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The urea concentration in tears of C. Thus, it was not posible to find urea at high concentrations in C. At first glance, the presence of glucose in the tear fluid might seem strange, representing a waste of energy.

However, it must be remembered that the lacrimal fluid is highly important for ocular surface protection. A previous evaluation of tears as a non-invasive biomarker fluid for glycemia failed to establish a direct correlation between the physiological profile and tear glucose concentrations (43, 44). The high concentrations of glucose in R.

Glucose was not found in the tears of C. A high concentration of glucose was observed in the tears of C. Among the components studied, existed lower interspecies variation in the tear-to-serum concentration ratio of glucose, suggesting correlated glucose concentration in these two fluids for all species in this study, except C. Cholesterol, a lipid that can be widely found in tears, is related to lubrication, structural maintenance, thermoregulation and bactericidal activity (20, 46, 47).

The tears of the studied animals had more cholesterol than human tears, reasserting the importance of this molecule for reptiles, birds and domestic animals (47, 48). The tears of birds of prey presented biochemical profiles with greater Euclidian distance, or less similarity, from the other species.

The similarity between the biochemical profiles of the other species was 88. This finding, in addition to the associations observed for animal habitat, suggest that species-specific particularities in lacrimal composition do not reflect a linear trajectory or history in animal evolution.

The cluster variability and exceptions observed in this study apparently do not correspond to the positioning of the animals, or different Classes of vertebrates, on the taxonomic scale.

Moreover, making evolutionary assumptions based on biochemical parameter reference values for tears and sera of vertebrate animals is difficult, because these parameters are profoundly affected by a variety of factors, such as changes in dietary habit, and interference in the quantitative expression of lipids, proteins and nitrogenous products as these are present in all body fluids, especially in reptiles and birds.

In this work, strict procedures and criteria for animal inclusion were rigorously maintained, with the aim of obtaining more reproducible and homogeneous results, but it must be assumed that all of the cited factors might cause interference bias. In conclusion, we suggest that variations in the tear composition of reptiles, birds and mammals are potentially associated with the type of environment in which the animal lives, and there are marked differences between tears from humans and tears from these animals.

A more precise study of these associations, through proteomics, glycomics and metabolomics analyses, may aid in the development of new therapeutics for human and veterinary medicine. Moreover, this study provides information that can serve as a basis for further analyses using animals in experimentally controlled environments. The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher. The studies involving human participants were reviewed and approved by Ethics Committee in Research of the Institute of Health Science, Federal University of Bahia.

The animal study was reviewed and approved by Ethics Committee on Animal Experimentation of the School of Veterinary Medicine and Zootechnology. Written informed consent was obtained from the owners for the participation of their animals in this study. AO, MA, and RP conceived and designed the study. AR, TB, and DC carried out investigation and methodology. The data validation was made by AO, RP, and MA. All authors have read and accepted the manuscript as it is presented to the journal.

AP is a research fellow from the National Council for Scientific and Technological Development (CNPq). RP is a technological development fellow from CNPq. AC is a Ph. Akat E, Arikan H. A histological study of the eye in Hyla orientalis (Bedriaga, 1890) (anura, hylidae).

Neagu A, Petraru O. Gilbert S, Epel D. Collin SP, Collin HB. A comparative SEM study of the vertebrate corneal epithelium.

Comparative study of the corneal epithelium in some reptiles inhabiting different environments. Lam P, Mangos S, Green JM, Reiser J, Huttenlocher A.