The Zoological Society is a building that can be found outside of your main zoo:
The Zoological Society allows you to unlock special regions that have various perks, for example:
The Zoo d'Amour, which increases the success rate of animal breedings there by 10%
The Serengeti, which allows you to keep African animals in large, mixed species enclosures, with a significant bonus to visitors
The Prehistoric Park, which allows you to keep various extinct animals in mixed-species enclosures, with a significant bonus to visitors
Additionally, unlocking the Zoological Society will allow you to flip 5 cards instead of the usual four in any boosters you open.
You can unlock the Zoological Society at level 15 and up for 150 Diamonds; at level 40 and up, you have the option of paying 15,000,000 ZD instead.