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
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.
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.
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
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.
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
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.
Facemire CF, et al. (1995). "Reproductive Impairment in the Florida Panther:
Nature or Nurture?" Environmental Health Perspectives 103(Supplement
Toppari J, et al. (1996). "Male Reproductive Health and Environmental
Xenoestrogens". Environmental Health Perspectives 104(Supplement 4):741-803.