Crocodilians and birds are the modern representatives of Phylum Archosauria. Although there have been recent advances in our understanding of the phylogeny and ecology of ancient archosaurs like dinosaurs, it still remains a challenge to obtain reliable information about their behaviour. The comparative study of birds and crocodiles represents one approach to this interesting problem. One of their shared behavioural features is the use of acoustic communication, especially in the context of parental care. Although considerable data are available for birds, information concerning crocodilians is limited. The aim of this review is to summarize current knowledge about acoustic communication in crocodilians, from sound production to hearing processes, and to stimulate research in this field. Juvenile crocodilians utter a variety of communication sounds that can be classified into various functional categories: (1) "hatching calls", solicit the parents at hatching and fine-tune hatching synchrony among siblings; (2) "contact calls", thought to maintain cohesion among juveniles; (3) "distress calls", induce parental protection; and (4) "threat and disturbance calls", which perhaps function in defence. Adult calls can likewise be classified as follows: (1) "bellows", emitted by both sexes and believed to function during courtship and territorial defence; (2) "maternal growls", might maintain cohesion among offspring; and (3) "hisses", may function in defence. However, further experiments are needed to identify the role of each call more accurately as well as systematic studies concerning the acoustic structure of vocalizations. The mechanism of sound production and its control are also poorly understood. No specialized vocal apparatus has been described in detail and the motor neural circuitry remains to be elucidated. The hearing capabilities of crocodilians appear to be adapted to sound detection in both air and water. The ear functional anatomy and the auditory sensitivity of these reptiles are similar in many respects to those of birds. The crocodilian nervous system likewise shares many features with that of birds, especially regarding the neuroanatomy of the auditory pathways. However, the functional anatomy of the telencephalic auditory areas is less well understood in crocodilians compared to birds.

Nucleus rotundus receives a major input from the optic tectum in crocodiles, Caiman crocodilus. Telencephalic projections of nucleus rotundus were studied in Caiman by means by the Fink-Heimer procedure after anodal, stereotaxic lesions. Efferent axons of nucleus rotundus assemble on the ventromedial aspect of this nucleus and swing ventrolaterally to enter the dorsal peduncle of the lateral forebrain bundle. These ascending fibers continue rostrally in the dorsal peduncle of the lateral forebrain bundle to enter the telencephalon where they remain restricted to a lateral portion of the lateral forebrain bundle. At more anterior levels, these fascicles turn dorsally, pass through the ventrolateral area, and terminate massively in a lateral part of the rostral dorsolateral area. The results of this experiment are compared with similar studies on thalamotelencephalic connections of diencephalic visual areas in other amniotes. Parallels in fiber connections of thalamic auditory and visual areas and the segregation of these modalities in the telencephalon of Caiman are discussed. These similarities in neural circuitry and synaptic elements of auditory and visual systems that synapse in the midbrain of Caiman form the basis for a different interpretation of sensory system organization in amniotes.

The torus semicircularis is a tonotopically organized auditory region in the midbrain of the crocodile, Caiman crocodilus (Manley, '71 ) . 'I"wo distinct regions of the torus can be distinguished in Caiman: an external nucleus, which is continuous with the deep layers of the optic tectum, and a central nucleus. Ascending connections of the central nucleus were studied with the light microscope by means of the Fink-Heimer procedure after unilateral, anodal stereotaxic lesions. Efferent axons leave the central nucleus in the lateral lemniscus to enter the tecto-reuniens tract where they course as far rostrally as nucleus Z. At this level, this bundle bifurcates. The majority of these fibers turn medially and then pass through and probably synapse on interposed neurons of nucleus Z, prior to their termination in nucleus reuniens. The remaining axons continue ventrally past nucleus Z to enter the ventral supraoptic decussation where they travel anteriorly to a level just posterior to the optic chiasm. Here the fascicles cross the midline and swing caudally, still in the ventral supraoptic decussation, until they reach nucleus Z of the contralateral side. At this level, these fibers enter the tecto-reuniens tract and turn medially, to pass through and perhaps end on the intercalated neurons of nucleus Z, prior to their termination in nucleus reuniens. The neuronal architecture of nucleus Z and that of the nucleus reuniens complex are described. The latter, a midline nuclear group, consists of two subdivisions: a pars centralis and a pars diffusa. Each of these subdivisions is segregated into two neuronal aggregates at the midline. Efferents of the central nucleus terminate massively in the pars centralis and, to a considerably lesser extent, in the pars diffusa of nucleus reuniens.The results of this study are compared with similar ones in pigeons and mammals. Parallels in the fiber connections of midbrain auditory and visual areas and the segregation of these modalities in the mesencephalon and diencephalon are discussed.It has been suggested that if a receptor has a special function, it should have a separate central representation (Lorente de N6, ' 3 3 ) . Audition, somatosensation, and vision each possess receptors that are special in that each is differentially sensitive to a particular form of energy. In most vertebrates, these modalities remain separate in the periphery. In many mammals, each of these modalities remains separate in its central connections. This feature of neural organization is not unique to mammals since auditory and visual systems of pigeons remain segregated not only in the periphery but also in the midbrain, thalamus, and telencephalon (Boord, '68; J. COMP. NEUR., 153: 179-198.

Certain aspects of thalamic organization in reptiles and mammals are reviewed. Features shared by the dorsal thalamus of reptiles and that of mammals include projection to the telencephalon, specific and non-specific non-telencephalic afferents, and input from the thalamic reticular nucleus. Differences between the dorsal thalamus of reptiles and that of mammals are the absence of reciprocal telencephalic efferents to the dorsal thalamus and lack of local circuit neurons in reptiles (with the exception of the dorsal geniculate complex in turtles) and their presence in mammals. A thalamic reticular nucleus is present in both reptiles and mammals. In both of these classes of vertebrates, this neuronal aggregate surrounds the dorsal thalamus along its lateral surface, projects to the dorsal thalamus, and is organized into sectors. In one group of reptiles, Caiman crocodilus, the sole reptilian group in which immunocytochemical features have been investigated in detail, the reticular nucleus contains at least three neuronal subpopulations: neurons immunoreactive for glutamic acid decarboxylase (GAD); neurons immunoreactive for parvalbumin; and cells that are not immunoreactive for parvalbumin or, probably, GAD. On the other hand, the reticular nucleus of mammals contains a single population of neurons immunoreactive for GAD, gamma amino butyric acid, and parvalbumin.