Prior to the invention of cameras, scientists and explorers would register the animals they encountered by drawing them. These illustrations were often made in watercolour because the medium was both portable and versatile. Before the disciplines of biology and ecology proliferated into the myriad fields we know today, a person who went out into nature to observe and catalog the things they saw was called a "naturalist".
Probably the most famous naturalist of all was Charles Darwin, who traveled the southern hemisphere in the 1830's aboard his ship, the Beagle. Another famous naturalist, and contemporary of Darwin's, was Alfred Russel Wallace (my dad's hero) who cataloged the flora and fauna of Indonesian Archipelago and the Amazon Basin. Making drawings of the organisms they encountered was crucial in their joint development of the Theory of Evolution as it allowed for side by side phenotypic comparison between species that didn’t share the same ecosystems.
This illustrative style tries to be accurate while at the same time being quick and efficient; hence the no-frills aesthetic. If your subject disappears into the forest you never know if or when you'll see another one!