Speckles of cryptic black‐headed gull eggs show no mechanical or conductance structural function

Abstract

The structural-function hypothesis provides an alternative to signalling-based predictions to explain the remarkable diversity observed in avian eggshell colour. According to the hypothesis, protoporphyrin, the common pigment of visible speckles, lubricates and thus strengthens the shell and simultaneously moderates gas transfer across it. Correlational evidence for the structural-function hypothesis in form of a coincidence of both shell thinning and reduced evaporation with eggshell speckles comes from a restricted set of species with limited calcium supply or little nest predation and no need for camouflage of the eggs. Here, we investigate whether protoporphyrin-based pigmentation similarly affects a species with cryptically marked eggs and ample dietary calcium, the black-headed gull, Larus ridibundus. Although shell thinning of speckles occurred, this effect was minimal compared with thinning through embryonic growth. Furthermore, speckled and plain areas of the shell did not differ in water vapour conductance through the shell. We conclude that protoporphyrin speckling does not fulfil a structural function in gull eggs. Instead, during shell formation where the protoporphyrin of speckles is deposited in place of calcite it could inflict a structural cost. We propose that the mechanical and water vapour conductance functions of shell speckling need to be evaluated as separate hypotheses and that both functions could, in fact, negatively affect each other.

A Review of the Relationship between Eggshell Colour and Water Vapour Conductance

“…This seemed to be an abnormal condition associated with the pesticide contamination but Jagannath et al (2008) made no association with shell thickness and strength. Despite this, Maurer et al (2011) cited this result as being equivalent to other studies regarding mass loss. However, thinner shell thickness per se does not necessarily indicate an increase in shell water vapour conductance and an increase in the rate of mass loss during incubation.…”

“…Therefore, variation in rates of mass loss of eggs in multiple natural nests cannot be used as a true reflection of variation in water vapour conductance of the eggshells. Hence, like Maurer et al (2011) I conclude that the evidence for the degree of maculation correlating with eggshell water vapour conductance is not as strong as Higham and Gosler (2006) and Sanz and García-Navas (2009) suggest.…”

Darker eggshell spotting indicates lower yolk antioxidant level and poorer female quality in the Eurasian Great Tit (Parus major)

“…Several studies measured eggshell reflectance using a spectrophotometer and expressed it as brightness and red or brown chroma (Avilés et al 2007, Martínez-de la Puente et al 2007, Hanley and Doucet 2009, Maurer et al 2011b, Holveck et al 2012, Duval et al 2013, Krištofík et al 2013. Others determined pigment concentration of the eggshell by chemical analysis (Maurer et al 2011b, Cassey et al 2012, Duval et al 2013, Brulez et al 2014, or analyzed digitized pictures of eggshells to determine the percentage of spotted eggshell surface (Martínez-de la Puente et al 2007, Sanz and García-Navas 2009, Bulla et al 2012, Holveck et al 2012, Brulez et al 2014), or to determine hue, saturation, and brightness derived from HSB and RGB (Martínez-Padilla et al 2010, Cassey et al 2012, Holveck et al 2012), or to determine other spotting pattern variables, such as pattern contrast and marking size (Stoddard et al 2012). Finally, many studies used visual scoring of photographs, and spotting pattern was expressed as spot intensity, spot size, and spot distribution (Gosler et al 2005, Sanz and García-Navas 2009, López de Hierro and De Neve 2010, Bulla et al 2012, Holveck et al 2012, De Coster et al 2013, Hargitai et al 2013, Brulez et al 2014.…”

Eggshell spotting does not predict male incubation but marks thinner areas of a shorebird's shells

“…We strongly recommend studies aimed at resolving the still open question of how maternal resources (i.e. micronutrients, trace elements, and pigments) allocated in pigment speckles and in different eggshell layers contribute to embryonic development during their depletion (Orłowski et al 2016), especially in view of the fact that embryoinduced shell thinning at pigment speckles is minimal compared to that in plain eggshell regions (Maurer et al 2011c) and that the innermost (mammillary) layer of an eggshell is eroded or decalcified during embryonic development (Karlsson andLilja 2008, Igic et al 2017). This suggests that certain elements sequestered into eggshell pigment speckles (especially in those located in the upper eggshell layers not subject to decalcification) may not be further utilized by developing embryos.…”

“…the maculated eggshell area) demarcate thinner areas of the shell (reviewed by Cherry and Gosler 2010). Such localized eggshell thinning at pigment spots has been reported in a few species in the Passeriformes (Gosler et al 2005), Charadriiformes (Maurer et al 2011a, 2011c, Bulla et al 2012, and Falconiformes (Jagannath et al 2008). This phenomenon has been invoked as evidence for the structural function of maculation in the mechanism compensating for Ca deficiency and/or strengthening the eggshell (see Cherry and Gosler 2010).…”