Frogs have moist skin, through which they absorb oxygen to supplement their lungs. Frogs must stay moist and most are found near water sources, although there are a few rare exceptions, such as the desert dwelling Australian water-holding frog, which burrow to retain moisture. Frogs tend to come in greens and browns, to blend in with their environment. Frogs with toxins, however, advertise this fact with bright displays of reds, yellows, and blues, called aposematic coloration. Poison dart frogs are the most famous for this and they collect their poison from the beetles and ants they consume. There are no venomous frogs, but there are many that are poisonous. To be venomous, the toxins need an active delivery method such as fangs, while poisonous animals use a more passive method, with their toxins being ingested or absorbed.