Yet some studies show higher cadmium levels in females than males with apparently similar exposure. In many cases, cadmium levels are higher in adults than in young animals, although this is not always the case. Likewise, once animals have reached maturity, cadmium levels may show a continued increase with age in some species, but not others. Geographical variations in cadmium levels within an animal group relate to both anthropogenic sources as well as natural sources of cadmium. Trophic level considerations differ among different classes of vertebrates, but there is evidence for bioaccumulation under some circumstances, particularly in birds.
Invertebrates are more susceptible to the effects of cadmium than are birds and mammals.
Seabirds, having been exposed to relatively high cadmium levels in their invertebrate marine foods, seem to have evolved a greater tolerance or abilty to handle cadmium that species living on or near land. The effects level for most mammals and most birds is well below the level that causes effects in seabirds. One problem with interpreting the effects of cadmium on animals is that while there is considerable literature on the levels of cadmium in nature, and some studies on the effects of cadmium from laboratory studies, few laboratory studies measure tissue levels. Thus it is difficult to determine whether the levels found in nature are detrimental, and the degree of sublethal effects and conversely it is difficult to determine whether levels associated with effects in laboratory studies are relevant to exposure under natural conditions.