As soon as you have two or more of an animal, they begin to mate, and each animal type has a displayed birth rate and death rate. If your birth rate is higher than death rate, the population will increase over time. When the death rate is higher than the birth rate, eventually, they will all die off.

More animals -> more babies.

Each animal can be upgraded in four different categories: Increasing birth rate, decreasing death rate, increasing sell price, and increase max population size. One caution about increasing the sell price, is that it also increases the buy price, and sell price is always 1/2 that of buy price. As far as the animals are concerned, you will be doing a combination of buying new types and upgrading old types to keep them relevant.

It seems to be the case that the more of an animal you have, the faster the population will grow, which intuitively makes sense. In addition to the factors of population, birth rate, and death rate, there is an additional factor: Mating rate. This is basically a counter of how many times per second the birth and death rates are applied, as modified by the population level.

Mating rate is increased principally by the biome statues you will build over time: each time you choose to watch an ad, you will receive either money or gems and the value seems to track with total zoo worth. These provide a 4% increase in mating rate for that biome (i.e. that set of 5 animals) for each level of statue. The higher level the statue, the longer it takes to upgrade another 4% mating speed — up to a maximum of 24% extra mating speed. Mating speed is also modified by the Unicorn rare animal, and unlocking species variants, either by paying for gems through microtransactions (e.g. 50 gems for $4.99), or buy saving up for them through the trading market.

The market is a sort of bounty system. Once unlocked, you will have a chance to trade a certain amount of an animal for either gold or gems (and there is a rare animal that increases the gems you get, which can save you $ instead of buying microtransactions). With bounty slots opening at 50,000, 1 million and 1 billion gold, these are a fairly minor part of the gameplay, though the only way to get the IAP currency, gems, without either making a purchase, or watching ads. I must say that I appreciate that Iron Horse Games has made the ads optional, and the way the gift rewards scale makes it feel worth it no matter the size of your zoo.

The market lets you trade in thousands of animals for a handful of teeny gems. No YOU’RE a monster.

The IAP currency, gems, can be used for one-time boosts (time warp, increased birth rates, increased passive income, etc), for buying species variants (thereby increasing the mating rate of that one animal), or for increasing the space you have available for rare animals (which seems to be rather at a shortage after a dozen or two prestiges — I encourage you to be picky with your rare animals).

Rare animals that seem appealing to me thus far, although I’ve not yet seen them all, are the ogre to increase bounty gem rewards, the mermaid to increase birth rates (helps a ton early game), Cerberus to increase the idle time (usually 2 hours, and Cerberus increases it by an hour to start), the phoenix which starts you with 25 of an animal as you unlock new species, and Shingami who returns a portion of the animal’s sell cost when they die rather than simply losing all potential profit from the animal’s death. I have not unlocked but am rather enamored with the dragon. While quite expensive (4–5x most others), he increases passive income by 15 times, which would make idle income much more competitive, and perhaps dominant.

I believe the most important rare animals to start are, in order, (1) Mermaid to keep your pricey new animals from dying out, (2) Shingami to begin recouping otherwise lost funds from animal deaths, (3) Cerberus to increase idle time so that you don’t have to check in as often, and (4) Phoenix to decrease waiting time and generally speed up runs. After that, I’d save up for (5) the Ogre to get the equivalent of free microtransactions and (6) the dragon to make bank on those sweet, sweet passive gains.