Ok so I don’t know if they fixed it since the other person tested it, but they do “half” the weight of items!
The thing is they do it differently other creatures. Instead of half-ing the weight per-item they half there TOTAL weight when on your shoulder. So if you have them hold a stack of berries that way 10., on your SHOULDER they will only at 5. weight! Hope this helps
DO NOT believe that this reduces weight on its inventory. As of today AUG 2020 I tested stacks of meats and berries etc, IT DOESNT. But instead it increases spoil timer of foods, meats and berries on its inventory. ⬆️ to prevent ppl from false informations!
Weight: Jerboa's total weight is HALVED on your shoulder. Ex. Let's say your crystal is 80 weight. If you have jerboa hold the crystal and put him on your shoulder it would weigh 40. Check top utility comment for his weather prediction!