infertile felines

Dunn is an associate of the Center for Investigative Reporting

Florida's panthers are in serious trouble. And some scientists see unsettling parallels between these large cats and other mammals, including humans. Already an endangered species, Florida's panthers are plagued by low sperm counts, abnormal sperm, undescended testicles, thyroid problems, depressed immune function, and congenital heart defects. Only 30 to 50 of the large cats survive. Until recently, inbreeding was blamed for their fertility problems. But many scientists think that manmade chemicals, like pesticides, are the real culprits.

The panthers' reproductive problems are the most severe of any cat species studied. Panthers have the lowest sperm counts, the lowest semen volume, and the highest number of abnormal sperm on record. In fact, over 90% of their sperm are abnormal. Most of the males also suffer from "cryptorchidism", meaning that one or both testes remain lodged in the abdomen. These undescended testes can contribute to sperm defects. They produce less sperm, and more defective sperm, than testes that descend properly into the scrotum. As a result of the defects, several of Florida's panthers are completely sterile and are unable to impregnate a female.

These fertility problems are eerily similar to reports of undescended testicles and plummeting sperm counts in humans. By some estimates, although the science is controversial, human sperm counts have dropped 50% over the last fifty years. What's more, a study in England showed an approximate doubling in the rate of undescended testes in three-month old boys between the 1950's and 1980's. Similar increases in cryptorchidism have been reported in Scotland and Denmark, although not in the U.S.

Blame it on Inbreeding

For Florida's panthers, inbreeding seemed like a logical source of the defects. Panthers live in the pine forests of south central Florida, where they are increasingly hemmed in by suburban development. In this small area, they are forced to mate with genetic relatives. It appears that the population now consists of only two genetic strains. As a result of inbreeding, these cats may be passing on dangerous recessive genes to their young.

Inbreeding can indeed produce defects like cryptorchidism, which is thought to stem from a recessive gene. Some domestic animals inbred for genetic "purity" pass this defect from generation to generation. In Florida's male panther cubs, cryptorchidism has increased at an astounding rate since 1975-80. At that time, around 15% of male cubs had an undescended testicle. In 1985-90, that number soared to 67%. Today, over 90% of the males are cryptorchid. The increase seemed to parallel an increase in inbreeding.

The Focus Turns to Chemicals

But many scientists think that manmade chemicals, not inbreeding, are the real source of the problem. They blame contaminants called "endocrine disrupters" which can alter reproduction in a wide range of animals. These chemicals disrupt the action of natural hormones, which are crucial for processes like sperm formation and testicular descent. In laboratory mice and rats, some of the chemicals can cause the same low sperm counts and undescended testes seen in Florida's panthers. What's more, they have caused population declines and poor reproductive success in other wildlife species. The chemical culprits include dioxin, p,p'-DDE (a breakdown product of the pesticide DDT), polychlorinated biphenyls (PCBs), and a host of others. Many of them are present in south Florida.

In humans, high doses of endocrine disrupting chemicals produce similar defects. From the 1940's to the 1970's, millions of American women took the drug diethylstilbestrol (DES). This synthetic estrogen, intended to prevent miscarriage, caused serious problems in infants exposed before birth. The most highly exposed sons often have low sperm counts, abnormal sperm, and undescended testicles. These are the same problems afflicting Florida's panthers.

And Florida's panthers show other signs of endocrine disruption. Many of the animals appear to have been "demasculinized" or "feminized" by abnormally high blood levels of estrogen. From 1986 to 1993, researchers collected blood from both free-ranging and captive panthers, and found a disturbing trend. The males had estrogen levels as high as in females--sometimes higher. While no one is quite sure what "normal" hormone levels are for these animals, Florida's panthers appear to have higher estrogen levels than cougars from the London zoo. Cougars can help indicate what may be normal for the Florida panther, since these panthers are a subspecies of cougar.

Furthermore, there is data to discredit the "inbreeding" theory. One study showed that the level of inbreeding in Florida's panther population is not so different from that seen in other groups of large cats who show little sign of cryptorchidism. The study compared captive male cougars in Florida, Texas, Colorado, Latin American, and North American zoos. It found undescended testicles in only two of more than fifty captive animals. And regardless of the amount of inbreeding, the defect has never been reported in any other species of nondomestic cat.

A History of Contamination

Contaminants have a history of affecting Florida's panthers. In July 1989, a four-year old female panther was found dead in Everglades National Park. She was one of only three prime breeding-age females in the area, and concern over her death prompted an investigation. Her tissues were analyzed for pesticides, PCBs, and other manmade contaminants. Toxic levels of mercury, an endocrine disrupter, were listed as the cause of death. Her fat also contained very high levels of two other endocrine disrupters: p,p'-DDE (57 ppm) and PCBs (27 ppm).

Raccoons are an obvious route of exposure. In the heart of Florida's citrus belt, where panthers live, DDT and other pesticides pollute many waterways. The aquatic food chain bioaccumulates these contaminants, which reach high concentrations in the fish. Raccoons feed on the fish, and panthers then feed on raccoons. Chemicals from a lifetime of meals get stored in the panthers' body fat. The panthers that eat more raccoons, and not white-tailed deer, have higher levels of contaminants in their bodies. During pregnancy, females pass the contaminants on to their young, where they can damage the developing reproductive system. Exposure after birth might also contribute to the Florida panther's loss of fertility.

In 1995, Florida's state wildlife agency had plans to import cougars from Texas to mate with the Florida panthers and increase their gene pool. Only time will tell whether contaminants or inbreeding are the cause of the panthers' woes, but many believe it is a deadly combination of both.

Further Reading

Facemire CF, et al. (1995). "Reproductive Impairment in the Florida Panther: Nature or Nurture?" Environmental Health Perspectives 103(Supplement 4):79-86.

Toppari J, et al. (1996). "Male Reproductive Health and Environmental Xenoestrogens". Environmental Health Perspectives 104(Supplement 4):741-803.


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