03 – ON THE CROP AND GIZZARD STONES (GASTROLITHS) OF BACKYARD CHICKENS: A GLASS CHEMISTRY

Ano 12 (2025) – Número 2 – Fulgurites, Gastroliths, Aluminium, Crônicas Artigos

ON THE CROP AND GIZZARD STONES (GASTROLITHS) OF BACKYARD CHICKENS: A GLASS CHEMISTRY

 

10.31419/ISSN.2594-942X.v122025i2a3PHCS

 

Marcondes Lima da Costa^1*

Glayce Jholy Souza da Silva Valente²

Pabllo Henrique Costa dos Santos²

¹Geosciences Institute and PPGPatri from Federal University of Pará, Belém, Pará, Brazil,  marcondeslc@gmail.com

²Faculty of Conservation and Restoration, Federal University of Pará, Belém, Pará, Brazil

                ^*Author for correspondence

Submitted on November 7, 2024; after revisions accepted on November 8, 2024

 

ABSTRACT

The pebbles from the chicken digestive tract (gastroliths) in their colorless, green and amber tones are tabular and sub-rounded, matte, with physical and x-ray diffraction characteristics typical of industrial bottle glass. Total and spot chemical analyzes carried out by XRF and SEM/EDS show that the raw material was quartz in the form of pure sand, possibly, and as pigments iron, chromium, perhaps sulfur compounds. Iron was the main pigments for the green gastrolith, and iron-chromium and sulfur for the amber. The chickens must have ingested glass pebbles from fragmented and comminuted bottles, due to the lack of stones and minerals of a compatible size (a few mm or more) for crushing and grinding food. It is known that there is no hard rock in more than 90% of the outcropping land in the state of Acre, and in Feijó nothing is known, except for those that have been arriving in recent decades from Rondônia, and used in thousands of tons, in paving the BR -364 and neighboring areas, and in civil construction in general. A true “geological contamination”. They will certainly reach the digestive tract of animals in Acre.

 

RESUMO

As pedrinhas do trato digestivo de galinha em suas tonalidades incolor, verde e âmbar são tabulares e subarredondadas, foscas, com características e difratométricas físicas típicas de vidros de garrafas industriais. As análises químicas totais e pontuais realizadas por XRF e SEM/EDS mostram que a matéria-prima foi quartzo na forma de areia pura, possivelmente, e como pigmentos ferro, cromo, talvez compostos de enxofre. Ferro foi o principal pigmento do gastrólito verde, e ferro-cromo e enxofre para o âmbar. As galinhas devem ter ingerido as pedrinhas de vidro de garrafas fragmentadas e cominuídas, pela falta de pedras e minerais em tamanho compatível (alguns mm ou mais) para trituração e moagem de alimentos. É sabido da ausência de rochas duras em mais de 90% dos terrenos aflorantes do estado do Acre, e em Feijó nada se conhece, exceto as que estão chegando nas últimas décadas procedentes de Rondônia, e empregadas em milhares de toneladas, na pavimentação da BR-364 e vicinais, e na construção civil em geral. Uma verdadeira “contaminação geológica”. Elas, certamente, em breve, vão chegar ao trato digestivo de animais no Acre.

 

INTRODUCTION

Occasionally you least expect, someone would appear with a folded piece of paper containing colored pebbles, which they had found in the internal bodies of free-range chickens in their backyards. Curiosity is always accompanied by the hope of being in front of a precious stone of high value, and the question, why do chickens eat pebbles. Most of those pebbles that we had access comprise a variety of quartz or even glass, with a diameter between 0.5 and 1.5 cm. In 2024, two samples of chicken pebbles found in the backyards of settlers in the municipality of Feijó, State of Acre, were received, visually appearing similar to each other. It was then decided to analyze these samples, which were mediated by Mrs. Rita Feitosa Pinheiro, from the city of Feijó.

Small stones found in chicken crops and gizzards, that is, in the digestive tract of these animals, are not new. They have been frequently found in these animals when raised freely, regardless of the country, but they are not restricted to chickens. They are also common in birds in general, in addition to modern reptiles, fish, and aquatic mammals. These pebbles or stones (stomach stones) are called gastroliths in the technical-scientific language (from the Greek gastir, stomach; and lithos, stones). According to Wings (2004), this term was introduced by Mayne in 1854, although stones in the digestive tract had been known for a long time, when the digestive function was recognized. Skoczylas (1978) defines gastroliths simply: “When the swallowed objects remain in the stomach, they are called gastroliths.” Wings (2004) distinguishes: bio-gastroliths (non-pathological invertebrate concretions), patho-gastroliths (every pathological stone formed in the stomach), and geo-gastroliths (swallowed pebbles and grit), terms that refer to the nature of formation of these stones. In ostriches from Germany and South Africa, an average of 1 kg of gastroliths per animal was found, corresponding to around 1,000 pebbles, larger than 1 mm in size, predominantly formed by quartz (Wings, 2004). And what is even more interesting is that gastroliths are also found in their fossil ancestors, going back more than 200 Ma.

As previously presented, the term gastrolith includes not only pebbles, that is, fragments of minerals and rocks (stomach stones), but, unfortunately, also various stones (kidney, bile or gallbladder patho-gastroliths), comprising aggregates of petrified millimetric crystals of calcium oxalates (e.g. whewellite, CaC₂O₄.H₂O, weddellite, CaC₂O₄.2H₂O), the dominant ones with up to 80%; phosphates of magnesium and ammonium (struvite, NH₄MgPO₄·6H₂O) and calcium (apatite, Ca₅[PO₄]3[OH,F,Cl]) with up to 10%, sulfates, carbonates, urates, cystine, among others (Baran, 2014; Karki & Leslie, 2023). These patho-gastroliths cause moderate to severe disorders not only in humans but also in several animals and can even lead to death. In turn, minerals of inorganic origin, found as pebbles in the digestive tract of birds, reptiles, and fish, current or fossil, can be the most varied, but varieties of quartz predominate, in addition to feldspars, aluminosilicates, and carbonates (Panichev and Seryodkin, 2022; Michael, 2021). The function of mineral gastroliths in the digestive tract, especially in the gizzard, is still much debated, although they have been studied exhaustively, with a few hundred articles already published (Wings, 2004). Most researchers agree that they help in crushing and grinding of the hardest foods, aided by the muscular system of the gizzard. They are also ingested instinctively to adjust the chemical composition of digestive electrolytes and to prolong and activate digestive enzymes, grind food, among others (Panichev and. Seryodkin, 2022). In remains of Triassic fossils, such as plesiosaurs, crocodilians, and pinnipeds (Long et al., 2006), these authors describe gastroliths found in the body of Ichthyosaur Panjiangsaurus, normally measuring between 0.6 and 2 cm, but some reach 3.7 cm and are formed of quartzites and other siliceous rocks, with carbonate venules. Or even buoyancy in aquatic animals appears to be negligible. Accidental ingestion of sediment is common, as is the overlap of numerous functions (Wings, 2004).   In gastroliths from Kenyan ostriches in addition to quartz was found also feldspar, obsidian, chert, among others.  In other situations, sandstones, limestones, siliceous rocks, volcanic rocks, and even glass and anthropogenic bricks were identified. Wings (2004) in his work counted 1150 gastroliths in the fractions 4-8mm and 8-20mm, of which 819 were quartz, 130 quartzite, 26 cherts, 22 glass and 153 other types of rock. Glasses are more frequently between 4 and 8 mm in diameter.

Gastroliths have a very variable external appearance promoted by extremely complex processes. Certain stones appear to be well polished (we think they confuse polishing with rounding), certainly due to the physical movements within the digestive tract, the abrasion caused by organic matter, especially its phytoliths, in addition to the action of digestive acids and enzymes (Wings, 2004). As you can see, it is a complex and intriguing world, which involves animal life from the first vertebrates in geological time (their corresponding fossils), which allows us to deduce the environmental conditions of that time and today, geology, and try to unravel the function of these stones inside the animals’ bodies.

In this work, as mentioned, the pebbles found in the digestive tract of chickens in the municipality of Feijó, state of Acre, will be addressed. With the results obtained, we hope to instigate the advancement of these studies, and show how great the importance of minerals is.

 

MATERIALS AND METHODS

Two specimens of green pebbles, two colorless and three of amber were made available by Mrs. Rita. After being photographed, one specimen of each color was used for non-destructive analyses, such as: density determination, partial chemical composition determination by portable XRF and micromorphological and semi-quantitative chemical analysis by SEM-EDS.

Density was determined using a digital pycnometer Anton Paar Ultrapyc 5000, Serial Number: 1050059383 Software Version: 1,002,020, nitrogen gas, target pressure of 19 psi; target temperature: 20^o C; flow mode: monolith; cell size: nano; preparation method: flow; final mode criteria: < 0.5%; running time: 7 to 9.5 minutes.

The qualitative partial chemical composition was carried out directly on the sample, which is much smaller than the field of incidence of the X-rays, which impairs the fidelity of the results in quantitative terms. Bruker’s S1 Turbo equipment was used, through the Geochemical analytical package. The equipment does not detect elements with an atomic weight less than or equal to that of sodium.

Semiquantitative micromorphological and microchemical analyzes were conducted by scanning electron microscopy, Hitachi TM3000 equipment, coupled with an Energy Dispersive Spectroscopy (EDS) Swift ED 300 system, under voltage acceleration from 5 to 15 kV and, with an SDD detector (161 eV-Kα) from the Lamiga laboratories of the Institute of Geosciences at the Federal University of Pará. The analysis was carried out under a low vacuum, with a non-metallized sample.

 

RESULTS AND DISCUSSIONS

Physical features

Three sets of colors were identified for the pebbles under investigation: colorless, green and amber. The length varies between 5 and 10 mm and the width between 2.5 and 8 mm. The density is 2.43 for colorless, 2.48 for green and 2.46 for amber, therefore lower for colorless and higher for green (Table 2). These density values ​​fit perfectly between ordinary bottle glass (Costa et al., 2024). The hardness is in the order of 5.5 to 6 on the Mohs scale. Colorless and green are transparent while amber is translucent.

The pebbles are characterized by shapes with a tabular appearance, with a contour approaching rectangular to trapezohedron, with low sphericity and a degree of rounding between sub-rounded and rounded, low polishing (matte), regardless of the color of the pebbles. The data regarding density, hardness, brightness and even diaphaneity allow them to be classified as common industrial bottle-type glass in their colorless, green and brown colors.

 

 

Figure 1 – The physical aspects of the seven stomach stones investigated: 3 amber, 2 green and 2 colorless.

 

Table 1 – Some physical features for investigated chicken stomach stones (gastroliths). Table 1 – Physical features

	Colorless	Colorless	Green	Green	Amber	Amber	Amber
Stones	1	2	1	2	1	2	3
Length (mm)	10	8	7,5	7	nd	8	5
Width (mm)	4	6	6	4	nd	2,5	3
Density (g/cm3)	2,4324	nd	2,4811	nd	2,4639	nd	nd
Hardness	5,5 – 6	5,5 – 6	5,5 – 6	5,5 – 6	5,5 – 6	5,5 – 6	5,5 – 6
Luster	Vitreous	Vitreous	Vitreous	Vitreous	Vitreous	Vitreous	Vitreous
Diafanity	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent
Surface	Matte	Matte	Matte	Matte	Matte	Matte	Matte
Rounding	4-5	4-5	4-5	4-5	4-5	4-5	4-5
Sphericity	Low	Low	Low	Low	Low	Low	Low

Rounding: 4: sub rounded; 5: rounded. Nd: not determined.

 

XRD mineralogy

The gastroliths investigated represented by the amber one are amorphous to XRD (Figure 2), as expected by the previously described physical features.

 

Figure 2 – XRD spectrum for amber chicken gastrolith, showing a typical pattern of amorphous structure.

 

Chemical Composition

Chemical analyzes obtained by handheld XRF show that they are formed, regardless of color, by SiO₂ (> 80%), the dominant component, CaO (~13%), in addition to Al₂O₃ (2 to 3%), K₂O (~1.0 %) and MgO (present, close to the detection limit of the equipment used, confirmed by SEM/EDS). Na₂O is not detected by this method, but was detected by SEM/EDS, as shown below. The potential chromophore chemical elements detected were Fe (0.352% of Fe₂O₃), Mn (0.02% of MnO), and Cr (0.02) in green; Fe (0.081% Fe₂O₃) in amber. The Cl content is surprising, in the order of 0.2% in amber, 0.14% in green, and 0.09% in colorless. In colorless glass the Fe₂O₃ content is the lowest (0.047%). S was also detected, but always at low values: 0.06% in amber, 0.05% in green, and 0.02% in colorless. Therefore, Fe and Cr must be the main chromophores for the green tone, and iron for the amber. Sulfur can also contribute to the amber color (Salesforce, 2013). This chemical composition, dominated by SiO₂ and CaO, meets the nature of industrial anthropogenic glasses, in which SiO₂ represents the fundamental raw material, formed only from silica, such as quartz, generally coming from white quartz sands or quartzites, while CaO, K₂O, MgO, and Cl, correspond to fluxes or additives, normally from limestone, soda ash and/or vegetable ash or equivalent.

Figure 3 illustrates the micromorphological features of the gastroliths investigated. They corroborate the amorphous nature, typical of glass, when magnified above 400 x). There is also a widespread presence of “gaseous” tonsils (irregular dark spots in the vitreous matrix, even more amorphous), which demonstrate that the material was melted and cooled quickly, trapping gases. Locally, these tonsils assume disproportionate relative sizes. All these micromorphological aspects are similar across the three types of gastroliths investigated, regardless of the inserted pigments.

The dominant mass of the colorless gastrolith (Table 2) according to SEM/EDS is composed of O, Si, Na, Ca, Al, K, and Mg, decreasing in this order, where O, Si, Na, and Ca represent more than 97% of the total composition. This confirms the significant presence of Na (which cannot be detected by portable XRF) and also Mg, which was detected by this method, but with imprecision. In general terms, this chemical composition is the same for green and amber gastroliths (Tables 3 and 4), however the Mg and K levels are higher in the green gastrolith, approximately double those in the colorless one. In turn, the Mg levels are relatively high in the amber.

This chemical composition is dominated by O, Si, Na, and Ca, that is, by SiO₂, Na₂O and CaO, in addition to smaller concentrations of Al (Al₂O₃), K (K₂O) and Mg (MgO), as previously discussed with the data by handheld XRF, reinforce that these gastroliths actually correspond to simple glass, like those from industrial bottles (Couri et al. 2020; https://www.oberk.com/packaging-crash-course/how-glass-bottles-are-made), in which the raw material was silica, probably quartzose, while Na₂O and CaO, Al (Al₂O₃), K (K₂O), and Mg (MgO), represent the nature of the flux (barrel, which is a Na carbonate, or albite-type feldspars; limestone or lime, which are classic in glass production). In this gastrolith samples (glass) carbonate and limestone can be excluded, since no carbon has been detected in their chemical composition. Al and Mg may also come from the intentional addition of aluminum and magnesium oxides to increase chemical durability.

The green gastrolith matrix is ​​the only one that contains iron, both in the mass and as grains or isolated phases of Fe and Cr oxides (Table 3). The iron contents in green gastroliths are the highest among the three types, as already demonstrated by analyses of XRF. These isolated phases of Fe and Cr oxides (light gray to white tones and high relief in the images), in which Fe contents are on the order of 6 times those of Cr, were also found in amber gastroliths (Table 4). They were incorporated, as previously stated, as pigments, and interpreted as newly formed phases during the glass production process. The use of iron as a green pigment in glasses is widely known (Salesforce, 2013; Couri et al. 2020). The raw material, based on white sand, may contain rare grains of these minerals, and even gold, as was seen in the colorless gastrolith. Couri et al (2020) mention gold as ruby ​​pigmentation, which is not the case here. It is concluded that iron was the main component for pigmentation of green glass and iron-chromium of amber, in agreement with what is known about glass coloring (Table 6). Normally, Mn also contributes to this color, but it was not detected. The chemical composition of these gastroliths, undoubtedly representative of glass, possibly bottles in their most common tones, colorless, green and amber, only partially resembles the sea glass described by Costa et al (2024), equivalent to black glass, diverging in the pigmenting agent.

 

 

Figure 3 – Some SEM photomicrographs showing the distinct microscopic textures of the tree color of gastroliths investigated.

 

Table 2 – SEM/EDS chemical composition of colorless stomach stones (gastroliths) found the chicken. 1. glass; 2. glass; 3. Fe phase in glass matrix; 4. Cr-magnetite grain in glass matrix; 5. Cr-magnetite grain with gold in glass matrix; 6. Amygdale in glass matrix.

 

All gastroliths studied show the constant presence of amygdales, which represent possible air bubbles, which are occupied by materials that encompass the chemical composition of the matrix plus the omnipresence of Cl, S and sometimes P (Table 2 analysis 5; table 4 analysis 5; table 5 analysis 4), always with Na or Ca at very high values ​​than the matrix average. Similar characteristics were found by Costa et al (2024) in sea glass from the Azores.

The ingestion of these glass fragments by chickens demonstrates that in their environment there was no availability of other resistant stone materials (rocks and minerals in macro grains), but certainly many discarded glass bottles, which over time broke or were broken. In Acre, this custom dates to the era of rubber plantations at the end of the 19th century, when drinks (beer, cachaça and brandy, soft drinks, sometimes wine) as well as medicines and perfumes, etc., were filled in glass bottles in these colors. main ones. Unable to collect them, they were discarded into the environment, suffering natural breakage and comminution, generating smaller and smaller fragments, until they reached the size of the capacity of the beaks of chickens raised freely in huge backyards. This situation continues to this day. Fragments of glass in the digestive tract of animals, even ceramic material, have already been found in other locations, as described by Wings (2004).

It is known that there are no hard rocks in more than 90% of the outcropping land in the state of Acre, and in Feijó this reaches 100%. It is not known, except for those that have been arriving in recent decades from Rondônia, and used in thousands of tons, in paving the BR-364, not only in Feijó, but throughout its extension and neighboring areas, and also in civil construction in general. A true “geological contamination” of Acre. They will certainly reach the digestive tract of animals in Acre. Just wait a little longer.

By way of illustration, table 6 presents the main pigmenting agents used in the production of glass, mainly various bottles. For those who want to know the modern production processes of glass bottles, it is recommended, for example, to read the article by Couri et al. (2020).

 

Table 3 – SEM EDS chemical composition of the green stomach stones found in chicken. 1. glass; 2. glass; 3. Fe phase in glass matrix; 4. glass; 5. Amygdale.

 

 

Table 4– SEM EDS chemical composition of the amber stomach stones found in chicken. 1. glass; 2. glass; 3. glass area; 4. amygdale; 5. glass with Fe and Cr (Cr-magnetite); 6. glass with much more Fe and Cr (Cr-magnetite grain).

 

Table 5 – Chemical pigments used in the glass bottle industry in Brazil cited by Couri et al. (2020).  Table 5 – Chromophores chemical elements

 

CONCLUSIONS

The colored pebbles (colorless, green, and amber) found in the digestive tract of backyard chickens in Feijó, Acre, correspond to gastroliths, which have several known functions, the main one being as a crusher and food grinder, although there are still many controversial ones. The three types (colorless, green, and amber) are, due to their physical, morphological, crystalline, and whole chemical and microchemical characteristics, as well as micromorphological, equivalent to industrial glass, similar to that of common bottles. For the chickens to have ingested these materials, the search for other hard materials such as stone/mineral was certainly due to a lack of options. It is well known that glass bottles were widely used in the not so distant past, and were discarded in backyards and along paths in rubber plantations and farms in state of Acre (Amazon region), and this procedure continues to the present, and even more intensively, since there is no recycling of them in these regions, and which is still restricted in much of Brazil. It is known that there is no hard rock in more than 90% of the outcropping land in the state of Acre, and in Feijó nothing is known, except for those that have been arriving in recent decades from Rondônia, and used in thousands of tons, in paving the BR -364 and neighboring areas, and in civil construction in general. A true “geological contamination”. They will certainly reach the digestive tract of animals in Acre in a short space of time.

 Acknowledgments

To Mrs. Rita Feitosa Pinheiro for the opportunity to introduce us and discuss the pebbles of chicken chat from a farm in Feijó, state of Acre, and thus instigate us to study them in more detail, the results of which were presented in this work. To CNPQ for scholarship and grant (Nr. 304.967/2022-0) for financial support.

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