There are four ways to die in Tokyo Jungle: combat, Hunger, Toxicity, and old age. Even when the player's animal has died, they'll gain control of another member of the pack, if there are any. Pack members, once you gain control of them, have full health and hunger (unless in combat, in which case health might be affected) and either zero Toxicity or 50 Toxicity depending on whether or not you're in a polluted area.
Perhaps the most common method of death; animals often meet their demise during a scrap with a Predator (or pack of Predators), or against an aggressive Grazer. Death at the hands of another animal can be avoided with quick wit, stealth, and combat prowess. However, this becomes increasingly difficult as the years tick by, with larger and more dangerous animals becoming more and more common. The player must learn how and when to fight and flee in tough situations, as well as how to circumvent threats altogether, if they wish to survive.
When an animal's Hunger gauge reaches zero, its Life will steadily decrease until it can find food or die. This method of death is avoided by eating often enough to keep the Hunger gauge from emptying. While this is fairly easy in the early years, it becomes increasingly difficult as time goes on. Easy-to-catch prey becomes more scarce, being replaced by hardier beasts. Plants also steadily increase in rarity. As the years progress, pollution becomes more and more common, and even food or water that the player does manage to find may be Toxic. While one can still consume tainted food and water, it will cause a spike in the Toxicity Meter, which may eventually lead to another form of death. It is up to the player's own discretion if the risk is worth it and when.
If an animal's Toxicity gauge reaches 100%, its Life will start to decrease until it drinks untainted water, eats untainted food, or uses a healing item. Death by Toxicity can be avoided by steadily moving away from poisoned areas. However, doing so becomes difficult as the later years approach, due to Toxicity becoming much more frequent. Consuming poisonous food or water jumps the Toxicity meter about 25% per food eaten and about 15% per drink of poisonous water. Rain and heat also slowly increase your Toxicity, but not nearly as fast as being in a polluted area.
An animal is considered old at about 14 years (Homo Erectus, Office Worker, and all Robot Dogs excepted). When that happens, its abilities are lowered and its maximum hunger will slowly start to drop. At age 20, the maximum hunger will hit zero. When the animal's maximum hunger hits zero, it will begin to lose health just as if it were starving. Eating and/or taking food items will no longer help at this point, however, healing items such as Pet Medicine can prolong your life enough to mate and change generations.