At day 21 postpartum, we weaned pups into 12 permanent gro
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.
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.
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).
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.
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