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Mertens was born in Saint Petersburg, Russia. He moved to Germany in 1912, where he earned a doctorate in zoology from the University of Leipzig in 1915. During World War I he served in the German army.
Mertens worked at the Senckenberg Museum in Frankfurt for many years, beginning as an assistant in 1919, and retiring as Director Emeritus in 1960. He also became a lecturer at Goethe University Frankfurt in 1932, and became a Professor there in 1939. Both jobs provided him with ample time for extensive travel and the study of lizards. He collected specimens in 30 countries.
Nine species and two subspecies of reptiles have been named after Mertens: Amblyrhynchus cristatus mertensi (San Cristóbal Marine Iguana), Amphisbaena roberti (Robert's worm lizard), Chalcides mertensi (Algerian three-toed skink), Cryptoblepharus mertensi (Mertens' snake-eyed skink), Erythrolamprus mertensi (Mertens' tropical forest snake), Liolaemus robertmertensi (Robert's tree iguana), Micrurus mertensi (Mertens' coral snake), Phalotris mertensi (Mertens' lizard-eating snake), Phelsuma robertmertensi (Mertens' day gecko), Tropidoclonion lineatum mertensi (Mertens' lined snake), and Varanus mertensi (Mertens' water monitor). Also Vanderhorstia mertensi (Mertens' shrimp goby) has been named after him.
Mertens died after being bitten while feeding his pet savanna twigsnake, Thelotornis capensis. Because no applicable antivenom existed at that time, he suffered for 18 days before dying. He kept a diary of his deteriorating condition, noting that it was "für einen Herpetologen einzig angemesene Ende [= for a herpetologist a singularly appropriate end]."
- Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). The Eponym Dictionary of Reptiles. Baltimore: Johns Hopkins University Press. xiiiISBN 978-1-4214-0135-5. ("Mertens", p. 176; "Robert", p. 223; "Robert Mertens", p. 223).
- Reptile species described by Mertens, according to the Reptile Database.
- Amphibian species described by Mertens, according to Amphibian Species of the World 6.0, an Online Reference. American Museum of Natural History.
- Reptile species named after Mertens, according to the Reptile Database.
- Reptile subspecies named after Mertens, according to the Reptile Database.
- Adler, Kraig (1989). Contributions to the History of Herpetology. Society for the Study of Amphibians and Reptiles (SSAR).