Guinea pig poos can give you some indication of their their health. Guinea pigs are very talented at hiding signs of ill health due to the fact that they are prey animals. In the wild any signs of weakness or disease would result in their rapid demise. Being able to pick up on subtle signs that something isn't quite right could dramatically improve the chances of recovery from disease. The five main things you should make note of are faecal number, size, colour, consistency and smell. Changes in any of these parameters can be an indicator of change in diet, appetite, gut motility, hydration status and infections.
Number As herbivores guinea pigs are designed to eat and poo a lot, since their diet has relatively low nutritional value. A 'normal' guinea pig will produce up to 100 poos a day. This number is naturally lower in older or less mobile guinea pigs. If the number of faecal pellets drops below 50 a day it may be a sign that something is wrong. Reasons for lower numbers of poos can be reduced food intake or slowed gut motility.
Size can vary a little from one pig to the next, with large adult males sometimes producing pellets 1.5 to 2cm long (average is closer to 1cm). They should be rounded at both ends, smooth and plump. If the poos are very small it is often an indicator that food intake is too low. If they are tear drop in shape (ie pinched in at one end) it can be a sign of dehydration, or altered gut motility.
Normal faecal colour is dark brown. If the faeces are greener in colour and soft it is likely to be the caecotrophs (faeces that are produced following one pass of the gut, they are eaten directly from the anus to pass through the GI tract for a second time). Caecotrophs are not normally seen in the cage, as they are eaten straight from the anus. If they are noted it will be a sign that your guinea pig is struggling to eat them, this may be due to age and spinal or hip pain. In addition some guinea pigs will be seen chasing other guinea pigs to eat their caecotrophs. This is normal in young, weaning guinea pigs, in order to populate their guts with 'good bacteria'. It can be normal in older guinea pigs too, or may indicate a problem with gut microflora or a vitamin deficiency.
Normal guinea pig poos should be firm and well formed, with a slight glisten. If the poos are dry and crumbly it is an indication of dehydration. If the poos are soft and easily deformed it is often a dietary problem. An ideal diet contains 80% hay or grass, with an eggcup of pellets, and fruit and veg as a treat only. Fruit and veg does not need to be given every day and should be a treat sized portion (not a plateful). If the faeces are watery it is an emergency and you should take your guinea pig to the vets immediately, it can be a sign of infection, including clostridial infectious, which can be rapidly fatal even with aggressive treatment. Some older guinea pigs, especially boars, have a reduction in muscular tone around the anus. This can allow the development of faecal impaction in a sac like pocket by the anus. Faecal pellets build up in here and a fungal infection can ensue, it can also make it difficult to pass more faeces and for the guinea pig to eat it's caecotrophs. It is common requirement for owners of older boars to empty this sack of faeces on a regular basis. A video on how to do this can be seen through facebook: https://www.facebook.com/theguineapigvet/videos/618920421892888/
Guinea pig poos should not normally smell. If they do it could be the caecotrophs that you're observing or a sign of infection.
Changes in guinea pig poos are rarely as a result of infection, it is much more commonly due to diet, inability to eat normally (teeth), or altered gut motility. If you are concerned that your guinea pigs poos are not normal seek veterinary advice.