Has anyone ever thought that Tsunade's rules for medical ninjas came from her summon??
After all... Med. Ninjutsu probably originates from the slug species, just like frog taijutsu, and snake shed skin jutsus originate from their species respectively. Sooo... If the slugs have a sage mode, they wouldn't use it unless it is absolutely necessary (they are healers above all else).
it is illogical to think that the slugs do not have a sage mode. With all that has been given, it should be obvious.
Edit:
Also, It is not absolutely necessary for one to have huge chakra reserves to become a sage. The whole goal of sage mode is to incorporate an equal amount of natural energy (equal to your spirit energy (yin) and equal to your physical energy (yang)) So, one would have 3 equal parts of energy. Yin, Yang, and Nature. It is hard to do this, and it may be "Impossible" for some. this is because it is hard to incorporate such a huge amount of energy (nature energy) into ones system. Also, some people might have an imbalance of yin/yang chakra, making their bodies incapable of using sage mode.
But. Having less or more chakra reserves makes little difference. One would just take in less, or more nature energy. So, if one has a lower chakra reserve, it would take less time to gather nature chakra and bring it into the system, than the time it takes for someone with a large amount of chakra.
A smaller chakra reserve would only mean a lesser time in sage mode, and a lesser time it takes to gather nature energy.
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For Jugo's clan-- it is almost like the people are born naturally receptive of nature energy. Well. Actually. They are born like that. They are born with 3 types of chakra instead of 2 (yin yang and nature, instead of just yin and yang)
They do not need to consciously absorb nature energy, because it naturally goes into the body's chakra system. this is similar to when one does not use chakra (after depleting it), and it naturally comes back over time.