Environmental enrichment is a major tool to refine housing conditions for captive animals. Since the benefits of environmental enrichment have been first described (1), the effects of different types of sensory, social, and structural enrichment have been investigated, covering a variety of different species living in laboratories, in zoos, and on farms [e.g., (2–9)]. Still, the main body of research on environmental enrichment has been conducted using laboratory rodents (10, 11).

While the results between the studies sometimes vary depending on the strain, sex, age, and differences in exposure to environmental enrichment (12–15), they support the general notion that environmental enrichment can increase the animals’ welfare. The wide range of reported beneficial effects through environmental enrichment include (but are not limited to): increased neurogenesis, learning and memory performance (16–19); promotion of species-specific behaviors and preventing the occurrence of behavioral disturbances like stereotypies (10, 20–24); facilitation of development of individual variation (25–27); as well as the reduction and mediation of anxiety, depression, and stress (17, 28–31). Together with the already mentioned factors, preference can likewise be used to assess welfare (32–34), and indeed, rodents show a preference for increased complexity and are even willing to work for access to additional enrichment (35).

Yet, given that laboratory rodents adapt very quickly to new conditions and environments, an initially beneficial impact of increased complexity might cease over time. One simple way of providing not only complexity but also novelty is to regularly exchange enrichment items, for example as part of the cage-changing routine. Indeed, researchers conducting enrichment studies have used novelty as part of their enrichment. However, it is impossible to disentangle whether reported welfare effects or preferences were due to the increased complexity or the novelty. That is because novelty was not systematically applied as a distinct enrichment strategy. Rather, novelty was used as part of the complex housing condition to further increase the contrast between the enriched and unenriched environments [see e.g., (11, 25, 36–38)].

So far, only a few studies have focused specifically on the effect of novelty on the welfare of laboratory rodents. For example, Abou-Ismail and Mendl (39) housed rats under two conditions, labeled complexity and novelty. In the novelty condition, the rats were provided five copies of the very same enrichment item, e.g., five ladders, which were exchanged weekly for five copies of another enrichment item, e.g., five shelters. In contrast, the rats from the complexity condition were offered five different items at a time which were not exchanged over the course of the 5 weeks. They found the complexity condition preferable over the novelty condition. Notably, their novelty condition did not always offer shelter or nesting material. Because nesting material has been identified as a major enrichment item for laboratory rodents (10, 40), their novelty housing condition without nesting material can arguably be considered impoverished compared to their complexity condition which always offered nesting material. In another experiment performed with mice, Gross et al. (41) did not find beneficial effects of novelty either. In their study, the shelters remained inside the cages for the duration of the experiment in the complexity housing condition, but in the novelty housing condition the shelters were replaced with a different kind of shelter every week (41). However, it has previously been argued that novel objects, especially when introduced into the home cage, might be perceived as a threat (42). Hence, to avoid potential adverse effects of novelty, it would be more cautious to avoid applying novel objects to the home cage but find ways to provide novelty outside the home cage (e.g., in an accessible extra cage or a play chamber).

With the present study, we aimed to further explore the potential benefits of novelty when providing structural enrichment. We compared the effects of three different enrichment conditions on mice, the most commonly used model species (43, 44). In contrast to the aforementioned studies, we used a double cage system (45), in which one cage served as home cage and the second one was used as extra cage. This system allowed us to provide the mice constant access to nesting material and shelter in their home cage, and additionally, to offer novelty in a voluntarily accessible extra cage outside the familiar home cage. To systematically compare the effects of novelty and complexity on the mice, the extra cages either provided a weekly changing set of enrichment items (novelty), a fixed set of enrichment items (complexity), or no additional enrichment items (space). To gain insights regarding the effects of enrichment condition on the mice’s welfare and their preferences, we monitored the mice’s spontaneous behavior, body weight, and extra cage usage. More specifically, if for example, novelty were superior to complexity, we would expect to see more behaviors indicative of good welfare, e.g., more play, and fewer signs of poor welfare, e.g., fewer stereotypies, in the novelty mice (32, 46, 47), and if for example, novelty were more attractive to the mice than mere complexity, we would expect that novelty mice enter the extra cages faster when access is provided and spend more time in the extra cage, as well as interacting more with the enrichment therein.

To test whether enrichment condition influenced the mice’s spontaneous behavior, body weight, and usage of the extra cage, we conducted behavioral observations and monitored the mice’s weight over the course of 5  weeks. We indeed found that enrichment condition influenced some of our outcome measures, namely relative time the mice spent in the extra cage, latency to enter the extra cage, and relative body weight.

Regarding relative time spent in the extra cage, we see that across all 5  weeks, mice from the complexity and novelty enrichment condition spent more time in their extra cages than mice from the space enrichment condition (Kruskal-Wallis test for Weeks 1–5: χ² = 16.294, p < 0.001; post hoc pairwise comparison for Weeks 1–5, complexity-space: Z = 2.519, p _adjusted = 0.012, novelty-space: Z = 3.991, p _adjusted < 0.001; Supplementary Table S1 and Figure 2A). This effect of enrichment condition on relative time spent in the extra cage, with mice spending more time in the structurally enriched extra cages compared to the structurally non-enriched extra cages from the space enrichment condition, can also be seen for all individually analyzed weeks, except during week 4 (for details please see Supplementary Table S1 and Figure 2A). The only time we detected a difference between the novelty and complexity enrichment condition was during week 3 (Kruskal-Wallis test for Week 3: χ² = 14.114, p _adjusted = 0.005; post hoc pairwise comparison complexity-novelty: Z = −2.558, p _adjusted = 0.011, Supplementary Table S1 and Figure 2A), showing that mice from the novelty enrichment condition spent more time in the extra cage than mice from the complexity enrichment condition.

Regarding latency to enter the extra cage, we found a statistically significant interaction between enrichment condition and week (LMM: F_2,1429.553 = 4.828, p = 0.008, Supplementary Table S2 and Figure 2B). Post hoc analysis indicated that over the weeks, the latency to enter the extra cage decreased significantly faster for mice from the complexity enrichment condition compared to mice from the space enrichment condition (post hoc pairwise comparison, space-complexity: b = −0.012 ± 0.004, t_1429.334 = 2.956, p _adjusted = 0.009, space-novelty: b = 0.010 ± 0.004, t_1429.702 = −2.345, p _adjusted = 0.050, Supplementary Table S3 and Figure 2B).

Regarding relative body weight, we also found a statistically significant interaction between enrichment condition and week (LMM: F_2,105 = 4.04, p = 0.020, Supplementary Table S2 and Figure 2C). Here, post hoc analysis revealed differences in the slopes for the week effect on mice from the space enrichment condition and mice from the novelty enrichment condition, indicating a decrease in relative body weight over the weeks in space mice compared to mice from the novelty enrichment condition (post hoc pairwise comparison: b = −1.290 ± 0.491, t₁₀₅ = −2.625, p _adjusted = 0.027, Supplementary Table S3 and Figure 2C). We also found a statistically significant effect of enrichment condition on relative body weight (LMM: F_2,134.1 = 3.37, p = 0.037, Supplementary Table S2 and Figure 2C), but post hoc analysis revealed no statistically significant effect after correcting for multiple comparisons (for details please see Supplementary Table S3 and Figure 2C). For all other outcome measures, we did not find statistically significant effects of enrichment condition on our outcome measures (for more details please see Supplementary Table S1 and Supplementary Figure S1).

Albeit not the focus of this study, we also analyzed the effect of strain and week on our outcome measures. We found that week influenced latency to enter the extra cage. Apart from the interactive effect of week with enrichment condition described above, week had a statistically significant main effect on latency to enter the home cage (LMM: F_1,1429.553 = 1004.212, p < 0.0001, Supplementary Table S2 and Figure 2B), showing that the latency to enter the extra cage decreased over the weeks. For all other outcome measures, we did not find statistically significant effects of week or strain after post hoc pairwise comparisons and correcting for multiple comparisons (for more details please see Supplementary Tables S4–S6 and Supplementary Figure S1).

Because of their rare occurrence, some spontaneous behaviors were only descriptively analyzed, namely agonistic behavior, stereotypic behaviors, and bar mouthing. Upon visual inspection of the data, we did not detect an effect of enrichment condition (for more details, please see Supplementary Table S7 and Supplementary Figure S2).

The present study aimed to explore whether novelty of environmental enrichment may offer beneficial effects beyond that of structural complexity alone. To this end, we allowed mice of two strains access to an extra cage offering either novelty, complexity, or space. Over the course of 5 weeks, we recorded spontaneous behaviors and body weight, as well as extra cage usage as indicators of welfare. With one exception during week 3, in which mice from the novelty condition spent statistically significant more time in their extra cages than mice from the complexity condition, we did not find distinct differences between the complexity and novelty conditions. However, we saw that mice spent more time in the structurally enriched extra cages (complexity and novelty) compared to the structurally non-enriched extra cages (space) and that mice from the complexity enrichment condition reduced their latency to enter the extra cage faster than mice from the structurally non-enriched condition (space).

This is in line with the study by Gross et al. (41), who investigated the effects of different housing conditions on stereotypic and anxiety-related behaviors in mice. They, too, did not find systematic differences between the novelty and complexity condition. In contrast, Abou-Ismail and Mendl (39) found that their complexity housing condition was superior to their novelty housing condition. This discrepancy between the study by Abou-Ismail and Mendl (39) on one hand and the present study on the other could be due to species-specific differences between mice and rats. However, we think that their findings are more likely related to the study’s experimental design. As already mentioned in the introduction, a plausible explanation is their non-permanent provision of nesting material in their novelty housing condition compared to their complexity condition, where nesting material was always available to the rats. The findings of the two studies led us to use a double cage system, in which we could provide constant access to nesting material in the mice’s home cages while offering different levels of enrichment in a voluntarily accessible extra cage rather than the home cage itself.

Our other results indicate a preference for the structurally enriched conditions (complexity and novelty) over the structurally non-enriched space condition, a preference which has been reported previously (35, 68). As it has been argued that preferences can be used to give welfare insights (32–34), a preference for the enriched extra cages over the structurally non-enriched space extra cage might indicate that additional space alone is less beneficial for the mice than increased complexity and novelty. Apart from the differences in preferences, relative body weight was the only other outcome measure affected by enrichment condition. However, we advise caution when interpreting this result. Post hoc pairwise comparisons did not reveal any statistically significant differences between the groups. Furthermore, our mice were all non-breeding, healthy, adult females, and unlike young or breeding mice, our mice did not need to gain weight, and their body weight fluctuations were mild and within the normal range.

Apart from a preference for the structurally enriched extra cages (complexity and novelty) over the structurally non-enriched extra cages (space) and the differences in body weight, our results did not reveal further effects of enrichment condition. Albeit not the focus of our study, as the rationale for including different mouse strains was to be better able to generalize our findings, we also tested for strain differences. In the literature, behavioral differences between C57BL/6J and DBA/2N mice have been reported (69). There are also studies showing that the effects of environmental enrichment can be strain-specific (29, 49, 70) and that the presence of the other strain can have an influence as well (45). In our study, however, we did not detect statistically significant differences between the strains. Even so, we cannot conclude that there were no strain differences, as our sensitivity analysis indicated that we could only detect strain effects of large effect sizes (Cohen’s f ≥ 0.481).

Overall, the spontaneous behaviors either did not show differences between the groups or the behaviors happened too rarely to be statistically analyzed. Nonetheless, the behavioral data we gathered gave some insights. For instance, the occurrence of bar mouthing, stereotypic and agonistic behavior was very low, and we saw play behavior in all three housing conditions despite the mice’s relatively advanced age (PND 167–202), albeit rarely. Play behavior has been reported to be a reliable indicator of good welfare, whereas weight loss, inactivity, and the presence of stereotypic and agonistic behavior are considered signs of impaired welfare (32, 46, 47). Together with the mice’s relatively stable weight, our observations indicated that the furnishing we offered in the home cages was already of a relatively high standard and provided the mice with good welfare, even though definite claims cannot be made here, as in addition to the provided enrichment in the home cages, all mice had regular access to at least extra space. As the authors of previous studies already pointed out, the provision of nesting material is already sufficient to improve the welfare of laboratory mice greatly (10, 41). Likewise, it has been argued that providing enrichment diversity, i.e., complexity, is probably more important for the animals’ welfare than providing novelty without diversity (39).

In conclusion, our study showed that additional structural enrichment was more attractive to mice than extra space alone. However, with only one result indicating that novelty was preferred over complexity, the present study does not allow us to determine whether or not enrichment novelty increased mouse welfare beyond the effects of enrichment complexity. We did not detect differences between the two enrichment conditions in mice, but it is still conceivable that there are effects of novelty beyond complexity, especially regarding extended periods and with a wider range of variables assessed. To answer this question is of great interest not only for mice and other lab animals. Rather, this concern regards all captive animals, be it in zoos, on farms, or in our own homes. Further research is needed to explore whether additional novelty may be more refining than environmental complexity alone.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was approved by the local (Amt für Gesundheit, Veterinär- und Lebensmittelangelegenheiten, Münster, Nordrhein-Westfalen, reference number: 39.32.7.1) and federal authorities (Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen “LANUV NRW”). The study was conducted in accordance with the local legislation and institutional requirements.

SK, SR, and NS conceived the study. LBi, NK, SK, SR, and NS designed the experiments. LBi carried out the experimental procedures. LBo and VK analyzed the data. LBo prepared the manuscript. All authors reviewed and approved the final version of the manuscript.