Animal Welfare Information Center Newsletter, Winter 1997/1998, Vol. 8, no. 3-4 *************************

Enrichment in Group-Housed Laboratory Golden Hamsters

by
Cheryl Arnold, Ph.D. and
Robert D. Westbrook, Ph.D.
Hendrix College, Conway, Arkansas

Summary

We provided group-housed golden hamsters with enrichment items (either a mason jar or pvc pipe) for 2 weeks at a time in an attempt to reduce aggression. Compared to controls, who had nothing extra in their cages, enriched hamsters showed more varied behavior and less aggression toward their cagemates. The hamsters preferred jars to pipes. Based on our observations, we recommend that when enrichment items are given to group-housed hamsters, multiple items be included in each cage and groups be monitored, as their aggression levels change with age.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Introduction

Laboratory golden hamsters (Mesocricetus auratus) are often group-housed. While this may seem odd for what is considered a solitary species (1), many golden hamsters do quite well under these conditions. We've even found that in spite of the common aggression and wounding, both male and female golden hamsters prefer social contact to solitary conditions (2,3). Given this state of affairs, we believe that laboratories which group-house their golden hamsters should find ways to reduce the aggression among cagemates. We tested two readily available, durable, and inexpensive items to see if they reduced aggression.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Subjects and Materials

We received six pregnant females (Sprague-Dawley, Indianapolis, Indiana) which all gave birth 2 days after arrival. We housed mothers with their litters in polypropylene cages (18.5 inches x 10 inches x 8 inches high) with cedar shavings, brown paper toweling (for nests), and free food and water.

[Photo:  PVC pipe and glass jars were evaluated as enrichment items] At day 21 postpartum, we weaned pups into 12 permanent gro ups of four (same-sexed) littermates and housed them in the same type of cages with cedar shavings and free food and water. Lights were on a 12 hours on 12 hours off cycle, and cages were changed twice a week. Each litter contributed one cage of females and one cage of males to the study, and no animals died.

For enrichment, we used mason jars and pvc pipes (fig. 1). We purchased the pint-sized, clear glass jars at a grocery store and the 2-inch diameter, white plastic pipes ("T" joints) at a hardware store.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Procedure

We rotated each cage through a 2-week treatment of the jar (J), pipe (P), and control (C) conditions using all possible treatment orders (JPC, JCP, PCJ, PJC, CJP, CPJ). This controlled for possible confounding between age and treatment. For example, if all hamsters had experienced the J condition last, and if aggression had been lowest in the J condition, we would not have known if this resulted from the jar or from older hamsters being less aggressive. In this part of the study, only one enrichment item was in each cage at any time.

We collected data on the last 2 days of each treatment to minimize novelty effects. Using one-zero scoring, we recorded whether or not hamsters showed object interaction or aggression during 60 consecutive 10-second intervals (10 minutes). Object interaction was defined as any hamster physically interacting with an enrichment item. We excluded non-contact, external sniffing of the item, and apparent incidental contact, such as leaning against the item while sleeping. Aggression was defined as two or more hamsters tumbling rapidly or "balling up" and biting one another. The experimenter observed each cage in a varied order for 10 minutes during the morning (0700-0900), afternoon (1300-1500), and evening (1900-2100) hours of both days. This provided a total of 60 minutes of data per cage per condition. During the dark periods, we used two 25-watt red lights for conducting observations.

For statistical purposes, we treated the hamsters in each cage as one subject (N=12) and we used the number of intervals in which a behavior was scored as a measure of frequency. It is worth noting that these frequencies are underestimated because we only recorded whether or not a behavior occurred (a score of one or zero) in each interval, and not how many times each behavior occurred.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Results and Discussion

[Bar Charts]Figures 2 and 3   [Bar Charts]Figures 4 and 5

The hamsters used both enrichment items, but preferred the jars. They interacted with the jars an average of 184 intervals, more than twice as often as they interacted with the pipes (85 intervals). As figure 2 clearly shows, object interaction decreased with age. Although we did not collect data on additional behaviors, our impression was that hamsters in the control conditions spent more time sleeping and eating. Enrichment appeared to increase species-typical behaviors like scent marking, gnawing, hoarding, and digging.

Hamsters used the items in different ways than we envisioned. Most commonly, they stood atop the jar or pipe while peering out from, or gnawing on, the stainless steel bar lids. Several groups used the jars for urinating or hoarding their food. These two tendencies made cage cleaning a lot easier. Young hamsters regularly slept together in the jar, but this behavior decreased as the animals grew. Jars were also scent marked, gnawed on, hid in, and defended. Pipes did not elicit nearly as many behaviors. Some hamsters spent a lot of time tunneling through them, but this was rare. Like jars, pipes were scent marked and gnawed on, but only occasionally hid in and rarely defended. Hamsters may have preferred jars because the jars greater height, as compared to pipes, made it easy to look outside the cage. Other reasons they may have favored the jars were the security of a single entrance, their transparency, or their versatility.

The effects on aggression were often dramatic, although groups varied greatly in their aggression levels. Because of this, we've reported median data in this section rather than averages. The median is the halfway point, and it gives a truer estimate of the central data point than the average, or mean, when extreme values are present and the data don't fit a normal curve. Overall, aggression decreased with age (7 of the 12 cages showed no aggression during data collection at 9 weeks) and with enrichment. Aggression dropped from a median of 24.5 intervals at 5 weeks to 3.5 at 7 weeks to 0 at 9 weeks (fig. 3). Median values were 9 intervals in the control condition, 3.5 in the pipe condition, and 2 in the jar condition (fig. 4). Enrichment items decreased aggression the most in young hamsters. Five-week old hamsters showed a median of 47 intervals of aggression in the control condition, but only 16 in the jar condition and 11 in the pipe condition.

Our control and jar conditions showed the pattern of highest aggression at 5 weeks, lowest aggression at 7 weeks, and slightly increased aggression at 9 weeks (see fig. 5). Our pipe condition did not. Seven-week old hamsters with pipes spent a median of 24 intervals fighting (up from 11), but 9-week old hamsters with pipes spent a median of 0 intervals fighting. One explanation for this puzzling result was our small sample size the four most highly aggressive groups just happened to be assigned to pipes at 7 weeks of age. Another possible factor was that by this time our females were cycling, and we may have collected data on these females on their more aggressive days. We did not attempt to verify day of cycle. Still, this unexpected aggression points to the importance of monitoring animals who are given enrichment. In our case, one group of females increased their aggression from two intervals with the jar (at 5 weeks) to 29 intervals with the pipe (at 7 weeks).

We ran a followup study to see if the preference for jars lasted. To do this, we removed enrichment items from all hamsters for 3 additional weeks and then added both a jar and a pipe to each cage for 2 weeks. Data were collected as before and showed that hamsters still preferred the jars (an average of 64 intervals spent in jars, and 43 spent in pipes) and aggression decreased further (median 0 intervals). It's also interesting that, whereas we found males to be the more aggressive sex in the first study (total aggressive intervals: males 336, females 152), we found the reverse in the follow-up study (total aggressive intervals: males 0, females 40). Hormonal changes are likely responsible for the fact that males and females often show different rates of aggression at different times in their lives. Although there are several differences between the living conditions we tested and the conditions that most hamster aggression studies use, our results support the notion that adult females are generally more aggressive than adult males in this species (1,4).


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Conclusion

Providing group-housed golden hamsters with simple enrichment items can substantially reduce cagemate aggression, especially when animals are weanlings. Because our enriched animals expressed more species-typical behaviors, enrichment appears beneficial for singly-housed hamsters as well. However, it seems important to determine whether singly-housed hamsters act more aggressively toward handlers when easily defendable items, like jars, are present, before a facility mandates their use. (For a discussion of hamster-human interaction, see reference 3).

In the current study, group-housed hamsters competed for a single enrichment item. Some colleagues have suggested that four items per cage (specifically, one jar per animal) would have been ideal. We disagree. Besides the space and safety considerations that multiple, movable jars would bring, we believe it is unlikely that animals would forego fighting and automatically establish individually "owned" jars. Arnold and Estep (2) observed that when five male hamsters shared a five-cage chamber provisioned with separate food and water supplies, most animals hoarded their food into a communal pile, slept together, and urinated in one or two locations. They did not act territorial. However, we agree that multiple items should reduce competition and might further reduce cagemate aggression. We recommend that caretakers closely monitor cages with enrichment items as the hamsters age and add items, or remove problem animals, when necessary.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

Acknowledgments

We'd like to thank students Kevin Hoggard for helping with the animals and Aaron Kemp for proposing the final preference test.


To: Summary | Introduction | Subjects and Materials | Procedure | Results and Discussion | Conclusion | Acknowledgments | References

References

  1. Murphy, M.R. (1977). Intraspecific sexual preferences of female hamsters. Journal of Comparative and Physiological Psychology 91:1337-1346.

  2. Arnold, C.E. & Estep, D.Q. (1990). Effects of housing on social preferences and behaviour in male golden hamsters (Mesocricetus auratus). Applied Animal Behaviour Science 27:253-261.

  3. Arnold, C. & Gillaspy, S. (1994). Assessing laboratory life for golden hamsters: Social preference, caging selection, and human interaction. Lab Animal 23(February):34-37.

  4. Siegel, H.I. (1985). Aggressive behavior. In The Hamster: Reproduction and Behavior, H.I. Siegel, (ed.), Plenum Press:New York.

This article appeared in the Animal Welfare Information Center Newsletter, Volume 8, Number 3/4, Winter 1998

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May 8, 1998
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