Final answer:
The white substance found on the heads of marine iguanas is uric acid, which is an adaptation allowing them to excrete nitrogenous waste efficiently in their arid habitat.
Step-by-step explanation:
The white stuff on the heads of marine iguanas is uric acid. Marine iguanas are a species of reptiles found on the arid islands off the western coast of South America. These reptiles, as well as birds and most terrestrial arthropods, convert toxic ammonia to uric acid or the closely related compound guanine, also known as guano, rather than urea. The process of converting ammonia to uric acid is much more complex and energy-intensive. The uric acid is water insoluble and is excreted as a white paste or powder, which can be found on the heads of these iguanas. This is a means for them to excrete nitrogenous waste without the need for a large volume of water, which is a beneficial adaptation on the arid islands where they live.