CRANIAL OSTEOLOGY OF CYCLARHIS GUJANENSIS (AVES: VIREONIDAE)

Abstracts

The small passerine Cyclaris gujanensis can tear into small pieces large or heavy-bodied preys that could not be swallowed whole such as frogs, snakes, bats and birds. However there are few studies on the cranial anatomy of this species. Thus, we focused on the description of the cranial osteology to contribute to the anatomical knowledge of this species and to make some assumptions about functional anatomy. The fossa temporalis is shallow but broad and the fossa of os palatinum is deepened. The os quadratum processes are long and thick. The os pterygoideum is enlarged and the upper jaw is strongly inclined ventrally (140°) with reference to the skull. The rostral extremity of rhamphotheca is hooked with ventral concavity to fit the mandible (pincer form). The mandible fossae are deepened and broad and its bulky medial process probably provides mandible stability and strong support to the muscles attached on it. All these peculiar characteristics probably indicate a considerable force in the C. gujanensis jaws and partially explain its distinctive feeding habit compared with the other Vireonidae. Nevertheless, new studies with functional approaches to analysis the forces of the muscle fibers and the cranial kinesis are needed to prove the hypotheses mentioned above.

Rufous-browed Peppershrike; Skull; Osteology; Anatomy.

O pitiguari (Cyclarhis gujanensis) é um pequeno vireonídeo com uma ampla variedade de presas, capaz inclusive de dilacerar presas maiores, como lagartos, morcegos e aves. No entanto, são escassos os estudos sobre morfologia craniana desta espécie, e, motivados pela sua maneira particular de alimentação, objetivou-se descrever a osteologia craniana para contribuir com os conhecimentos anatômicos dessa espécie, além de discutir alguns aspectos morfofuncionais relacionados à sua alimentação. A fossa temporal é ampla, porém pouco profunda, já o osso palatino apresentou fossas profundas. Os processos do osso quadrado são longos e espessos. O os pterygoideum é robusto e a maxila superior é fortemente inclinada (140°) em relação ao crânio. A extremidade rostral da ranfoteca é curvada acentuadamente, com formato côncavo em sua porção ventral para se encaixar com a mandíbula (forma de pinça). A mandíbula possui, além de fossas amplas e profundas, um processo medial bastante desenvolvido, que provavelmente dá suporte aos músculos que movimentam as maxilas. Tais características peculiares na osteologia craniana de C. gujanensis, poderiam explicar parcialmente o seu hábito alimentar diferenciado quando comparado com outros Vireonidae. Porém, novos estudos, com abordagens funcionais, cinese craniana e análises da força das fibras musculares da mandíbula são necessários para comprovar as afirmações mencionadas acima.

Pitiguari; Crânio; Osteologia; Anatomia.

INTRODUCTION

The Rufous-browed Peppershrike, Cyclarhis gujanensis (Gmelin, 1789), is a well distributed passerine in the Neotropical region where it occupies a wide range of habitats (Sick, 1997). Although it is considered an omnivorous bird, the literature revealed a peculiar foraging behavior. This little bird can capture and tear big preys as frogs, snakes, bats and birds (Moojen et al., 1941; Sick, 1997; Ghizoni-Jr et al., 2000; Andreau & Fernandes, 2010).

C. gujanensis uses the beak to tear apart large prey items held down with the foot (Snow, 1964). However, the morphofunctional system of the feeding apparatus in C. gujanensis is little known. Only few studies on its anatomy are found (Beecher 1953, 1978; Raikow, 1978; Orenstein & Barlow, 1981), but none provided a detailed description of the cranial osteology and the morphological functions of the feeding apparatus.

Donatelli et al. (2014) stand out the importance to study the movements of the beak to apprehension and dilacerations of preys through an analysis from the jaw apparatus. Such approaches could enable us to understand this unique foraging strategy by this little passerine when compared to other heavy-bodied and large carnivorous birds. Thus, we provided the description of the cranial osteology of feeding apparatus in C. gujanensis to contribute to the anatomical knowledge of this species and to discuss some aspects about functional anatomy and feeding habit of this bird.

MATERIALS AND METHODS

We studied the cranial and jaw osteology based on eight adult specimens of C. gujanensis (Gmelin, 1789). We compared the descriptions with three specimens of Vireo olivaceus (Red-eyed Vireo) (Linnaeus, 1766) and two of Vireo chivi (Chivi Vireo) (Linnaeus, 1766). The material analyzed is from the collections of the Museum of Zoology, University of São Paulo (MZUSP: Vireo chivi 9A392, 9A393), the Museum of Natural History Taubaté (MHNT: C. gujanensis 630 and Vireo olivaceus 192, 808, 1066) and the Laboratory of Ecology, Systematics and Conservation of Neotropical Birds - UFMS (LESCAN: C. gujanensis 1, 2, 3, 11, 12, 13, 14).

We focused in bones associated with jaws movements and their morphological functions are based on previous studies (Bock, 1960, 1964; Cracraft, 1968; Richards & Bock, 1973; Zweers, 1974; Zusi, 1984; Gennip, 1986; Baumel et al., 1993). The inconspicuous structures were described using an Olympus SZX12 stereo-microscope (Olympus, Tokyo). We used a protractor (180°) to calculate the angle of inclination between the upper jaw and the arcus jugalis. Specific measurements of all specimens were taken with a digital caliper to the nearest 0.01 mm: snout-vent length (SVL).

We followed Nomina Anatomica Avium (Baumel et al., 1993) for the osteological terminology and some nomenclature modifications proposed by Burton (1984) and Posso & Donatelli (2005). The jaw apparatus were studied concurrently but this manuscript refers only to the osteological analysis. The analysis of the jaw muscles will be presented in another manuscript (Previatto & Posso (in press)).

RESULTS

The frontal region (F) (Fig. 1) shows a little conspicuous and rostrally suture in the zona flexoria craniofacialis (ZFC) (Fig. 1). In the orbital portion, the lateral extremities of frontal medially are curved (concave) with an average width of 1.2 mm and the parietal region (os parietale) (P) (Fig. 1) has an average width of 1.9 cm. It gives a stout head to the C. gujanensis. This difference is bigger in V. olivaceus and V. chivi (0.6 to 1.5 cm). The rostral apex of the upper jaw (ossa maxillae) (OM) (Figs. 1 and 3) shows a pronounced curvature when the ramphotheca is preserved, where we also observed a small concavity for engagement of the jaw (pincer form). The upper jaw is inclined ventrally at an angle of approximately 140° (Fig. 2) to the arcus jugalis and it is about half the total length of the skull (from 17.1 mm to 37.2 mm). The arcus jugalis (J) (Figs. 1 and 2) has an average of 16 mm in length, slightly less than half the total length of the skull, and its bones are completely fused to each other. The arcus jugalis shows strong curvature in C. gujanensis (140°) when compared with V. chivi and V. olivaceus (160°).

Figure 1
Dorsal view of the skull of C. gujanensis - F: os frontale; J: arcus jugalis; OM: ossa maxillae; P: os parietale; PrPO: proc. postorbitalis; ZFC: zona flexoria craniofacialis

Figure 2
Lateral view of the skull of C. gujanensis - Bp1: bony projection 1; Bp2: bony projection 2; E: os ectethmoidale; FST: fossa subtemporalis; FT: fossa temporalis; J: arcus jugalis; L: os lacrimale; LF: lateroesfenoidal fossa; PrE: proc. esquamosalis; PrPO: proc. postorbitalis; PrPT: proc. pterygoideum; PrD: proc. dorsalis; PrOtQ: proc. oticus os quadratum; PrOrQ: proc. orbitalis; PT: os pterygoideum

Figure 3
Ventral view of the skull of C. gujanensis - CDC: condylus caudalis; CDL: condylus lateralis; CDM: condylus medialis; CL: crista lateralis; FC: fossa choanalis; FV: fossa ventralis; LCL: lamella caudolateralis; OM: ossa maxillae; PA: os palatinum; PT: os pterygoideum

The bony projection 1 (facies nasalis) (Bp1) (Fig. 2) of the os ectethmoidale (E) (Fig. 2) is large and it fuses with os lacrimale. It is also observed in V. chivi and V. olivaceus. Apparently the os lacrimale (L) (Fig. 2) also fuses with the frontal region. The bony projection 2 (facies orbitalis) (Bp2) (Fig. 2) is larger in its ventral portion covering partially the os palatinum bones and it is in contact with the arcus jugalis, but not ventrally beyond it.

The fossa temporalis (FT) (Fig. 2) is shallow, but deepened in its rostroventral portion, dorsoventrally enlarged (embracing the dorsal portion of the proc. postorbitalis) and rostrocaudally shortened. The fossa temporalis is more reduced and deepened in V. chivi and V. olivaceus. The fossa subtemporalis (FST) (Fig. 2) is swallowed in all species studied.

The proc. squamosalis (PrE) (Fig. 2) is long, with about half the distance of its insertion onto the skull to the arcus jugalis (4.5 mm by 8.3 mm) in C. gujanensis. It is laterally flattened and in its dorsal portion shows a semi-spiral form. The proc. postorbitalis (PrPO) (Figs. 1 and 2) is laminar (rostrocaudally flattened) and reduced, since its length from the distance between its origins in the skull to the arcus jugalis shows an average size of 3.3 mm (about one fifth of the distance of its insertion onto the skull to the arcus jugalis). In V. chivi and V. olivaceus the proc. postorbitalis is shorter (1/6 from its origin in the skull to the arcus jugalis) than in C. gujanensis (1/5). In the pars caudalis orbitae there is a narrow lateroesfenoidal fossa (LF) (Fig. 2) housing the first component of the M. adductor mandibulae externus rostralis medialis. Ventromedially we observed the area muscularis aspera, place of origin of the M. pseudotemporalis superficialis.

The os palatinum bone (PA) (Fig. 3) is wider in its caudal portion, tapering as it approaches its rostral portion. Its caudolateral expansion is continuous with the crista lateralis (CL) (Fig. 3) with a slight ventral curvature. This crest extends to the caudal portion of the palate and it ends almost perpendicular to the boundary with the proc. pterygoideum of os palatinum, forming the lamella caudolateralis (LCL) (Fig. 3). Dorsomedially the os palatinum is flattened, where some fibers of the M. pterigoideus dorsalis medialis are originated. The fossa choanalis (FC) and ventralis (FV) (Fig. 3) are both narrow and deep, but the first is narrower and deepened as it approaches to the proc. pterygoideum of os palatinum. The dorsal surface of the os palatinum lacks the lamela choanalis due to this bone is ventrally curved and also the fossa ventralis is deep. In C. gujanensis the os palatinum is caudally wider and its fossa ventralis is very deeper than in V. chivi and V. olivaceus. The proc. pterygoideum (PrPT) (Fig. 2) is narrow, long, and completely fused with the os pterygoideum. The os pterygoideum is robust (average of 8 mm and about 1/5 of the total length of the skull) in C. gujanensis.

Cyclaris gujanensis presented the proc. orbitalis os quadratum (PrOrQ) (Fig. 2) with a similar length to other vireos studied, but is wider and thicker, especially in its basal portion. The proc. oticus os quadratum (PrOtQ) (Fig. 2) is wide. Moreover the sulcus intercotylaris is deepened and the condylus medialis (CDM) (Fig. 3) is larger than the condylus lateralis (CDL) (Fig. 3) and caudalis (CDC) (Fig. 3).

The bones of the mandible are completely fused. The pars simphisialis reaches about one third of the total length of the mandible (from 9.1 mm to 27.75 mm). In the intermediate portion, the angulus mandibulae (AM) (Fig. 4) is ventrally inclined at an angle of 150°. The proc. coronoideus 1 (PrC1) (Fig. 4) and 2 (PrC2) (Fig. 4) are reduced, but the latter is more prominent. The tuberculum pseudotemporalis (TPT) (Fig. 4), the local insertion for the M. pseudotemporalis superficialis is reduced. Both fossa medialis (FMM) (Fig. 4) and fossa lateralis (FLM) (Fig. 4) are broad and deepened. The fossa lateralis mandibulae showed similar size in the three species of vireos studied, but is deeper in C. gujanensis.

Figure 4
Lateral view of the mandible of C. gujanensis - AM: angulus mandibulae; FLM: fossa lateralis; FMM: fossa medialis; PrC1: proc. coronoideus 1; PrC2: proc. coronoideus 2; PrLM: proc. lateralis; PrRA: proc. retroarticularis; TPT: tuberculum pseudotemporalis

The tuberculum intercotylaris (TiC) (Fig. 5) is prominent. The fossa articularis quadratica (FAQ) (Fig. 5) is broad, being deeper in C. gujanensis than in the other vireos studied. The cotyla medialis (Com) (Fig. 5) is deeper than the cotyla lateralis (Col) (Fig. 5) due to its articulation with the large condylus medialis of the os quadratum.

Figure 5
Dorsal view of the mandibule of C. gujanensis - Col: cotyla lateralis; Com: cotyla medialis; FAQ: fossa articularis quadrática; PrMM: proc. medialis mandibulae; PrLM: proc. lateralis mandibulae; PrRA: proc. retroarticularis; TiC: tuberculum intercotylaris

The proc. medialis mandibulae (PrMM) (Fig. 5) is longer and thicker in the basal portion compared with V. chivi and V. olivaceus. The proc. lateralis (PrLM) (Figs. 4 and 5) is thick and prominent at the base but it apex is thin. The proc. retroarticularis (PrRA) (Figs. 4 and 5) is short and narrow.

DISCUSSION

Cyclarhis gujanensis has strong curvatures of the beak and arcus jugalis, combined with the stout head and heavy and hooked beak. These strong curvatures can also be found in parrots (120 to 130°) (Thompson, 1899), a well-known strong-billed group of birds. In addition, when the ramphotheca is preserved, we observed a ventral curvature in its rostral end (pincer form) that could provide more power to peck and to cut preys in small pieces.

The os lacrimale is completely fused with the os frontale and ectethmoidale. This bony is absent or vestigial in many Passeriformes (Cracraft, 1968), including Vireonidae, due to the large os ectethmoidale, and it is generally supported by the arcus jugalis. The ectethmoidale is large and thick (pneumatic) in frugivorous species of Sturnidae due to the broad development of the adductor muscles, especially the M. pseudotemporalis superficialis (Beecher, 1978). However, our results showed that in C. gujanensis and Vireo the large os ectethmoidale does not appear to be modified by the reduced M. pseudotemporalis superficialis. The same results were observed by Orenstein & Barlow (1981).

Orenstein & Barlow (1981) argue that the fossa temporalis was restricted to lateral portion of the skull as a result of the dorsal position of the crista subtemporalis in C. gujanensis, thereby preventing the fossa temporalis could extend to the caudal region of the skull. The same result was observed in this study. However, this fossa is more reduced and deeper in V. chivi and V. olivaceus than in C. gujanensis. In Dendrocolaptidae and Furnariidae (Donatelli, 1997 and Donatelli & Marceliano, 2007) this fossa is broad, longer than wide, except close to rostral region of the proc. squamosalis for Furnariidae. Richards & Bock (1973) found a reduced fossa temporalis in Loxops, located between the proc. postorbitalis and the proc. squamosalis.

The proc. squamosalis provides the broadest area of origin for large external adductor muscles in birds (Baumel et al., 1993). Similar to C. gujanensis, it is also long in Dendrocolaptidae (Donatelli, 1997) and Furnariidae (Donatelli & Marceliano, 2007), but shorter and narrower in V. chivi and V. olivaceus. Richards & Bock (1973) stated that the size of proc. postorbitalis is related to the development of M. adductor mandibulae externi rostralis temporalis and lateralis. Indeed, these muscles are larger in C. gujanensis than in V. chivi e V. olivaceus and also in other vireos (Orenstein & Barlow, 1981).

The fossa ventralis of the palatine bones origins the ventral os pterygoideum muscles system which connects the palate to the laterocaudal, medial and caudomedial portions of the mandible (Bock, 1964; Bulher, 1981). This fossa is deepened in C. gujanensis probably due to selective pressures to tear larger preys played by the complex and bulky os pterygoideum muscles system (Orenstein & Barlow, 1981).

The os pterygoideum plays an important role in the jaw apparatus to transmit movement from the os quadratum to the os palatine and the other way around (Gennip, 1986). This bone is robust in C. gujanensis, similar to the other passerines (Donatelli, 1997, and Donatelli & Marceliano, 2007) and the other Vireonidae studied. Although the os pterygoideum primarily serves as the transmission mechanism of muscle movements, we observed few and tenuous muscles inserted on the proc. dorsalis. However, this process is useful to prevent bone disarticulation and to ensure the articulation between os palatinum and pterygoideum (Zweers, 1974).

The os quadratum plays important role in the jaw apparatus on birds. It connects some of its most important elements, namely the skull, mandible, os pterygoideum and arcus jugalis. In addition it is a bone of origin and insertion for several muscles (Gennip, 1986). In fact, the os quadratum plays a fundamental role in the skull kinesis, causing the force to be transmitted by the os pterygoideum and os palatinum to the upper jaw (Zusi, 1984). Cyclaris gujanensis presented the os quadratum orbital process with a similar length to other vireos studied, but is wider and thicker, especially in its basal portion.

The both large lateral and medial condylus and deepened cotylas of the upper jaw are providing protection against mandible lateral disarticulations (Bock, 1960). According to Bock (1960) these features provide better support to prevent disarticulation of the mandible when the beak is opened. Therefore, it is an advantage for C. gujanensis since it needs a stronger more fixed mandible and so that the most powerful biting and crushing become more stabilized.

The deep fossa lateralis mandibulae and articularis quadratic showed similar size in the three species of vireos studied, but is deeper in C. gujanensis. According to Zusi (1984) these depressions are critical to the jaw apparatus articulation because they houses large adductor external muscles that elevate the mandible.

In C. gujanensis the proc. medialis mandibulae is longer and thicker in the basal portion compared with V. chivi and V. olivaceus. Similar results were observed in Furnariidae (Donatelli & Marceliano, 2007). In Icteridae, Bock (1960) found the process long and narrow, and he states it is a site for insertion to the muscles of the depressor system and of the os pterygoideum. Besides, Bock (1960) also argues that when this process is very long in birds, he could be in contact with the basitemporal plate and it serves as a support to avoid possible disarticulation of the mandible. Despite the large development of this process, it is difficult to argue that in C. gujanensis it provides more stability to peck without morphofunctional approaches.

CONCLUSION

The broad and deep depressions of the palatine and mandible, where the muscles from the adductor and pterygoideus systems are lodged, combined with long processes (proc. orbitalis os quadratum, proc. medialis ossa mandibulae and proc. squamosalis) to support an origin or insertion to these muscles.

The position of the arcus jugalis with the upper jaw resulting in a very oblique angle provides strength to the prehensile in order to capture and to dilacerate large preys. On the other hand, the fossa temporalis, the region where the most developed external adductor muscles are found, is restricted to the lateral skull. In fact, it is dorsoventrally wide and deep only in its rostroventral region.

The strong, fused, hooked and curved bill, associated with long and large processes and fossae, where the adductor muscles are housed, probably allow C. gujanensis to tear large preys into small pieces. These anatomical studies could partially explain its peculiar feeding behavior in comparison with other Vireonidae. However, further studies on functional approaches and analyzes of muscle fibers forces are necessary to corroborate/refute the hypotheses mentioned above.

ACKNOWLEDGMENTS

We are grateful to Luis F. Silveira (Museu de Zoologia da Universidade de São Paulo) and to Herculano Alvarenga (Museu de História Natural de Taubaté) to permit the osteological descriptions in loco. We also thank to André V.C. Levorato by the high-quality illustrations and Maria F.M. Ortiz by the English version of the manuscript.

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Publication Dates

• Publication in this collection
  2015

History

• Accepted
  08 Apr 2015