Final answer:
Across the Eocene-Oligocene boundary, Catarrhines may have experienced a reduction in size as an adaptation to cooler temperatures, following ecological principles like Bergmann's rule.
Step-by-step explanation:
The size of Catarrhines, or Old World monkeys, at the Eocene-Oligocene boundary experienced a transition influenced by evolutionary and environmental changes. During the Eocene epoch, there was a trend for some primates, which includes Catarrhines, to evolve larger brains and eyes while their muzzles became smaller. By the end of the Eocene epoch, cooler temperatures or competition from the first monkeys led to the extinction of many prosimian species, ancestors to modern primates. Lower temperatures may have also resulted in a decrease in body size for some surviving primate lineages to adapt to the new conditions at the onset of the Oligocene period. While not explicitly mentioned in the provided references, general patterns of mammalian response to cooler climates can include a reduction in size, which falls in line with the ecological principle known as Bergmann's rule, stating that animal sizes increase with colder climates. Hence, a decline in temperatures at the Eocene-Oligocene boundary likely influenced the size and survival of Catarrhines during this period.