A major environmental problem getting increasing attention is the effect of synthetic chemicals called endocrine disruptors that interfere with the normal functions of an organism's hormone system and can therefore cause reproductive, behavioural and neurological problems, as well as interfere with the immune system and is also a cause of tumours.

Last week I looked at how these synthetic chemicals are formed, some of the common types of disruptor chemicals and their general effects on organisms. Of course not all organisms will be affected in the same way, for example, invertebrates have hormone and proteins unique to their group, and not found in vertebrate animals. However when it comes to organisms in the wild, the issue of endocrine disruptors becomes complicated. A major problem in predicting how endocrine disruptors affect the environment and those organisms that call it home is that most of the research done has been on laboratory animals. Another problem is lack of knowledge, because although research is being done on animals in the wild, there are many species whose body systems and their reactions to pollutants that have not been researched. This week I will focus on the factors influencing how these chemicals affect animals in the wild.

A factor that affects how animals in the wild react to endocrine disruptors is how they develop as an embryo. For instance, some animals are viviparous, that is, they are live births, so that the offspring are born looking like miniature copies of their parents. So for the entire time during their development they were inside their mother, if their mother was exposed to these disruptors, so was the offspring. However, in species which are contained in eggs, the amount of contact with the mother and environment is limited. Once in the environment, the egg also provides protection to the developing embryo inside, by acting as a physical barrier against these disruptors.

Some animals also go through radical changes as they move from one life stage to another, for instance, the metamorphosis in amphibians from tadpoles to frogs. Some invertebrate groups also moult or exhibit limb regeneration, pigmentation, pheromone production and diapause. These processes further complicate how endocrine disruptors would affect these animals.

In order to better understand the effect of endocrine disruptors on animals, I will deal with the four main groups; mammals, birds, reptiles and amphibians in turn as each group has specific traits that influence how these chemicals affect them.

The first group is mammals, and those whose diet consists of mainly fish might be more susceptible to endocrine disruptors, for reasons, such as their dependence on aquatic organisms for food, therefore they are constantly exposed to disruptors that may be found in aquatic environments. Their position in the food chain means that they may be subject to bioaccumulation. Simply put, organisms low in the food web become contaminated from their environment or by eating other contaminated plants or animals, as one moves up the food web, contaminated organisms eat other contaminated organisms. Therefore the amount of toxins – in this case the endocrine disruptors, have reached quite high concentrations when they are consumed by mammals. Finally they may be aspects of their reproductive physiology that need to be taken into account – such as embryonic diapause. This condition occurs in females of some animal species, such as, kangaroos, where around the time she gives birth she also becomes receptive to males and mates. The embryos that result develop as far as a hollow ball of cells and enter a suspended state of animation caused by the secretion of the hormone prolactin.

Another group of animals that may be affected by these disruptors are birds. Again, like mammals, the biology of birds includes some unique traits that may make them susceptible to endocrine disruption. One such trait is their high metabolic rate and can consume as much food as a mammal of similar size, thus increasing their exposure to contamination via food. Migration by some species also increases their risk of exposure to contaminants.

Birds also undergo periods where large amounts of energy are expended, like during migration, courtship, breeding and parental care. These periods are also accompanied by periods of starvation. Birds respond to these situations by mobilising stored lipids. Therefore they are more susceptible to lipophilic contaminants, those that are attracted to the fats in the body.

With respect to reptiles, as mentioned earlier, some species lay eggs and that they shell gives some form of protection. However, not all species produce eggs with tough shells, so that contaminants are able to cross through the shell to the developing embryo. Also some species bury their eggs, therefore, increasing the risk of contamination from the substrate, for instance, chemicals that have leached through the soil and remained there. In temperature dependent sex-determination (TSD) reptiles, like crocodiles, many turtle and lizard species, it has been shown that the incubation temperature determines if the offspring that hatches will be either male or female. However, research has shown that in addition to this, steroid hormones such as oestrogen and testosterone and in turn endocrine disruptors, play a major role in sex determination.

Finally in the amphibians, that spends time in both water and on land, is exposed to both aquatic and terrestrial sources of endocrine disruptors. Their semi-permeable skin makes it a good substrate for these disruptor chemicals to pass through. Also many amphibian species hibernate, during which time they remain submerged in the substrate thus making them susceptible to contaminants stored there.

The problem of endocrine disruptors in our environment has shown us how unique our "wild" biodiversity is. Their unique traits and life stages greatly influence how these synthetic chemicals affect them compared to its influence on laboratory animals. One of the major drawbacks in fighting this problem is the lack of knowledge with respect to the life history, ecology and behaviour of many species. Therefore it can be seen that much research is needed in this area if we are to protect our biodiversity from this threat.