Forster was the naturalist on James Cook's second Pacific voyage, during which he was accompanied by his son Georg. 

His Descriptiones animalium was completed within a month of returning to England with Cook, but remained unpublished until it was  edited by Hinrich Lichtenstein and published in 1844. It contains some very detailed descriptions of the Cape animals, Promontorium Bonae Spei (pp. 362–410), also a listing of the animals of Madeira and Ascension.

"From a scientific point of view, Forster’s most important work would have been the Descriptiones animalium - but these were only rediscovered and published in 1844 by Hinrich Lichtenstein (1780–1857), the director of the Berlin Natural History Museum. The Descriptiones were a zoological survey and description of the animal species discovered on the world voyage with Cook. In the manuscript, Forster had ordered the animals according to their geographical origin, and described them using the Linnaean method. As the manuscript remained unpublished during his lifetime, Forster could not reap the fruits of his labor and even had to watch other naturalists such as Johann Friedrich Gmelin and John Latham claim the first descriptions of animals he had actually already recorded“ (Mariss, Johann Reinhold Forster and the making of Natural History of Cook’s Second voyage …. 2019, 25).

Digital facsimile from Biodiversity Heritage Library at this link.