In addition to the chemical-specific properties that affect bioaccumulations, the animals themselves differ in the degree a chemical is accumulated. Some of the most important factors that differ among animals that result in differences in bioaccumulation (and BAF) are lipid content, feeding ecology, habitat use, reproduction, age, biotransformation ability, and energy demand.
Lipid content is important as this is the organic phase into which the organic compounds dissolve. However, some compounds such as metals and fluorinated chemicals are associated to the animal's proteins in the muscle. The animal's feeding ecology is important as this determines the exposure to the contaminants from the diet, just like habitat use is important in an environment where the chemicals are not evenly distributed, but may depend on water masses, depth, sediment type, etc. Reproduction is important as an elimination pathway, especially for female mammals, due to maternal transfer of lipid-soluble contaminants from the mother to the offspring. Often this results in a buildup of contaminants in male mammals with age, as they do not have this pathway of elimination, whereas females reach a steady-state level sooner. Biotransformation ability is the ability of an animal to transform the accumulated chemical into another, preferentially a more-water-soluble compound that can be eliminated. This biotransformation ability depends on the enzymatic activity of the animal, and this is highly species specific. The difference in biotransformation results in a pattern of contaminants that differ widely from the lower end of the food web to animals that occupy higher trophic levels. In general, coldblooded species have a poor ability to biotransform the mixture of chemicals they have absorbed from the diet or from the water, and they therefore often reflect the pattern of chemicals seen in the water or the diet. Warmblooded species on the other hand have a greater enzymatic ability, and can modify to different degrees the accumulated contaminant mixture, resulting in a contaminant pattern of persistent compound and persistent metabolites formed in the biotransformation process. Warmblooded animals also have a higher energy demand than coldblooded species, due to their requirement of a high and stable body temperature. This higher energy demand results in a higher bioaccumulation than in coldblooded species; for a fish and a bird with the same diet and body size, the bird will accumulate higher contaminant levels than the fish.
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