Scientists studied the gut bacteria of wild and captive-bred European hares to understand why animals raised in captivity struggle when released into nature. They found that captive hares have very different gut bacteria compared to wild hares, missing important bacterial types that help break down wild plant foods. When researchers tried feeding captive hares fresh grass to prepare them for the wild, it didn’t significantly change their gut bacteria to match wild hares. This suggests that simply adding grass to their diet isn’t enough—scientists may need to develop more specialized food plans that match what wild hares naturally eat to help captive animals survive after release.
The Quick Take
- What they studied: Whether feeding captive hares fresh grass could change their gut bacteria to be more like wild hares’ gut bacteria
- Who participated: 45 European hares total—some living in the wild and some raised in captivity. Researchers analyzed the trillions of bacteria living in their digestive systems
- Key finding: Adding grass to captive hares’ diet made only small changes to their gut bacteria and didn’t make them significantly more similar to wild hares’ bacteria. Wild hares had bacteria that captive hares completely lacked, including a group called Spirochaetota
- What it means for you: If you work with wildlife conservation or animal reintroduction programs, simply feeding captive animals grass before release may not be enough to prepare them. More personalized dietary approaches based on what wild animals naturally eat might be necessary for success
The Research Details
Researchers collected gut samples from 45 European hares—some living wild and some raised in captivity. They used advanced genetic technology called genome-resolved metagenomics, which is like taking a detailed census of all the bacteria living in the hares’ digestive systems. They identified and studied 860 different bacterial genomes, with 87% being completely new to science. Some of the captive hares were then fed fresh grass in addition to their regular pellet-based diet for a period of time, and researchers re-sampled their gut bacteria to see if the grass changed their microbial communities.
Understanding gut bacteria is important because these microscopic organisms help animals digest food and absorb nutrients. When captive animals are released into the wild, their gut bacteria may not be equipped to handle wild foods, which could cause digestive problems and reduce survival chances. By studying whether dietary changes can prepare captive animals’ gut bacteria before release, scientists can develop better strategies for successful animal reintroduction programs.
This study is reliable because it examined a large number of bacterial genomes (860) with high-quality genetic analysis. The researchers compared wild and captive animals directly, which is a strong research approach. However, the study included only 45 hares total, which is a relatively small sample size. The discovery that 87% of bacterial species were new to science suggests the researchers used thorough, detailed methods. The study was published in a peer-reviewed scientific journal, meaning other experts reviewed the work before publication.
What the Results Show
The most important finding was that wild and captive hares have significantly different gut bacteria communities. Wild hares possessed bacteria that captive hares completely lacked, including an entire bacterial group called Spirochaetota. Additionally, wild hares had greater capacity to break down amino acids (building blocks of proteins) and certain sugars from plant material—skills their gut bacteria had developed to handle wild foods.
When researchers added fresh grass to the captive hares’ diet, the results were disappointing. While the grass did cause some minor changes to their gut bacteria, these changes were not statistically significant (meaning they could have happened by chance). Most importantly, the grass supplementation did not move the captive hares’ gut bacteria closer to resembling wild hares’ bacteria communities.
The researchers identified 860 different bacterial genomes in total, which is an enormous catalog of microbial diversity. The fact that 87% of these were previously unknown to science highlights how much we still have to learn about wildlife microbiomes. This discovery alone is valuable for understanding microbial diversity in nature.
The study revealed that the differences between wild and captive hare microbiomes weren’t just about having different bacteria present or absent—the bacteria also had different functional abilities. Specifically, wild hares’ bacteria were better equipped to process the complex plant materials found in natural diets. This suggests that captive hares’ bacteria have essentially ‘forgotten’ how to efficiently digest wild foods because they’ve been eating processed pellets for generations.
This research builds on growing evidence that gut microbiota plays an important role in animal reintroduction success. Previous studies in other species have suggested that microbiome mismatch could contribute to reintroduction failures, but this is one of the first detailed studies to examine whether simple dietary interventions can fix the problem. The findings suggest that earlier assumptions about the ease of ‘rewilding’ captive animals’ digestive systems may have been too optimistic.
The study included only 45 hares, which is a relatively small number for drawing broad conclusions. The research focused only on European hares, so results may not apply to other species or even other hare populations. The grass supplementation period wasn’t specified in detail, so we don’t know exactly how long hares were fed grass or how much they consumed. Additionally, the study only measured changes in gut bacteria—it didn’t track whether these bacterial differences actually affected the hares’ health, survival, or ability to digest food in practice.
The Bottom Line
For wildlife conservation programs attempting to reintroduce captive-bred hares: Simply adding grass to a pellet-based diet is unlikely to adequately prepare animals for wild conditions (low confidence for this specific intervention). Instead, develop more targeted dietary interventions based on detailed analysis of what wild hares naturally eat in their specific habitat (moderate confidence). Consider microbiome assessment as part of pre-release evaluation protocols (moderate confidence). Further research is needed to identify which specific foods or dietary components would effectively reshape captive hares’ gut bacteria.
Wildlife biologists and conservation programs working on hare reintroduction should pay close attention to these findings. Zoo nutritionists preparing captive animals for release should consider this research when planning pre-release diets. Researchers studying other captive-bred species may find similar microbiome challenges. General animal lovers should understand that preparing captive animals for the wild is more complex than previously thought. However, this research is primarily relevant to professional conservation efforts rather than individual pet owners.
Changes to gut bacteria typically occur over weeks to months, depending on the dietary intervention. However, this study suggests that simple grass supplementation may not produce meaningful changes even over extended periods. More targeted dietary interventions might require several months of consistent feeding before significant microbiome shifts occur. Complete adaptation to wild-like microbiota may take even longer or may require multiple generations in the wild.
Want to Apply This Research?
- For conservation programs using this research: Track dietary composition weekly (percentage of pellets vs. natural foods), monitor fecal consistency and digestive health daily, and measure body weight and condition scores bi-weekly. If microbiome testing becomes available, track bacterial diversity metrics monthly.
- Implement a gradual dietary transition protocol that moves beyond simple grass supplementation toward species-specific natural foods identified through wild hare diet analysis. Document and track which food items are consumed and in what quantities. Create a pre-release feeding schedule that progressively increases natural food exposure over 8-12 weeks.
- Establish baseline microbiome profiles for all captive hares before dietary intervention. Collect fecal samples at regular intervals (weekly or bi-weekly) to track bacterial community changes. Monitor digestive health indicators (fecal output, consistency, animal behavior) continuously. If possible, conduct follow-up microbiome analysis post-intervention to assess whether targeted dietary changes produced better results than grass supplementation alone.
This research describes findings from a scientific study on hare gut bacteria and dietary interventions. These findings are relevant primarily to professional wildlife conservation and reintroduction programs, not to individual pet care or human health. The study was conducted on a small sample of European hares and may not apply to other species or populations. While the research suggests that simple grass supplementation is insufficient for preparing captive hares for wild release, this does not constitute veterinary or conservation advice. Anyone involved in animal reintroduction programs should consult with wildlife biologists, veterinarians, and conservation experts before implementing dietary or other interventions based on this research. This summary is for informational purposes and should not replace professional guidance from qualified experts in wildlife management.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.