The discoveries made by Béland Martineau, and De Guise during
their investigation of the St. Lawrence belugas raise broader questions
relevant to animal populations everywhere.
was the case in the St. Lawrence, researchers have commonly blamed
the decline and disappearance of wildlife populations on human disruption
of their habitat, excessive hunting, fishing, and trapping, or on
the introduction of aggressive foreign species that overwhelm native
competitors. All these forces are unquestionably at work in the
global loss of animal species, but biologists find that they do
not explain all the declines.
In light of the growing evidence that many synthetic chemicals disrupt
hormones, impair reproduction, interfere with development, and undermine
the immune system, we must now ask to what degree contaminants are
responsible for dwindling animal populations. Could hormone disruptors
account wholly or in part for some losses that have been blamed
on classically invoked factors such as habitat loss or overexploitation?
Have overexploited species failed to rebound after protection because
synthetic chemicals are impairing reproduction?
these questions has already prompted surprising reassessments, even
regarding one of the most closely monitored animal populations in
the United States-the critically endangered Florida panther. The
decline of the big cat, which has come to symbolize the effort to
restore the much abused Everglades, had been blamed on reproductive
problems caused by inbreeding, human encroachment, road kills, and
mercury contamination. Seeking to halt the panther's slide toward
extinction, state and federal officials constructed thirty-six specially
designed wildlife underpasses along Alligator Alley, a highway running
across the Everglades where a number of panthers have been killed.
panthers' range in southern Florida, which includes Everglades National
Park and the Big Cypress swamp, lies downstream from major agricultural
areas and consequently suffers from pesticide and fertilizer pollution.
But until quite recently, no one had considered synthetic chemicals
a factor in the panthers' plight.
first clue came in 1989. Prompted by the death of an apparently
healthy female in Everglades National Park, federal and state wildlife
agencies began a study of the remaining panthers, which number no
more than fifty. Wildlife specialists concluded that the female
had died from mercury poisoning, which they attributed to the fact
that Florida panthers prey heavily on raccoons and are therefore
linked through the raccoon to the aquatic food chain where mercury
and other contaminants accumulate. But the study showed the panthers
had a host of other problems as well. These included apparent sterility
in some males and females, an extraordinary level of sperm abnormalities,
low sperm count, evidence of impaired immune response, and malfunctioning
thyroid glands. Thirteen out of seventeen males had undescended
testicles, and records on the population showed that the incidence
of this problem had increased exponentially in male cubs since 1975.
Those investigating the panthers attributed their poor reproduction
and related symptoms to lack of genetic diversity resulting from
inbreeding in the tiny population.
as U.S. Fish and Wildlife Service contaminant specialist Charles
Facemire became aware of the emerging information on hormone-disrupting
synthetic chemicals, he began to question whether bad genes were
really the problem. In his research, he had found that the panthers
were not particularly inbred compared to other large felines and
that their genetic diversity was, in fact, slightly above average.
At the same time, he was learning that undescended testicles are
a known consequence of prenatal hormone disruption.
the panthers had suffered hormone disruption in the womb, he learned,
it might be evident in the hormone ratios in their blood-specifically
in the relative levels of testosterone, the typically male hormone,
and estrogen, the typically female hormone. One would expect males
to have far higher levels of testosterone, but an analysis of the
panthers' blood found ratios that seemed peculiar indeed and suggested
that many of the males had been "feminized." Two males
had far more estradiol, a form of estrogen, in their blood than
testosterone. In several others, estradiol was present at nearly
equal levels to testosterone. Although such ratios appear highly
abnormal, no definitive conclusion is possible until further work
determines normal hormone ratios in closely related mountain lions
hormone disruption theory took on even more power when Facemire
reviewed archived data on contaminants in the animals, for the records
showed that the panthers carry high levels of several synthetic
chemicals that are known to disrupt hormones. Besides lethal levels
of mercury, the fat of the female found dead in 1989 contained 57.6
parts per million of DDE, a breakdown product of the pesticide ,
DDT, as well as 27 parts per million of PCBs, a persistent industrial
chemical. At the same time, new findings by Environmental Protection
Agency reproductive toxicologist Earl Gray indicate why DDE may
be affecting the development of male panther cubs. DDE has long
been described as a weak estrogen, but Gray's studies have demonstrated
that it is also a potent blocker of male hormones.