[5] After a stay in Paris, Rengger decided, together with doctor M. Longchamp from the canton of Vaud, to undertake a research trip to South America.
So they went up to the Entre Ríos Province, waited for eight months in Corrientes because the leader Artigas had blocked the stream traffic, and didn't reach Asunción until July 30, 1819.
[5] Afterwards, Rengger began to explore the wildlife of Paraguay and described himself the way he conducted his research: I lived for six years in these lands whose capital, Asunción, was my usual abode.
At the same time, I spared neither effort nor cost of keeping alive animals and raising them in our dwelling, which gave me much new information about their behavior and their temperament, and especially about the changes that they undergo with age.
Despite this scientific activity, there was a lack of contact with educated men and correspondence with foreign countries, especially with the homeland, because the dictator had all incoming and outgoing letters embezzled, so that the relatives of Rengger received only a little news of his situation.
After a short stay in Paris, where he became acquainted with Alexander von Humboldt and Georges Cuvier, Rengger arrived in Aarau on March 16.
Rengger then completed his work of the natural history and published his book on the mammals of Paraguay, which met with great interest in scientific circles (1830).