1071

Are Pollutants Affecting the SexualDevelopment of Florida’s Alligators?Alligators are among the most interesting of animals for abiologist to study. Their ecology is closely tied to the envi-ronment, and their reptilian biology offers an interestingcontrast to that of mammals like ourselves. Studies of alliga-tor development offer powerful general lessons well worthyof our attention.

In no area of biology is this more true than in investiga-tion of alligator sexual development. The importance is notbecause sexual development in alligators is unusual. It is not.As with all vertebrates, sexual development in alligatormales—particularly development of their external sexual or-gans—is largely dependent on the androgenic sex hormonetestosterone and its derivatives. In the alligator embryo,these steroid hormones are responsible for the differentia-tion of the male internal duct system, as well as the forma-tion of the external genitalia. After the alligator’s birth, an-drogenic hormones are essential for normal maturation andgrowth of the juvenile male’s reproductive system, particu-larly during puberty.

The strong dependence of a male alligator’s sexual de-velopment on androgens is not unusual—mammals showthe same strong dependence. So why should researchers beinterested in alligators? In a nutshell, we humans don’tspend our lives sloshing around in an aquatic environment,and alligators do. Florida alligators live in the many lakesthat pepper the state, and, living in these lakes, they are ex-posed all their lives to whatever chemicals happen to enterthe lakewater by chemical spills, industrial wastes, and agri-cultural runoff.

The androgen-dependent sexual development of alliga-tors provides a sensitive barometer to environmental pollu-tion, because the androgen response can be blocked by aclass of pollutant chemicals called endocrine disrupters.When endocrine-disrupting pollutants contaminate Floridalakes, their presence can be detected by its impact on thesexual development of the lakes’ resident alligators. Just asthe death of coal miners’ canaries warn of the buildup ofdangerous gas within the shaft of coal mines, so disruptionof the sexual development of alligators can warn us of dan-gerous chemicals in the environment around us.

One of the great joys of biological research is being ableto choose research that is fun to do. Few research projectsoffer the particular joys of studying alligators. With StateGame Commission permits, researchers go to lakes in cen-tral Florida, wait till after dark, then spend the night on thelake in small boats hand-capturing the animals. The cap-tured animals are confined in cloth bags until sex can bedetermined, body measurements made, and blood samplescollected, and then released (see photo above).

For over ten years, Louis Guillette of the University ofFlorida, Gainesville, and his students have been carryingout just this sort of research. Their goal has been to as-sess the degree to which agricultural and other chemicalshave polluted the lakes of central Florida, using as theirgauge the disruption of normal sexual development inalligators.

To assess hormonal changes that might be expected toinhibit male sexual development, Guillette’s team lookedat the relative ratio of androgens (which promote maledevelopment) to estrogens (which promote female devel-opment) in each captured alligator. Some male endocrinedisrupters act like estrogens, while others decrease nativeandrogen levels. In either case, the ratio of estrogen toandrogen (the E/A ratio) increases, producing a more estrogenic environment and so retarding male sexual de-velopment. Particularly after puberty, the growth of malealligators’ external sexual organs is very dependent upon ahigh-androgenic environment. Any pollutant that raisesthe E/A ratio would be expected to markedly inhibit this development.

Part XIVRegulating the Animal

Body

Catching alligators is a job best done at night. Alligators inFlorida lakes, like the one shown here in the hands of ProfessorGuillette, seem to be experiencing developmental abnormalities,perhaps due to pollution of many of Florida’s lakes by endocrine-disrupting chemicals.

Rea

l Peo

ple

Doi

ng R

eal S

cienc

e

rav31208_ch54.qxd 7/19/01 1:39 PM Page 1071

The Experiment

Guillette’s team first looked at animals in two lakes and thenexpanded the research to look at animals from several otherlakes. Alligators were initially collected from LakeWoodruff National Wildlife Refuge and from Lake Apopka.Lake Woodruff is a relatively pristine lake with no agricul-tural or industrial runoff. Lake Apopka, on the other hand,is a large eutrophic lake exposed to various agricultural andmunicipal contaminants. In 1980, the lake experienced apesticide and acid spill from a chemical company, and has ahistory of pesticide contamination by DDT.

Clear comparisons of alligators collected from differentlakes required that animals be captured as nearly as possibleat the same time, to minimize possible variation due to pho-toperiod, temperature, and nutrition. This experimental re-quirement led to truly prodigious feats of alligator catchingby the research team. On a single night in 1994, 40 male al-ligators were hand-captured from Lake Woodruff. The fol-lowing night, 54 males were captured from Lake Apopka. Ina broader study of seven lakes carried out the following year,528 animals were captured during a 17-day period.

The external genitalia and total body length were mea-sured on captured animals. Body-length is a good indicatorof the age of the alligator. Alligators begin puberty at about3 years of age, and this must be taken into account whenmaking comparisons.

Blood samples were taken from each animal in order todetermine the plasma levels of estrogen and an androgen.Investigators measured plasma concentrations of estradiol-17β (E) and testosterone (A). The researchers estimated theE/A ratio, to determine if the internal environment was an-drogenic or estrogenic.

The Results

In most of the seven lakes studied, female alligators showeda much higher E/A ratio than males (graph a above), a nor-

mal result. The exceptions are Lake Griffin and LakeApopka, the most polluted of the lakes. The larger E/A ratioobserved in male alligators caught from these two lakes indi-cates an estrogenic hormonal environment in these animalsrather than the normal androgenic one.

Does this estrogenic environment have an impact on ju-venile sexual development? Yes. Researchers observed thatpostpuberty juvenile males from Lake Apopka and LakeGriffith (where E/A ratios were elevated) exhibited stuntedreproductive organs compared to those found in LakeWoodruff and other lakes (graph b above).

Prepuberty males did not show this effect, exhibiting thesame size external reproductive organs whatever the E/Aratio. This is as you would expect, as organ growth occursprimarily after puberty, in response to androgenic hormonesreleased from the testes.

A primary contaminant found in alligators’ eggs in LakeApopka is p,p'-DDE, a major metabolite of DDT. p,p'-DDEhas been shown to bind to androgenic receptors, and func-tions as an antiandrogen in mammals. The presence of p,p'-DDE reduces the androgenic effect in cells, creating a moreestrogenic environment.

The researchers also measured levels of plasma testos-terone. The plasma levels of testosterone were signifi-cantly reduced in alligators from Lake Apopka comparedto the control animals removed from Lake Woodruff.These reduced levels of plasma testosterone from the LakeApopka alligators also act to reduce the E/A ratio, produc-ing the observed abnormalities in reproductive structures.

1.0

1.5

E/A

rat

io2.0

Griffin Woodruff Jessup Apopka Okeechobee Orange Monroe

0.5

0.0

(a) Lake

FemaleMale

Pen

is ti

p le

ngth

(m

m)

9

8

76

543

2

1

(b) E/A ratio

Older juveniles (3–7 years old)Younger juveniles (under 3 years old)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Alligator sexual development inhibited by contamination. (a) Ratio of estrogen/androgen (E/A) plasma concentrations in large juvenilealligators 3–7 years old. A relatively larger ratio in males (*) is atypical and indicates an estrogenic hormonal environment, as opposed tothe expected androgenic hormonal environment. (b) Sexual development in male alligators, measured by penis length as a function of E/Aratio. In small juveniles under 3 years old, there is no apparent influence. In older juveniles 3–7 years old, there is a pronounced effect,higher E/A ratios retarding sexual development.

rav31208_ch54.qxd 7/19/01 1:39 PM Page 1072