Chemosignals related to reproductive and social status (pheromones) carry messages between opposite-sex and same-sex individuals in many species. Each individual must distinguish signals relevant to its own social behavior with conspecifics from signals used by other (heterospecific) species relevant to their social behavior. In male hamsters, the medial amygdala responded in a categorically different way to conspecific stimuli (socially relevant) and heterospecific stimuli (not socially relevant but serving similar purposes for other species), and may play an important role in this decision. Immediate-early gene responses to conspecific chemosignals and heterospecific chemosignals were characteristically different. The categorical responses, generated by chemosensory input from the vomeronasal organ and (probably) GABA inhibition within the amygdala, were not apparent at more peripheral sensory levels. This is the first evidence for an important role of the amygdala, a limbic structure known to be involved in social and emotional behavior, in discrimination of species specificity in chemosignals.

The causes and timing of piglet mortality were studied in different farrowing systems. In the first experiment 198 litters were recorded in three systems, two of which allowed the sows to move freely, and the third restricted them in conventional crates. More piglets were weaned from the conventional crates than from the open systems and they grew more quickly. More than half the liveborn mortality occurred during the first four days after parturition. In the open systems, 17 per cent and 14 per cent of the piglets born alive were crushed, compared with only 8 per cent in the crates. In the second experiment, 29 sows and litters were studied in detail in a communal pen system during the first seven days of lactation. Three-quarters of the liveborn mortality was due to crushing. The total number of piglets dying per litter, including stillbirths, was significantly associated with the total litter size and the sow's parity. The percentage liveborn mortality was significantly associated with the parity and body length of the sows and with the within-litter variation in the birth weight of the piglets. Individual birth weight was closely associated with percentage survival. Only 28 per cent of piglets weighing less than 1.1 kg at birth survived to seven days.

The human vomeronasal organ (VNO) has been the subject of some interest in the scientific literature and of considerable speculation in the popular science literature. A function for the human VNO has been both dismissed with ridicule and averred with conviction. This question of VNO function has been needlessly tied to the separate question of whether there is any place for pheromone communication among humans, a topic that is itself bogged down in conflicting definitions. This review is an attempt to weigh the evidence for and against human VNO function, to deconvolve that question from the question of pheromone communication and finally to provide a working definition of 'pheromone'. Further experimental work is required to resolve the conflicting evidence for and against human VNO function but chemical communication does appear to occur among humans. However, several examples reported in the literature do not meet the proposed definition for communication by pheromones: 'chemical substances released by one member of a species as communication with another member, to their mutual benefit'.

SUMMARYPrevious workers had identified the vomeronasal organ, on anatomical evidence, as an accessory olfactory organ, present in most terrestrial vertebrates. Lesion experiments had demonstrated its importance in sexual behaviour in the hamster. However, the sequestered position of the vomeronasal receptor epithelium within the organ raised questions concerning the access of olfactory stimuli. Using electrophysiological and pharmacological methods we have now demonstrated the following.

Many species employ chemical signals to convey messages between members of the same species (conspecific), but chemosignals may also provide information to another species (heterospecific). Here, we found that conspecific chemosignals (male, female mouse urine) increased immediate early gene-protein (IEG) expression in both anterior and posterior medial amygdala of male mice, whereas most heterospecific chemosignals (e.g.: hamster vaginal fluid, steer urine) increased expression only in anterior medial amygdala. This categorization of responses in medial amygdala conforms to our previously reported findings in male hamsters. The same characteristic pattern of IEG expression appears in the medial amygdala of each species in response to conspecific stimuli for that species. These results suggest that the amygdala categorizes stimuli according to the biological relevance for the tested species. Thus, a heterospecific predator (cat collar) stimulus, which elicited behavioral avoidance in mice, increased IEG expression in mouse posterior medial amygdala (like conspecific stimuli). Further analysis suggests reproduction related and potentially threatening stimuli produce increased IEG expression in different sub-regions of posterior medial amygdala (dorsal and ventral, respectively). These patterns of IEG expression in medial amygdala may provide glimpses of a tertiary sorting of chemosensory signals beyond the primary-level selectivity of chemosensory neurons and the secondary sorting in main and accessory olfactory bulbs.

Responses of single neurons in the olfactory bulb of anesthetized hamsters were recorded extracellularly while odors of defined concentration and time course were delivered to the olfactory system at constant flow. Responses could be either excitatory or suppressive, as judged by the first distinguishable change in firing rate during odor delivery. However, when the time course of the response was examined in more detail, approximately one-third of all tests and one-half of the tests at high concentration resulted in complex temporal patterns of firing rate that involved both increases and decreases with respect to spontaneous activity. Approximately two-thirds of all tests produced responses where increased firing rate preceded any distinguishable suppression. Excitatory and suppressive responses were each classified into four groups according to their temporal patterns. Different patterns were not equally represented in the data and the proportions of patterns elicited by the same odor changed with stimulus intensity. Complex responses, where temporal patterns included periods of firing rate above and below spontaneous rate, were increasingly common and intensity was increased. Magnitude of response is difficult to define when a single response includes both increases and decreases of firing rate but more than half of the neurons that responded to more than one stimulus concentration clearly had nonmonotonic intensity-response functions. Forty-one out of 101 neurons were classified as output cells because they could be driven at short constant latency by lateral olfactory tract stimulation. Their responses were not clearly different from the remaining cells that could not be classified as output cells. The contribution of the inhibitory circuits of the olfactory bulb to the generation of patterned response and to changes in pattern with intensity are discussed. The lateral inhibitory circuits of the bulb appear to be sufficient to explain the data presented here.

The vomeronasal system projects to the accessory olfactory bulb (AOB), to the medial (Me) and posterior medial cortical nuclei (PMCN) of the amygdala, to the bed nucleus of the stria terminalis (BNST), and to other central structures shown to be important in mating behavior, including the medial preoptic area (MPOA). In these experiments c-fos expression was used as a marker of neural activity to identify the contribution of vomeronasal sensory input during mating behavior in male golden hamsters, either intact or with vomeronasal organs removed (VNX). Inexperienced hamsters were either stimulated with a receptive female and allowed to mate, exposed to female hamster vaginal fluid (HVF), which contains stimuli known to act through the VN system, or placed in a clean cage alone. Densely stained Fos-positive nuclei were evident in mated animals in the central VN pathway [AOB, Me, posterior medial BNST (pmBNST)] and a VN target area (MPOA). HVF-exposed animals showed Fos expression in the AOB, Me, and BNST but not MPOA. Unstimulated animals showed almost no activation. Most VNX animals exposed to females did not mate, but performed intense chemoinvestigation. They had few Fos-positive nuclei in any of these areas except the caudal pmBNST. A few VNX animals that did mate had patterns of Fos activation that were similar but less intense than those of intact mating animals, suggesting a selective activation of VN central pathways during mating regardless of VN sensory input. The main olfactory system showed low levels of Fos expression in all animals (stimulated and unstimulated). Fos expression in the MPOA and rostral pmBNST was seen only in mated animals, suggesting that these regions are concerned with mating performance or its consequences, rather than the chemosensory input that triggers it. Fos expression in the caudal encapsulated pmBNST was evident in all groups of animals that performed chemosensory investigation, regardless of VN status or mating, suggesting that this region either directs or responds to chemosensory investigation.

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