Pollutant-induced abnormal behaviour

Pollutant-induced abnormal behaviour refers to the abnormal behaviour induced by pollutants. Chemicals released into the natural environment by humans impact the behaviour of a wide variety of animals. The main culprits are endocrine-disrupting chemicals (EDCs), which mimic, block, or interfere with animal hormones. A new research field, integrative behavioural ecotoxicology, is emerging.^[1]

This topic is of special concern for its conservation and human health implications and has been studied greatly by animal behaviourists, environmental toxicologists, and conservation scientists. Behaviours serve as potential indicators for ecological health. Behaviour can be more sensitive to EDCs than developmental and physiological traits, and it was the behaviour of eagles that first drew attention to the now well-known dangers of DDT.^[2] However, behaviour is generally difficult to measure and can be highly variable.

Behaviours which are critical for survival, such as reproductive and social behaviours, and cognitive abilities like learning can be affected directly or indirectly by chemical pollutants— many examples have been documented, and their chemical culprits have been identified.

EDCs known to alter behaviour^[2]

• Atrazine (pesticide)
• Bisphenol A
• Carbaryl
• Cypermethrin
• DDT
• DEHP
• Dioxin
• Endosulfan
• Fenarimol
• Fenitrothion
• Kepone
• Lead
• Mercury
• Methoxychlor
• Nonylphenol
• PCBs
• Vinclozolin
• 17β-trenbolone (agricultural pollutant)

Determining the link between such pollutants and altered behaviours often requires both field studies and laboratory studies. Field studies are useful in determining whether behavioural changes appear with pollution levels occurring in the environment, while laboratory studies can be used to clarify the mechanisms connecting an environmental pollutant to specific behavioural changes.

Mechanisms^[2]

EDCs affect the synthesis, storage, release, transport, clearance, receptor recognition, binding, or post-receptor responses of hormones. This results in either stimulative or inhibitive effects, resulting in overproduction or underproduction of hormones. The effects of hormones on behaviour have been well studied, and often produce direct behavioural effects by acting on the central nervous system. Indirectly, behaviours may be altered by hormones influencing an animal's metabolism or other important processes.

Since behaviours also influence hormones, chemical pollutants that induce behavioural changes may also affect hormone levels, which could result in more behavioural or other changes.

Applying Tinbergen’s four questions^[1]

Studies into the mechanisms underlying behavioural adjustments fall into a category of animal behaviour research described by Tinbergen.

Studies of animal behaviour typically pertain to one of Tinbergen’s four questions, and these can be applied to studies regarding chemical pollution. Questions of causation focus on how pollutant-exposure disrupts the mechanisms behind normal behaviour. For example, when differences in sexual behaviours were noted in wildlife after the introduction of DDT, biochemical experiments on rats were able to show that the pollutant was inhibiting androgen binding to androgen receptors.^[3]

Secondly, questions of ontogeny consider how exposure disrupts the development of behaviours. An example is when researchers examined the effects of an aerosol on the spatial learning of mice.^[4] Thirdly, questions of adaptation consider how behavioural modifications resulting from exposure will influence fitness. Scientists have investigated the reproductive success of white ibises exposed to Methylmercury, for instance.^[5] Lastly, questions of phylogeny consider how phylogenetic history might predetermine sensitivity or resistance to pollutants in a particular behaviour. This could include investigating how animals that are better at learning might be better at avoiding toxins in the environment.

Effects on reproductive behaviours

Reproductive behaviour effects may involve changes in courtship and mating behaviours, mate choice, or changes in nest building.^[2] Most studies on this topic have been conducted on fish and birds. For example, treating adult male zebra fish with biphenol A for 7 weeks resulted in decreased courtship behaviour of females.^[6] 17β-trenbolone exposure in adult guppies and mosquitofish also altered female mate selection, as they preferred unexposed males.^[6] Guppies treated with atrazine during breeding and through gestation were less likely to engage in and showed fewer numbers of courtship displays and other reproductive behaviours. Additionally, females preferred untreated males.^[6]

Studies on birds show significant effects of EDCs on mating songs and displays. For example, treating female zebra finches with PCBs before egg laying resulted in a size reduction in the song centres of the chick’s brains.^[6] Methylmercury exposure at environmental levels for 3 years in male white ibises resulted in increased homosexual behaviour, decreased rates of key courtship behaviours, and less attractiveness to females.^[5] Mammals are also susceptible, and effects on individuals have been shown to have transgenerational and even population-level consequences. Illustrating this, female rats three generations removed from vinclozolin exposure show changes in mate preference, preferring unexposed mates, while male rats do not, and this could have complex effects on the population.^[7]

Conservation implications

Chemical-induced changes in animal behaviour often have consequences for wild populations. The effects of concern aren’t limited to reproductive effects, which have obvious implications for population vitality. For example, frogs exposed to pesticide-levels found in the environment demonstrate hyperactivity, whip-like convulsions, and depressed avoidance behaviour, which may increase their vulnerability to predation.^[2]

As well, guppies from crude oil-polluted environments are less exploratory after both short-term and long-term exposure. This may weaken their foraging efficiency and resource-use diversity, thus posing a threat to the population viability.^[8] This topic is therefore quite important for understanding how human-impacts on the environment may threaten populations. Additionally, if abnormal behaviours can be used as indicators of toxic pollution, then this provides a much more accessible mode of toxicology science. Therefore, there is potential for engaging citizen scientists in environmental research.

References