Myoglobins are proteins found in muscle fibers and they store and carry oxygen. They also bind carbon monoxide (CO). Myoglobins of Loxodonta africana and Elephas maximus are different from myoglobins of most other mammals. Most significantly elephant myoglobins react with CO nearly eight times more strongly than other myoglobins. This means that elephants housed close to expressways (where emission of CO from motorvehicles is greatest) would be affected by the toxic gas more than other animals would. On the other hand, elephant myoglobin resists oxidation to a greater extent than normal myoglobins and, thus, it is more stable to actions of certain toxins.
Mizukami, H., & Bartnicki, D. E. (1986). Unusual Myoglobin of Elephant. Elephant, 2(2), 80-81. Doi: 10.22237/elephant/1521732015