Question: In Tamil, there is a proverb that translates to something like this:

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> A tiger will not eat grass, no matter how hungry it is.
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It essentially is used to describe a situation in which no matter how dire of a situation you're in, you will not use your last resort. In this case, it's used to describe that even if a tiger is starving to death, it won't eat grass (because it's a carnivore). I really couldn't find any sort of English idiom that can accurately describe this Tamil proverb. **Please post all related English idioms as answers, even if they are only partially accurate.**

A possible situation I might use this in: Alice was a hardworking student who believed that she could receive an A in her class without using extra credit. 

I might use the idiom to describe Alice in this situation, as she refuses to use her last resort (extra credit), no matter desperate she is (because she believes she can earn an A without extra credit.)

Answer: 
**"a tiger won't change his stripes"** is a saying, with some variations like: "a **tiger/leopard/zebra** can't change its stripes to spots".

* a person's character, especially if it is bad, will not change.
* certain personal traits are unchangeable.

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> It means that [a person's character, especially if it is bad, will not change, even if they pretend it has changed.](http://idioms.thefreedictionary.com/A+leopard+can%27t+change+its+spots)
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> * "I doubt very much that marriage will change Chris for the better. A leopard doesn't change its spots."
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**EDIT** - If you mean that Alice has an advantage that other people don't know about, you may say that **"she keeps an extra ace up her sleeve"** or **"...a card up her sleeve"**. (keeping it back until the time is right)
“Old habits die hard” is also a common proverb used nowadays. From my understanding, I believe it means that it’s very hard for someone to change especially if it was from something bad, though change is constant so is habit.