Olfactory discrimination of estrous condition by the male golden hamster (Mesocricetus auratus)

After repeated exposure to receptive and nonreceptive females, male golden hamsters were tested for olfactory preferences in a four-choice olfactometer. Males discriminated between the odor of anesthetized females in different stages of their estrous cycle when the airstreams carrying the stimulus odors were diluted. Previous failures to demonstrate such an ability were probably due to ceiling effects resulting from laboratory testing. Males preferred the odor of females on the day before receptivity (a day coincident with maximal scent marking by the female) and least preferred the odor of females on diestrus-1 (the day on which females attacked and chased males during pretesting encounters). Detection and quick response to an impending estrus would be especially important for males of a solitary and promiscuous species in which there is a first male mating advantage.     

Mating-induced inhibition of receptivity in the female golden hamster. I. Short-term and long-term effects

Female golden hamsters (Mesocricetus auratus) which have actively terminated mating with one male will respond in succession to two or more novel males. However, mating with the first male results in a reduction in receptivity shown toward a second male, with the size of the decrease being proportional to the amount of copulatory stimuli provided by the first male. Furthermore, the decreased receptivity shown toward the second male results in his achieving fewer ejaculations. These effects are more pronounced when the second mating occurs 5 min or 8 h after the first mating than when it occurs 1 or 4 h later, suggesting both short-term and long-term inhibitory effects. Copulation with a single male also effects a 40% shortening of behavioral receptivity. In a promiscuous species such as the hamster, prolonged copulation by the male may enhance the short-term inhibition of female receptivity, thereby reducing the risk of subsequent sperm displacement. The long-term effect occurs too late to affect sperm competition and may, instead, function to reduce the risk of predation as a result of unnecessary further mating.     

Contrasting visuomotor functions of tectum and cortex in the golden hamster

Summary Visually-guided orientation (spatial localization) and visual discrimination were dissociated by means of brain lesions in the golden hamster. After ablation of visual cortex, hamsters failed to discriminate visual patterns, but showed nearly normal ability to localize an object in space by means of vision. Ablation of the superior colliculus produced opposite effects: these animals were completely unable to orient to the position of a visual stimulus, but nevertheless showed excellent pattern discrimination.

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Zusammenfassung Visuell gesteuerte Orientierung (räumliche Lokalisation) und visuelles Unterscheidungsvermögen beim Goldhamster werden durch Hirnläsionen funktionell getrennt. Nach Entfernung der Sehrinde können die Tiere keine visuellen Musterunterscheidungen, wohl aber noch in fast normaler Weise visuelle Objektlokalisationen vornehmen. Entfernung des Colliculus superior führt zu entgegengesetzten Effekten: die Tiere können sich nicht mehr nach der Position eines visuellen Reizes orientieren, zeigen aber trotzdem ausgezeichnete Musterunterscheidung.

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During the period of the reported research, the author was supported by a U.S. Public Health Service predoctoral fellowship. Additional support was received from the National Institute of General Medical Sciences under their Training Grant STl GM 1064-04 BHS.For a more detailed report of this work, and reports of further experiments, see Schneider (1966).     

Dexamethasone can prevent stress-related litter deficits in the golden hamster

This study was designed to examine the effects of social subordination during early pregnancy in the golden hamster (Mesocricetus auratus). Primiparous females were mated to proven breeders and stressed during early pregnancy. Females were housed singly throughout gestation except for Days 4, 5, and 6 when they were paired for 10-min intervals three times each day with another female matched for age, weight, and day of pregnancy. Within each of the pairs, one female was consistently dominant to the other. Controls were exposed to a novel area instead of a conspecific. At parturition, all pups were counted, sexed, and weighed. There were no significant differences between control and dominant females' litter sizes and sex ratios. Subordinate females produced significantly smaller litters than control or dominant dams and significantly lower sex ratios than control dams. Subordinates produced fewer males than control or dominant dams, but there were no differences in the number of females produced. The entire experiment was repeated exactly except that females received a small dosage of dexamethasone in their drinking water on Days 3-7 of pregnancy. In this second group, there were no significant reductions in mean litter size, sex ratio, or pup weights among litters born to subordinate dams. These results suggest that subordinate dams produce smaller litters via selective resorption of males in utero and that the typical adrenal response to stress mediates this response.     

Line-bisectioning and obstacle avoidance: Evidence for separate strategies

Previous studies have frequently applied a combination of line-bisection tasks (in which participants indicate the middle of a line) and obstacle avoidance tasks (in which participants move their hand between two obstacles) with the aim of revealing perception–action dissociations in certain neurological disorders, such as visual form agnosia and optic ataxia. However, valid conclusions about the underlying processing pathways can only be drawn if participants apply the same strategy in both tasks (i.e. finding the middle between the obstacles). Yet, this assumption has never been tested directly. In this experiment, we investigated whether participants perform obstacle avoidance and line-bisectioning using similar strategies by manipulating the position of the obstacles and the start position of the hand relative to the obstacles. Our results indicate that the lateral hand position during obstacle avoidance does not only vary as a function of obstacle location but also strongly depends on the start position. Moreover, participants showed increased sensitivity to obstacle shifts occurring closer to the hand's start position. In contrast, during line-bisectioning the sensitivity to obstacles shifts was unaffected by the hand's start position. The findings suggest that during obstacle-avoidance the need to keep a safe distance from the obstacles is balanced with the requirement to minimise energetic demands. In contrast, the main intention during line-bisectioning is to move to the perceived midpoint as accurately as possible. The fact that very different constraints underlie trajectory planning in both tasks implies that caution has to be taken when interpreting differences in performance levels.     

Evidence for separate representations for action and location in implicit motor sequencing

We examined sequential learning of actions in an experiment in which four different actions (push, twist, pinch, switch) were placed at four horizontal locations. At transfer, participants responded to a sequence that required performing the same sequence of actions at different locations and to a different sequence of actions at the same sequence of locations. Participants with explicit knowledge demonstrated only learning the sequence of response locations. However, participants with implicit knowledge learned the sequence of actions just as well as the sequence of locations, and performance on individual sequences was just as good as performance when both sequences were presented. These results demonstrated that two types of sequences, one of actions and another of response locations, can be learned simultaneously, suggesting that parallel representations are involved in implicit motor skill acquisition.     

Lactose ingestion in the adult golden hamster (Mesocricetus auratus)

Adult mammals generally demonstrate a lower preference for the disaccharide sugar lactose than for any other common sugar, and because adults typically have low levels of the intestinal enzyme lactase, lactose ingestion may cause gastrointestinal distress. The lactose intake of adult golden hamsters was examined in three experiments; the main findings were: (a) hamsters allowed to choose between tap water and lactose solutions (maximum concentration = 32% weight/volume) over a 20-day period showed a clear preference for lactose solutions and ingested substantial quantities of lactose (up to 3 g/100 g body weight/day) without noticeable adverse effects; (b) hamsters consuming a single diet with lactose added (maximum concentration = 50%) over a period of days ingested up to 3.42 g/100 g body weight/day of lactose without noticeable adverse effects; (c) both hamsters with prior exposure to lactose solutions and those without such exposure consumed similar amounts of 32% lactose solution over an 8-day period, suggesting that hamsters' lactose intake does not depend on the occurrence of adaptation. It is suggested that the fermentative capacity of the hamster's pregastric pouch may underlie this animal's unusual tolerance for lactose.     

Determinants of mating success in the golden hamster (Mesocricetus auratus): I. Male capacity

Rested adult golden hamsters produced a mean of 13 ejaculations and then showed a modified copulatory pattern consisting of long intromissions (10-30 s of intravaginal thrusting) during which no sperm transfer occurs. When a second (fresh) female was introduced, 58% of the males produced at least one more ejaculation, and 17% of the males produced one further ejaculation when placed with a third female. In similar three-female tests conducted 24 hr later, all males ejaculated (M = 4 ejaculations); this level of ejaculatory output was maintained over a subsequent 10-day period of daily testing. Four ejaculations ensured a nearly 100% pregnancy rate and maximum litter size in the first females. Second and third females, however, received fewer ejaculations and subsequently showed reduced fertility and produced smaller litters. Most males showed high levels of long intromissions on all days and with all females. The physiological significance of these long intromissions may be to trigger the progestational response. Examination of the rate of recovery from sexual exhaustion indicated that 2 or 8 hr of rest resulted in fewer than 50% of the males being capable of one further ejaculation. However, most of these single ejaculations were accompanied by long intromissions and resulted in successful pregnancies. Four to eight days were required for full recovery of ejaculatory capacity. This study showed that the male golden hamster has a higher ejaculatory output and more rapid recovery from sexual exhaustion than other small rodents that have been studied. Thus, the limits of male capacity appear to vary widely among species.     

Dyadic social behaviour in the hamster: Evidence for contextual mediation

Dyadic social interactions among 34 adult male golden hamsters reared in either standard wire cages (Experiment 1) or cages with sandy substrates (Experiment 2) were observed for 15 min on consecutive days in two neutral arenas with either a bare Plexiglas or a sandy substrate. The tests showed that in the arena with the sandy substrate the hamsters spent less time engaged in agonistic encounters, initiated fewer agonistic encounters of all kinds and engaged in fewer agonistic encounters of a potentially physically damaging nature (i.e. violent encounters) than they did in the arena with the Plexiglas substrate. These effects were the same in animals reared under either condition. This finding argues against the behavioural effects being due to the novelty of a sand substrate to laboratory-reared hamsters. Consequently, the social behaviour of hamsters in laboratory settings is highly labile across various contexts. Therefore, it is necessary to examine the behavioural effects of contextual variables if general behavioural principles are to be derived from laboratory indices of social behaviour in hamsters, and possibly a number of other species.     

Sequence learning by action and observation: Evidence for separate mechanisms

In the Serial Reaction Time (SRT) task, participants respond to a set of stimuli the order of which is apparently random, but which consists of repeating sub‐sequences. Participants can become sensitive to this regularity, as measured by an indirect test of reaction time, but can remain apparently unaware of the sequence, as measured by direct tests of prediction or recognition. Some researchers have claimed that this learning may take place by observation alone. We suggest that observational learning may be due to explicit acquired knowledge of the sequence, and is not mediated by the same processes which give rise to learning by action. In Expt 1, we show that it is very difficult to acquire explicit sequence knowledge under dual task conditions, even when participants are told that a regular sequence exists. In Expt 2, we use the same conditions to compare actors, who respond to the sequence during learning, and observers, who merely watch the stimuli. Furthermore, we manipulate the salience of the sequence, in order to encourage learning. There is no evidence of observational learning in these conditions, despite the usual effects of learning being demonstrated by actors. In Expt 3, we show that observational learning does occur, but only when observers have no secondary task and even then only reliably for a sequence which has been made salient by chunking subcomponents. We conclude that sequence learning by observation is mediated by explicit processes, and is eliminated under conditions which support learning by action, but make it difficult to acquire explicit knowledge.     

Effects of toxicosis on the predatory behavior of the golden hamster (Mesocricetus auratus)

The purpose of this study was to measure the effects that toxicosis had on the hamster's predatory behavior and to compare these results with those found in previous studies with grasshopper mice. A 0.15 M LiCl injection caused hamsters to develop a greater aversion and to inhibit their feeding and attack responses more frequently toward a house cricket than did a similar injection of NaCl. The added presence of an almond coating on the cricket prolonged the number of days that a hamster exhibited an aversion toward the prey and an inhibition to attack. Essentially, the inhibitory effects from toxicosis in the hamster paralleled those found with the grasshopper mouse. The differences in these inhibitory effects are attributed to inherent differences in the attack responses of the two rodents.     

Dynamic illusion effects in a reaching task: evidence for separate visual representations in the planning and control of reaching.

The effects of an orientation illusion on perception and 2 different actions were investigated. An 8-cm x 2-cm cylindrical bar was placed in front of participants at various orientations. A background grating was used to induce an orientation illusion. In a perception task, the illusion affected participants' ability to align the bar with their sagittal planes. In one reaching task, a similar effect of the illusion was found on the choice between 2 possible grasping postures. In a second reaching task involving a single grasping posture, the orientation illusion affected the orientation of the hand at the beginning of the reach but not near its end. The authors argue that reaching trajectories are planned and initiated through a context-dependent representation but are corrected on-line through a context-independent representation. The relation of this model to a more general dichotomy between perception and action is discussed.     

Saccade size control in reading: Evidence for the linguistic control hypothesis

In Experiment 1, it is shown that during reading the eye makes larger saccades near long words than near short words. The effects are reduced when the subject’s peripheral vision is diminished by the use of a moving “window” centered on the subject’s fixation point, outside of which letters are replaced by Xs. In Experiment 2, it is shown that even if linguistic predictions are kept constant, the eye tends to make longer jumps when approaching THE than when approaching a three-letter verb. This “THE-skipping” effect is weaker if THE is compared with an auxiliary (HAD, WAS, or ARE) than if it is compared with a less frequently occurring verb (ATE, RAN, MET). It follows that knowledge of the lexicon can combine with information from peripheral vision fast enough to influence saccade size from moment to moment.     

Food hoarding behaviour in the golden hamster (Mesocricetus auratus): Effects of body weight loss and hoard-size discrimination

Two experiments investigated some of the factors that influence food hoarding behaviour in the golden hamster (Mesocricetus auratus). In Experiment 1, hamsters given limited access to food failed to increase the amount of food they consumed at each meal. Instead, they exhibited a significant increase in the amount of food taken to hoard during the limited access period, and this permitted hamsters who were allowed to retain their hoard to maintain body weight at levels comparable to free-feeding controls. Hamsters that were not allowed to retain hoarded food rapidly lost body weight. There was also an inverse correlation between body weight and amount of food taken to hoard in all hamsters given limited access to food, suggesting that body weight loss was one factor directly related to increased hoarding during periods of limited food accessibility. Experiment 2 investigated the effect of hoard-size discrimination on food hoarding in hamsters by artificially manipulating hoard size. The results suggested that hoarding behaviour could be influenced by artificially depleting and repleting the hoard, and this factor appears to influence hoarding in the absence of any significant changes in body weight. These results are discussed in relation to the kinds of adaptive feeding strategies that hamsters may utilize in their natural environment.