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A re-evaluation of the chemical composition of avian urinary excreta

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Abstract

Osmoregulation in birds is complicated, with different organs acting concurrently to regulate this physiological process. Of particular interest is how the urinary excretions of birds can remove excess nitrogen while minimizing the need for dietary water and balancing the physiological demands of oviparity. It has long been concluded from chemical analyses, and more recently from genetic studies, that uric acid is the principal constituent of urine in Aves. However, research has also demonstrated that waste material may be modified in the ceca immediately prior to it being expelled. Here, we quantify the chemical composition of the urine component of excreta of six avian species using X-ray diffraction techniques to test the hypothesis that it is principally composed of uric acid, as commonly reported. None of the analyzed samples were found to contain uric acid. Instead, a variety of compounds including ammonium urate, struvite (magnesium ammonium phosphate), and two unknown compounds, were found. Our results show that the uric acid pathway is indeed the system by which nitrogen is removed in these birds, but that additional modification occurs in the urine prior to excretion. These results raise questions for future research on the urinary excretions of birds, including identification of the unknown compounds found in the present study.

Zusammenfassung

Eine Neubewertung der chemischen Zusammensetzung des Urins bei Vögeln

Osmoregulation bei Vögeln ist ein komplizierter Prozess, bei dem verschiedene Organe gleichzeitig zur Regulierung ihrer Physiologie aktiv sind. Von besonderem Interesse ist dabei, wie bei Vögeln mit dem Urin überschüssiger Stickstoff ausgeschieden und gleichzeitig der Bedarf an Flüssigkeit aus der Nahrung minimiert sowie die physiologischen Anforderungen der Oviparie ausgeglichen werden können. Chemische Analysen in Kombination mit neuesten genetischen Studien haben schon länger dargelegt, dass Harnsäure den Hauptbestandteil des Urins bei Vögeln bildet. Jedoch haben zusätzliche Untersuchungen ergeben, dass Abfallstoffe in den Blinddärmen unmittelbar vor der Ausscheidung umgewandelt werden. In dieser Studie quantifizieren wir die chemische Zusammensetzung der Urin-Bestandteile der Vogelexkremente von sechs Arten mittels Röntgenstrukturanalysen, um die Hypothese zu testen, dass dieser hauptsächlich aus Harnsäure besteht, wie stets berichtet wird. In unseren Ergebnissen konnte kein Beleg für Harnsäure in den analysierten Proben gefunden werden. Stattdessen konnten wir eine Vielzahl an verschiedenen Bestandteilen identifizieren, unter anderem Ammoniumurat, Struvit (Magnesium-Ammonium-Phosphat) und zwei unbekannte Verbindungen. Obwohl der Abbau zu Harnsäure weiterhin den Stoffwechselweg darstellt, über den Stickstoff ausgeschieden wird, zeigen unsere Ergebnisse, dass zusätzliche Umwandlungen vor der Ausscheidung stattfinden. Diese Ergebnisse werfen Fragen für zukünftige Untersuchungen auf, einschließlich der genaueren Bestimmung der hier entdeckten unbekannten Verbindungen.

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Acknowledgments

We wish to honor the late Dr. Robert (Bob) Folk for discussion that inspired this work; his kindness, creativity and broad interests will be remembered. John, Marcy and the staff at Austin Zoo provided the urine samples. Chris Torres provided helpful discussion and comments on the manuscript.

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This work received no project-specific funding.

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Authors and Affiliations

  1. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas, Austin, TX, 78756, USA

    Nicholas M. A. Crouch & Julia A. Clarke

  2. Department of Chemistry, College of Natural Science, The University of Texas, Austin, TX, 78712, USA

    Vincent M. Lynch

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  1. Nicholas M. A. Crouch
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Correspondence to Nicholas M. A. Crouch.

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Crouch, N.M.A., Lynch, V.M. & Clarke, J.A. A re-evaluation of the chemical composition of avian urinary excreta. J Ornithol 161, 17–24 (2020). https://doi.org/10.1007/s10336-019-01692-5

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