The avian taste system

Abstract

Taste or gustation is the sense evolving from the chemo-sensory system present in the oral cavity of avian species, which evolved to evaluate the nutritional value of foods by detecting relevant compounds including amino acids and peptides, carbohydrates, lipids, calcium, salts, and toxic or anti-nutritional compounds. In birds compared to mammals, due to the relatively low retention time of food in the oral cavity, the lack of taste papillae in the tongue, and an extremely limited secretion of saliva, the relevance of the avian taste system has been historically undermined. However, in recent years, novel data has emerged, facilitated partially by the advent of the genomic era, evidencing that the taste system is as crucial to avian species as is to mammals. Despite many similarities, there are also fundamental differences between avian and mammalian taste systems in terms of anatomy, distribution of taste buds, and the nature and molecular structure of taste receptors. Generally, birds have smaller oral cavities and a lower number of taste buds compared to mammals, and their distribution in the oral cavity appears to follow the swallowing pattern of foods. In addition, differences between bird species in the size, structure and distribution of taste buds seem to be associated with diet type and other ecological adaptations. Birds also seem to have a smaller repertoire of bitter taste receptors (T2Rs) and lack some taste receptors such as the T1R2 involved in sweet taste perception. This has opened new areas of research focusing on taste perception mechanisms independent of GPCR taste receptors and the discovery of evolutionary shifts in the molecular function of taste receptors adapting to ecological niches in birds. For example, recent discoveries have shown that the amino acid taste receptor dimer T1R1-T1R3 have mutated to sense simple sugars in almost half of the living bird species, or SGLT1 has been proposed as a part of a T1R2-independent sweet taste sensing in chicken. The aim of this review is to present the scientific data known to date related to the avian taste system across species and its impact on dietary choices including domestic and wild species.

Keywords: avian species; diet; gustatory system; preference; taste receptors.

FIGURE 1

Topographical distribution of taste buds in the chicken oral cavity. The schematic view of the internal surface of the oral cavity is shown as a flat surface with the upper and lower surfaces next to each other in an “open book”-type view. Black dots represent taste buds. Lingual and/or pharyngeal papillae refer to mechanical (and not taste) organelles. The tip of the tongue is keratinized and does not contain taste buds. The main density of taste buds is in the upper palate. Other parts of the oral cavity in chickens with presence of taste buds include the root of the tongue and oropharynx, the base of the tongue and the soft oral mucosa in the mandible on both sides of the tongue. Taste buds gather in groups of 1–10 to form clusters, and these clusters are broadly distributed on the palate and the base of the oral cavity mainly around salivary ducts. This figure was created based on the data published by Kudo et al. (2008), Rajapaksha et al. (2016) using Adobe Illustrator 24.0.

FIGURE 2

Classification of avian taste buds. (A) Type I is an ovoid taste bud enwrapped by follicular cells. (B) Type II have an elongated shape. (C) Type III has no follicular cells. This figure was created based on the descriptions from Bath (1906), Rowland et al. (2015) using Adobe Illustrator 24.0.

FIGURE 3

Taste transduction via cranial nerves in birds. Cranial nerves (CN) V, VII, IX, X, and XI transmit taste information from the taste buds to the brain. Sensory ganglia are not shown. Figure was created using Adobe Illustrator 24.0.