Page 394 - Environment: The Science Behind the Stories

air was safe for them to breathe, studying how wild animals Following Guillette’s work, researcher Tyrone Hayes
respond to pollution and other hazards can help us detect envi- (Figure 14.14b) found similar reproductive problems in
ronmental health threats before they do us too much harm. frogs and attributed them to atrazine. In lab experiments,
Wildlife toxicologists use a variety of approaches in their male frogs raised in water containing very low doses of the
research. When scientists were zeroing in on the impacts of herbicide became feminized and hermaphroditic, develop-
DDT, one key piece of evidence came from museum collec- ing both testes and ovaries. Hayes then moved to the field to
tions of wild birds’ eggs from the decades before synthetic look for correlations between herbicide use and reproductive
pesticides were manufactured. Old eggs from museum col- impacts in the wild. His field surveys showed that leopard
lections had measurably thicker shells than the eggs scientists frogs across North America experienced hormonal problems
were studying in the field from present-day birds, indicating in areas of heavy atrazine usage. His work indicated that atra-
that something was thinning the present-day shells. zine, which kills plants by blocking biochemical pathways in
Often wildlife toxicologists work in the field with ani- photosynthesis, can also act as an endocrine disruptor.
mals to take measurements, document patterns, and generate
hypotheses, before heading to the laboratory to run controlled
manipulative experiments to test their hypotheses. The work Human studies rely on case histories,
of two of the pioneers in the study of endocrine disruptors epidemiology, and animal testing
illustrates the approaches embraced in wildlife studies.
Biologist Louis Guillette studied alligators in Florida In studies of human health, we gain much knowledge by study-
(Figure 14.14a) and discovered that many showed bizarre repro- ing sickened individuals directly. Medical professionals have
ductive problems. Females had trouble producing viable eggs, long treated victims of poisonings, so the effects of com-
young alligators had abnormal gonads, and male hatchlings had mon poisons are well known. Autopsies help us understand
too little of the male sex hormone testosterone while female what constitutes a lethal dose. This process of observation
hatchlings had too much of the female sex hormone estrogen. and analysis of individual patients is known as a case his-
Because certain lakes received agricultural runoff that included tory approach. Case histories have advanced our understand-
insecticides such as DDT and dicofol and herbicides such as ing of human illness, but they do not always help us infer the
atrazine, he hypothesized that chemical contaminants were dis- effects of rare hazards, new hazards, or chemicals that exist
rupting the endocrine systems of alligators during their devel- at low environmental concentrations and exert minor, long-
opment in the egg. Indeed, when Guillette and his co-workers term effects. Case histories also tell us little about probability
compared alligators in polluted lakes with those in cleaner and risk, such as how many extra deaths we might expect in a
lakes, they found the ones in polluted lakes to be suffering far population due to a particular cause.
more problems. Moving into the lab, the researchers found For such questions, which are common in environmen-
that several contaminants detected in alligator eggs and young tal toxicology, we need epidemiological studies, large-scale
could bind to receptors for estrogen and reverse the sex of male
embryos. Their experiments showed that atrazine appeared
to disrupt hormones by inducing production of aromatase, an
enzyme that converts testosterone to estrogen. CHAPTER 14 • Envi R onm E n TA l H EA lTH A nd T o xi C ology

(a) Louis Guillette taking blood sample from alligator (b) Tyrone Hayes in lab with frog
Figure 14.14 Wildlife studies examine the effects of toxic substances in the environment. Research-
ers Louis Guillette (a) and Tyrone Hayes (b) found that alligators and frogs, respectively, show reproductive
abnormalities that they attribute to endocrine disruption by pesticides. 393

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