Involvement of the avian song system in reproductive behaviour

Abstract

The song system of songbirds consists of an interconnected set of forebrain nuclei that has traditionally been regarded as dedicated to the learning and production of song. Here, however, we suggest that the song system could also influence muscles used in reproductive behaviour, such as the cloacal sphincter muscle. We show that the same medullary nucleus, retroambigualis (RAm), that projects upon spinal motoneurons innervating expiratory muscles (which provide the pressure head for vocalization) and upon vocal motoneurons for respiratory-vocal coordination also projects upon cloacal motoneurons. Furthermore, RAm neurons projecting to sacral spinal levels were shown to receive direct projections from nucleus robustus arcopallialis (RA) of the forebrain song system. Thus, by indicating a possible disynaptic relationship between RA and motoneurons innervating the reproductive organ, in both males and females, these results potentially extend the role of the song system to include consummatory as well as appetitive aspects of reproductive behaviour.

Keywords: canary; cloacal motoneurons; copulation solicitation display; nucleus retroambigualis.

Figure 1.

(a) Canary CSD showing the proposed pathway HVC–RA–RAm–cloacal motoneurons. (b) The full sequence of pathways for appetitive (singing) and consummatory (cloacal focus) reproductive behaviour. DM, dorsomedial nucleus of the intercollicular complex.

Figure 2.

(a) a longitudinal section of sacral spinal cord showing cloacal (mSC) motoneurons (green) retrogradely labelled by an injection of CTB 488 into the muscle. (b) RAm axons (red) in relation to mSC motoneurons (blue-green) in the ventral horn (vh) of the sacral spinal cord (female; transverse section). (c) Confocal projection of some contents of (b). (d) RAm neurons retrogradely labelled by an injection of CTB into the ventral horn of an upper sacral spinal cord segment (male). (e) RAm neurons retrogradely labelled from a similar injection of CTB (brown) embedded within the terminal field of BDA-labelled RA axons (black; female). An RA terminal field in RAm in a male canary, at approximately the same rostrocaudal level as the section shown in (d), is shown in fig. 3a of Wild et al. [12]. (f) Neurons in female RA (f) and DM (g) retrogradely labelled from an injection of CTB 555 centred on RAm (MLd in (g) = avian auditory midbrain). Scale bars, 100 µm. At right of each group of photographs are shown coronal hemi-sections housing the nuclei shown in figure 1. The sites of the injections are indicated by tracer-filled pipettes, and retrograde and anterograde directions of intra-axonal travel are indicated by arrows.