immunity lay eggs with "darker" pigment spots.
However, the ability of eggshell color to reliably signal female physical condition to other birds has been questioned, especially as eggshell color may not see empty nests of passeriformes, and the "structure-function" hypothesis may be the most applicable hypothesis to explain the pigmentation of blue and large eggs.
Calcium is a limited resource for eggshell formation in breeding passerine, and for insectivorous and flower-grained birds, their regular diet does not provide 25% of the calcium content required for eggshell formation.
Therefore, birds must eat extra calcium-rich food, while large birds can store calcium in their bones, and small passerines must collect calcium daily during egg laying to obtain sufficient resources to form eggs.
The structural function hypothesis has been speculated to explain the spots on eggs of the great and has been tested on other species, such as the blue and the European piebald flycatcher.
Structure inside the eggshell
During egg laying, females of many species of small passerine birds change their dietary preferences, including calcium-rich foods, an important nutrient that determines eggshell strength.
Protoporphyrin is the main pigment that makes up the macula of the eggshell of many birds, and can enhance the structural integrity of the eggshell under conditions of dietary calcium deficiency.
The relationship between eggshell thickness, calcium percentage and protoporphyrin concentration of large and blue was tested by adding calcium.
Compared to female large without added calcium, calcium-added female large, but not blue, have thicker egg-laying shells with higher calcium concentrations, and pigment spots appear in areas with thinner eggshells, but this is not the case with supplemental birds.
In blue, pigmented eggshells are thinner than adjacent unpigmented eggshells, with or without bird supplementation, and there is no significant relationship between protoporphyrin and eggshell calcium concentrations in both species, although the eggs laid by blue contain higher amounts of protoporphyrins.
However, it is suggested that protoporphyrin pigmentation may be consistent with areas with thinner eggshells, and that it may play an important structural role when calcium is limited in the natural diet, but pigment spots may be indispensable to eggshells when calcium is abundant, possibly fulfilling alternative functions.
Eggshell appearance function
Many bird eggshells are found in appearance, but the functional significance of this macula remains unclear, with two key pigments thought to be responsible for the coloration and pattern of bird eggshells.
These are protoporphyrin IX, responsible for yellow-red and biliverphyllin, blue-green hues, protoporphyrin produced during heme biosynthesis, present in the calcite and stratum corneum of the eggshell, usually localized as spots, inside the eggshell or in different layers on it.
Internalized protoporphyrin spots are considered evidence of their structural function rather than external signaling or concealment, as it is not necessarily visible from the outer surface.
Since the molecular structure of protoporphyrin is similar to that of lubricants used in solid-state engineering, the deposition of protoporphyrin can increase the strength of eggshells by acting as a shock absorber in the eggshell matrix.
This has led to the structural-functional hypothesis, which has been proposed that when dietary calcium is deficient, protoporphyrins are deposited in eggshells to strengthen.
Although calcium additives have been used to test the effect on eggshell spots, it has not been determined whether it causes an increase in eggshell calcium concentration while a decrease in protoporphyrin concentration.
If protoporphyrin pigmentation plays an important role in the structural strengthening of eggshells when feed calcium is deficient, there is a negative correlation between eggshell calcium and protoporphyrin concentration.
To the best of what is currently known, this relationship has not been tested, and studying the total protoporphyrin content rather than the location of pigment spots may be more effective in determining the strengthening properties of protoporphyrins.
Unlike large birds, which can store calcium in their bones, small passerines must collect calcium daily during spawning to obtain sufficient resources to form eggshells.
More than 90% of calcium-rich foods, including snail shells and calcareous sand, are consumed before or during egg laying.
This calcium-specific foraging behavior has been observed in a variety of species, such as in the great, where eggshell mineral content is strongly influenced by soil calcium content, and eggshell mineral content is higher when females nest in areas with higher soil calcium content.
Eggshell strength is determined by the combination of eggshell thickness and eggshell matrix tissue and is important to reduce damage to eggshell integrity, as this can lead to dehydration and death of the embryo.
Female blue finches with added CA laid eggs, and the eggshell was thicker and had a wider range of pigment spots, but had no effect on spot size and intensity, and protoporphyrin pigment spots were found in areas where structural strengthening was more important in the eggshell, in which the eggs were thinner.
summary
The effects of calcium addition on eggshell thickness, calcium and protoporphyrin concentration in large and blue were studied, focusing on the prediction of the structural function hypothesis.
We learned about the eggshells of calcium-added females and non-calcium-added females, and based on the structural function hypothesis, we predicted the shells of two supplemented females to lay eggs:
(1) thicker, (2) higher mineral (i.e. Ca) content, (3) the pigmented region of the eggshell is thinner than the non-pigmented region, (4) we quantified the eggshell protoporphyrin content and predicted from the structural function hypothesis that it was negatively correlated with calcium concentration and eggshell thickness.
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