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It has long been known that rats do not have a gallbladder, though other species including humans, monkeys, cows, reptiles, dogs and mice, all have a gallbladder.

In this paper from almost 100 years ago, scientists were studying what differences the presence or absence of a gallbladder gives the composition of bile. The authors found (what we now appreciate very well) that bile from mice who have a gallbladder was much more concentrated than rat bile - i.e. one of the functions of the gallbladder is to concentrate bile.

I know this is a very speculative question, but is it known whether having or lacking a gallbladder confers some sort of evolutionary or biologic advantage (or disadvantage)?

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    $\begingroup$ It is perhaps worth noting that rats aren't unique in this respect. Although most rodents and lab mice species have a gallbladder, there are other rodents (e.g. species of mice and gophers) that lack a gallbladder. Equines, most camelids, cervids, cetaceans, and many birds also lack a gallbladder. $\endgroup$ – Harry Vervet Mar 5 '16 at 20:59
  • $\begingroup$ @HarryVervet completely agree - thanks for including these other species. It seems so random that some species have a gallbladder and some do not. $\endgroup$ – Vance L Albaugh Mar 5 '16 at 22:56
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    $\begingroup$ Just providing a possible thought here: it might also be worth considering that the disadvantage of having a gallbladder includes the potential development of life-threatening diseases in the gallbladder. Infection of the bile duct, duodenum or pancreatitis can occur due to the formation of gallstone, which is formed from bile components. Having a gallbladder allows bile to concentrate, possibly increasing the risk of developing gallstone-induced diseases. $\endgroup$ – hello all May 7 '16 at 9:48
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    $\begingroup$ You only need a gallbladder if you have a diet rich in concentrated fats or engage in binge eating (aka most carnivores), otherwise the liver can produce bile on demand just fine. leaf herbivores and other animals that eat low concentrations of fat or forage continuously like rats don't need the gallbladder. $\endgroup$ – John Dec 6 '16 at 16:19
  • $\begingroup$ Just a thought: A gallbladder not only has advantages. For example if humans contract Salmonella infections some get chronic infections. And "excrete" (life)long these bacteria with their faeces. And as a matter of fact its the gallbladder where the Salmonellae are nesting. Maybe this could be a reason why it can be advantageous to have none, especially when you have an urban rats diet?! $\endgroup$ – Rudi_Birnbaum Jun 10 '17 at 15:31
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There are many different theories on this aspect of evolution, some of them are proved and others are just conjectures. Materials and research in this particular aspect is very few. Here I tried to provide an answer in reference to the particular animal you mentioned, i.e. rat.

Rats don't have a gallbladder possibly due to the following reasons -

  1. The concentrating power of bile in rat's liver is high, so the main function of bile concentration by gallbladder is unnecessay in them. This is probably the most supported theory regarding this.

  2. Rats frequently take their food so they require a continuous supply of bile. This eliminates the necessity of bile storage.

  3. Harbivore animals relatively lack fat in their food. This reduces the need of large amount of bile salts in the intestine.

References -

  1. https://theparadigmshiftgroup.com/animals-no-gallbladder/
  2. Zoology textbooks
  3. http://www.sciencedirect.com/science/article/pii/S0375090613000323?np=y
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As I understand the main purpose of the gallbladder is to ensure the full digestion fatty or otherwise difficult or complex foods to digest in support of the liver. Both rats and mice breastfeed for about the same amount of time 4-5 weeks. but mice don't have gallbladders; it's likely not a simple matter of ingesting less fat in their diets. Most mammals including humans developed gall bladders through evolution to aid in the digestion of fatty diets into adulthood. Rats instead developed larger cecum for the slow digestion of seeds and grains, which works in them because of their very specific gut microbiome.

http://www.drturumin.com/en/GallAcids_en.html

**http://www.differencebetween.net/science/difference-between-a-human-digestive-system-and-a-rat-digestive-system/

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    $\begingroup$ Please give the answer directly and provide references for it rather than giving a reference instead of an answer. $\endgroup$ – AlexDeLarge Jul 14 '16 at 19:17
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It's probably due to the fact that rats have an usually "harder do digest" diet. A gallbladder means a source of extra bile at the time of digestion. As you said mice have one but smaller, possibly rats used to have one of the same size, but as they adapted to urban live, bigger ones became advantageous.

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    $\begingroup$ Any sources you can add to this? $\endgroup$ – AliceD Mar 20 '17 at 1:49

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