Hamster over-marking

Johnston, R.E., Chiang, G. and Tung, C. (1994) The information in scent over-marks of golden hamsters. Anim. Behav. 48, 323-330. 

Cohen, A.B., Johnston, R. and Kwon, A. (2001) How golden hamsters (Mesocricetus auratus) discriminate top from bottom flank scents in over marks. J. Comp. Psychol. 115, 241-247. 

Scent over-marking is a specialized type of marking behavior in which one individual places its scent so that it covers or partially covers previously deposited scents of conspeci-fics. In this chapter I first review our research that shows that golden hamsters have amazing abilities to analyze such over-marks to determine which individual s scent is on top. Next I present experiments in which we examine how they do this. Finally, I briefly discuss the functions of over-marking and of the abilities to determine the top-scent individual. 

I first review experiments showing that hamsters and voles have the previously unsuspected ability to determine which of two individual s scent marks is on top in an overmark and then present experiments that investigate how they do this. At the end I speculate on the functions of these abilities and of over-marking. 

The first set of experiments with hamsters suggested that scent masking had occurred. After hamsters investigated (on 5 separate trials) a scent over-mark in which the top scent of one individual covered the bottom scent of another individual, they investigated the top scent of the mark less than a novel scent, indicating memory for the top scent, but they investigated the bottom scent the same amount as a novel scent, suggesting that they did not remember the bottom scent (Johnston et al., 1994). That is, these results supported the hypothesis that the top scent masks information in the bottom scent. 

Because we observed that hamsters rarely completely covered the scent of another, we then examined what perceivers remembered after investigation of partially overlapping scent marks of two individuals (which we presented in a crossed pattern). To our surprise, we found the same effect as in the first set of experiments that is, subjects showed evidence of remembering just the top scent of the over-mark, even though they did investigate the uncovered parts of the bottom individual s odor (Johnston et al., 1995). These... 

SENSORY EVALUATION OF SCENT OVER-MARKS HOW DO HAMSTERS TELL WHICH SCENT IS ON TOP ... 

One of the cues used by the visual system for perception of depth or relative distance of two objects from the observer is occlusion. That is, if one object occludes a second object, it is perceived as being in front of that second object. Many vaginal marks of hamsters are deposited as linear streaks or lines, and we hypothesized that hamsters might be able to use olfactory occlusion to determine which scent was on top. That is, in an over-mark the top scent is a continuous streak if it masks the bottom mark in the region of overlap, the underlying scent mark will appear to be discontinuous (interrupted). If hamsters can keep track of the spatial layout of the two scents they could potentially use this continuous versus interrupted information as a means to determine which individual s scent is on top, and thus preferentially remember the top scent. 

Individuals should also be sensitive to the marks and over-marks of like-sex conspe-cifics. If a male hamster or vole, for example, consistently notices his scent marks being over-marked by another male, he should recognize this individual as more of a threat than another neighbor that rarely over-marks in his area. Reactions to different like-sex rivals may depend on the knowledge that an individual gets about them through their scent marking behavior (see Hurst Rich, this volume)... 

Consequently, we predicted that female hamsters with LOT transections would (a) show greatly reduced investigation of social odors, (b) demonstrate impaired discrimination of individual odors, (c) fail to demonstrate normal preferences for odors of males over those of females and (d) show impairments in scent marking toward male and female odors similar to those reported in male hamsters following similar transections (Macrides etal., 1976). 

LOT cuts dramatically decreased overall olfactory investigation as shown by reductions in sniffing behavior in tasks for both individual discrimination and sex preferences. Moreover, female hamsters with LOT cuts did not discriminate between individual male odors, did not prefer male odors over those of females, and rarely flank marked in response to odors. However, vaginal marking in response to odors was only partially reduced by LOT transections. 

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