Scientists compared weight patterns in two types of zoo primates with very different digestive systems: lemurs and colobine monkeys. Lemurs digest food in their hindgut (back of intestines), while colobines have a special multi-chambered stomach that ferments food first. Researchers expected colobines to stay naturally slimmer because of their unique digestion, but surprisingly, both groups had similar rates of obesity in zoos. The study looked at weight records from over 8,600 individual primates across 24 species. The findings suggest that a primate’s digestive system alone doesn’t protect it from weight gain in captivity, and that diet quality—especially fiber content—matters more than scientists previously thought.

The Quick Take

  • What they studied: Whether primates with different types of digestive systems naturally resist obesity differently in zoos
  • Who participated: Over 8,600 individual primates from 24 different species living in zoos: 7,187 lemurs (14 species) and 1,443 colobine monkeys (10 species)
  • Key finding: Contrary to expectations, both lemur and colobine species showed similar rates of obesity in zoos—about 36% of lemur species and 40% of colobine species had weight problems. The special digestive system of colobines didn’t protect them from gaining excess weight.
  • What it means for you: If you work with zoo animals or care about primate welfare, this suggests that simply relying on a species’ natural digestion won’t prevent obesity. Instead, zoos need to carefully control what and how much they feed primates, focusing on diets with more fiber that match what these animals eat in the wild.

The Research Details

Researchers gathered weight measurements from zoo records for 14 lemur species and 10 colobine monkey species. They compared these zoo weights to measurements of wild primates to determine what a ‘healthy’ weight range should be for each species. They then calculated what percentage of zoo animals in each species were overweight, underweight, or at a healthy weight. The team also looked at whether other factors—like how much the males and females differed in size, how much the species ate leaves versus other foods, or whether they had seasonal weight changes—could explain why some species got heavier than others.

This approach is important because it tests a real-world hypothesis about whether biology alone can protect animals from obesity. By comparing zoo animals to wild ones, researchers could see what ’normal’ weight looks like and identify which species actually have weight problems. This helps zoos understand whether they need to change feeding practices.

The study examined a large number of individual animals (over 8,600), which makes the findings more reliable. However, the researchers relied on existing zoo records, which may not have been collected in a standardized way across all facilities. The study compared multiple species, which strengthens the conclusions but also means individual species might behave differently than the overall pattern suggests.

What the Results Show

The main surprise was that colobine monkeys—which have a special four-chambered stomach designed to ferment plant material—did not show lower obesity rates than lemurs, despite scientists expecting them to. About 40% of colobine species had weight problems compared to 36% of lemur species. This was unexpected because researchers thought the colobine digestive system would naturally limit how much they’d eat and prevent obesity.

Interestingly, colobines did show higher rates of being underweight, especially two species with the most complex stomachs (proboscis monkeys and red-shanked douc monkeys). This suggests their special digestion might make them prone to eating too little rather than too much, which could cause different health problems.

The researchers found that simple factors didn’t explain obesity patterns. Whether a species was very different in size between males and females, whether they ate mostly leaves, or whether they had seasonal weight changes—none of these clearly predicted which species would struggle with weight in zoos.

The study found no clear historical trends showing that obesity in these species has gotten worse or better over time in zoos. This suggests the problem isn’t getting solved on its own. The researchers also noted that sample size (how many animals of each species were in zoos) didn’t seem to affect obesity rates, meaning even species with many individuals in zoos still had weight problems.

Previous research had emphasized that lemurs were particularly prone to obesity in zoos, while colobines were nicknamed ‘slim monkeys’ in some languages, suggesting they naturally stayed thin. This study challenges that assumption by showing colobines have similar obesity rates. The findings suggest that earlier observations may have been incomplete or that the special digestive systems don’t work the same way in captivity as they do in the wild.

The study relied on weight records from zoos, which may not have been measured consistently across different facilities or time periods. Some species had very few individuals in zoos, making it harder to draw firm conclusions about them. The researchers couldn’t directly study how the digestive systems actually worked in the zoo animals, so they couldn’t confirm whether digestion was functioning differently than expected. Additionally, the study couldn’t account for all the different feeding practices across various zoos, which likely varies significantly.

The Bottom Line

Zoo managers should monitor the weight and body condition of primates regularly and continuously, rather than assuming certain species will naturally stay slim. Feeding programs should be adjusted to include higher fiber content that more closely matches what these primates eat in the wild. This may help prevent obesity across all primate species, regardless of their digestive type. (Confidence: Moderate—the study shows the problem exists but doesn’t test specific diet solutions.)

Zoo managers, veterinarians who work with primates, and animal welfare organizations should pay attention to these findings. Anyone involved in primate care or nutrition should reconsider assumptions about which species need weight management. This is less relevant to people who don’t work with animals, though it’s interesting for those curious about animal health.

Changes in zoo feeding practices would likely take weeks to months to show results in animal weight, similar to how it takes time for humans to see weight changes. Long-term monitoring over years would be needed to determine if dietary changes actually prevent obesity.

Want to Apply This Research?

  • If you manage zoo primates, track weekly body weight measurements and body condition scores (a visual assessment of how thin or heavy an animal looks) for each individual. Record this alongside detailed feeding logs showing what foods were offered and in what quantities.
  • Gradually increase the fiber content in primate diets by adding more leafy vegetables, hay, and whole plant materials while reducing processed foods and simple carbohydrates. Document the specific diet composition (percentage of leaves, fruits, vegetables, and other foods) to match wild diet patterns more closely.
  • Create a long-term tracking system that monitors weight trends over months and years for each species in your care. Compare your animals’ weights to the healthy ranges established in this research. Set species-specific weight management goals and review them quarterly to see if dietary changes are working.

This research describes patterns in zoo animals and does not provide medical advice for individual primates. Zoo managers and veterinarians should consult with primate nutrition specialists and veterinary professionals before making significant changes to feeding programs. This study identifies a problem but doesn’t test specific dietary solutions, so any dietary changes should be made gradually and monitored carefully with veterinary oversight. If you have concerns about a specific animal’s weight or health, consult a veterinarian who specializes in primates.