Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers
 
 

 

 

An excerpt from Chapter 9, Chronicle of Loss

 

 
 


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.

As 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?

Asking 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.

The 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.

The 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.

But 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.

If 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 and cougars.

The 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.


 

 

 

 

 

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