Crocodiles Their Ecology, Management, And Conservation

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ISBN 2-88032-987-6

A Special Publication of the Crocodile Specialist Group of the Species Survival Commission of the International Union for
Conservation of Nature and Natural Resources

International Union for Conservation of Nature and Natural Resources Avenue du Mont Blanc, CH-1196. Gland, Switzerland


ISBN 2-88032-987-6

A Special Publication of the Crocodile Specialist Group of the Species Survival Commission of the International Union for
Conservation of Nature and Natural Resources

International Union for Conservation of Nature and Natural Resources Avenue du Mont Blanc, CH-1196, Gland, Switzerland

(c) 1989 International Union for Conservation of Nature and Natural Resources Reproduction of this publication for educational and other non-commercial purposes is authorized without permission for the copyrigh holder, provided the source is cited and the copyright holder receives copy of the reproduced material. Reproduction for resale or other commercial purposes is prohibited without prior written permission of the copyright holder. ISBN 2-88032-967-6 Published by: IUCN, Gland, Switzerland.


Abercrombie, C. L. IE. Population dynamics of the American alligator


Brazaitis, P. The forensic identification of crocodilian hides and products


Gorzula, S., and A. E. Seijas. The common caiman


Hutton, J. M., and G. F. T. Child. Crocodile management in Zimbabwe


de Klemm, C, and D. Navid. Crocodilians and the law


Magnusson, W. E. Paleosuchus


Messel, H., and G. C. Vorlicek. Growth of Crocodylus porosus in the wild in

northern Australia


Messel, H., and G. C. Vorlicek. Status and conservation of Crocodylus porosus

in Australia


Messel, H., and G. C. Vorlicek. Ecology of Crocodylus porosus in northern



Messel, H., and G. C. Vorlicek. A model for the population dynamics of

Crocodylusporosus in northern Australia


Thorbjarnarson, J. B. Ecology of the American crocodile, Crocodylus acutus


Waitkuwait, W. E. Present knowledge on the west African slender-snouted

crocodile, Crocodylus cataphractus Cuvier 1824 and the west African dwarf crocodile

Osteolaemus tetraspis, Cope 1861


Whitaker, R., and Z. Whitaker. Ecology of the mugger crocodile


Whitaker, R., and Z. Whitaker. Status and conservation of the Asian crocodilians



This volume is a special publication of the IUCN/SSC Crocodile Specialist Group. The papers in this volume originally were prepared as chapters in a volume on crocodilians which was to be commercially published. The members of the CSG first committed to producing the commercial volume at the 7th Working Meeting in Caracas, Venezuela, in 1984. That commitment was renewed at the 8th Working Meeting in Quito, Ecuador, in 1986. Unfortunately, conflicting responsibilities prevented many of the authors from meeting their obligations to the publisher on schedule and the project had to be abandoned. Rather than scrap publication of the chapters that were turned in on time, most of the authors agreed to have their papers published in this special volume in the CSG Proceedings series.
Publication of this volume was supported by contributions from Professor Harry Messel and the University Foundation for Physics, University of Sydney, Australia; the Nixon Griffis Wildlife Conservation Fund of the University of Florida Foundation, Gainesville, U.SA.; and Jacques Lewkowicz of Soci6t6 Nouvelle France Croco, Paris. The opinions expressed herein are those of the individuals identified and are not the opinions of the International Union for Conservation of Nature and Natural Resources or its Species Survival Commission. Phil Hall was scientific editor and managing editor, Rhoda Bryant was copy and style editor.
The International Union for Conservation of Nature and Natural Resources (IUCN) was founded in 1948, and has its headquarters in Gland, Switzerland; it is an independent international body whose membership comprises states, irrespective of their political and social systems, government departments, and private institutions as well as international organizations. It represents those who are concerned at man's modification of the natural environment through the rapidity of urban and industrial development and the excessive exploitation of the earth's natural resources, upon which rest the foundations of his survival. IUCN's main purpose is to promote or support action which will ensure the perpetuation of wild nature and natural resources on a world-wide basis, not only for their intrinsic cultural or scientific values but also for the long-term economic and social welfare of mankind.
This objective can be achieved through active conservation programs for the wise use of natural resources in areas where the flora and fauna are of particular importance and where the landscape is especially beautiful or striking, or of historical, cultural, or scientific significance. IUCN believes that its aims can be achieved most effectively by international effort in cooperation with other international agencies, such as UNESCO, FAO, and UNEP, and international organizations, such as World Wild Fund for Nature (WWF).
The mission of IUCN's Species Survival Commission (SSC) is to prevent the extinction of species, subspecies, and discrete populations of fauna and flora, thereby maintaining the genetic diversity of the living resources of the planet. To carry out its mission, the SSC relies on a network of over 2,000 volunteer professionals working through 100 Specialist Groups and a large number of affiliate organizations, regional representatives, and consultants, scattered through nearly every country in the world.

Clarence L. Abercrombie, III
Wofford College, Spartanburg, South Carolina Florida Cooperative Fish and Wildlife Research Unit,
University of Florida, Gainesville, Florida
Vanity of vanities, saith the Preacher, vanity of vanities, all is vanity.
Ecclesiastics I:2.
The necessary title of this chapter suggests more vanity than I would prefer to confess. To begin with, I am not familiar with "t—he American Alligator". Indeed, modern research indicates that despite relative genetic homogeneity (Adams et al. 1980), the beast varies in demographically important ways from place to place–and perhaps from year to year. That is to say, alligators strongly reinforce the natural historian's fear of generalizations. To make matter worse, I am not particularly confident even about any single-frame "snapshot" of an alligator population at time t; therefore, to project a dynamic "movie" over t, t+1,...., t+n seems indeed the height of vanity. In other words, most of what I personally know about alligators focuses rather narrowly on Florida animals And I admit that even for these populations I cannot offer decent life tables, much less expressions of density dependence. Nevertheless, scientific ignorance about alligators is not unbounded. Some facts are known quite well enough, and these facts necessarily entail general demographic consequences. I shall review these facts, and I shall even venture in some instances to speculate beyond the confines of available data. Still, I hate to promise more than I can deliver, and in an age of longer titles, I would have called this chapter "Alligator Life History: Meditations from a Demographic Perspective." Thus my general strategy is rather simple. I shall examine the alligator literature for relevant life-history data. Supplementing this information with observations recently conducted in Florida, I shall attempt to establish broad ranges for values of several important demographic parameters. This will permit semi-informed guesses about what manner of demographic beast the alligator must be. In other words, my essay's objective is to employ what is known about alligators while speculating on matters which are not yet understood. In particular, I have in the back of my mind three presently unanswered questions, all of considerable scientific and managerial significance: What are alligator survival rates? How do alligators respond to alterations in density? And how are alligators populations affected by demographic catastrophes? Again I admit at the outset that I can do little more than merely raise these interesting questions. But I want to start you readers thinking about them because you all will be the folks eventually to work out the solid answers.



Actually I would have been unwilling to attempt even this modest task without considerable assistance, but, fortunately, alligator biologists have been very willing to share their time and insights. Colleagues that come readily to mind are Tommy Hines, Terri Jacobsen, Mike Jennings, Wayne King, Wendell Neal, Jim Nichols, Jane Packard, Franklin Percival, David Scott, Dave Taylor, Phil Wilkinson, and Allan Woodward. The most creative advice came, of course, from Paul Moler–when he could tear himself away from his eternal pursuit of the noble Pseudobranchus. The writing of this essay was partially supported by a faculty research grant from Wofford College. And, finally, I need publicly to thank the Spartanburg, South Carolina, K-Mart for selling a word processor that even I could afford.
The traditional first cut at alligator population dynamics has been to establish a population size structure and to interpret that structure by way of growth rates into a life table (Nichols et al. 1976b). In Florida we have been deterred from that approach by two basic difficulties. It is, to begin with, exceedingly hard to determine a population's size structure. The general problems in night-light counts are well known (Woodward 1978, Magnusson 1983, Wood et al. 1983), and even if those problems were entirely solved, the counts could provide no information on the demographically crucial sex ratios. Understandably leary of night counts, alligator managers have focused instead on harvest structures. Unfortunately, however, harvests are generally biased against some size classes (Hines 1979, Taylor and Neal 1984). Furthermore, Florida observations suggest that harvest is also seriously biased by sex, a point amplified by Ferguson and Joanen (1983).
The saddest note it, of course, that determining a size structure is the easier half of the lifetable battle. Within the next few years histological techniques for alligator age determine will probably be developed, but presently it is no fun at all to figure growth rates. In Florida, for example, we have learned that growth rates vary from area to area. Within a given area, they vary from year to year. Within a single area-year, they vary from microhabitat to microhabitat. And when all obvious space, time, and habitat variables are controlled, growth rates appear to vary stochastically from gator to gator!
All of this reinforces my reluctance to inflict an empirical growth curve upon an observed population structure (though you really should take a look at Taylor and Neal [1984]). Nevertheless, there is a very real sense in which limited, certain knowledge about growth tells us a great deal concerning the demographic nature of alligators. A newly-hatched alligator is approximately 25 cm in length and weighs about 50 g. If it is a male, a hatchling can eventually grow to be over 4 m long and may increase its weight by 7,000%. Females are significantly smaller; nevertheless, they seldom attain reproductive maturity at much less than 2 m (about 35 kg). This extraordinary increase from hatchling to adult size, a well-known fact, provides a reasonably firm jumping-off point for an analysis of alligator demography.
Let us consider a hypothetical alligator from north-central Florida, where climate dictates a 6.5-month (about 200 day) growing season. Investigations in Florida indicate that age at maturity is not necessarily constant across a given population, and it certainly is not the same throughout the alligators entire range. Mcllhenny (1935) speculated that females might mature in 6-7 years. Although a specially fed captive gator was observed to lay eggs at under 5 years of age (Whitworth 1971), I am reluctant to believe that wild animals successfully nest at ages less than the 9-10 years suggested by Chabreck and Joanen (1979). At the other extreme is the 18-plus years given by Fuller (1981) for North Carolina animals, a figure echoed by Jacobsen (pers. comm.) for alligators



in nutritionally impoverished portions of the Florida Everglades. I shall eventually return to this age- at-maturity question, but for now let us simply assume that a hypothetical female alligator in north-central Florida has, at around age 12 (length about 1.9 m), just reached reproductive adulthood. If we grant her membership in a numerically stationary population, then she can do her part in maintaining the population's stationary size if she produces in her lifetime exactly one daughter that lives long enough to begin her own reproductive career. To see how she might do this, let us consider a simple model of our newly matured female's lifetime production. If D is the expected number of daughters that will survive to begin their own reproductive careers, then


D = (Y) (N) (P),

where Y is the expected number of years before our newly-matured female dies or becomes reproductively senile; N is the expected number of hatchling daughters our female will produce annually across all Y year; and P is the probability that a given hatchling daughter will survive to begin her own reproductive career. (Demographers will note that what I call D would in conventional notation be R[0], the net reproductive rate, calculated in terms of new reproductiveage females rather than hatchling females. Furthermore, I have chosen the nonstandard approach of analyzing the net reproductive rate rather than the finite rate of increase, because the former is calculable in a more easily explained manner from the alligator data we possess).

Each factor in this simplistic equation is actually a combination of many demographic parameters. Let us therefore dissect Equation (1) and indicate apparently reasonable ranges for parameters values.

Y: Expected Years between Maturity and Senescence or Death

Let L represent the probability that a reproductively mature female lives from one year to the next. (Technically the demographer would prefer to talk about L[t], which would represent agespecific survivorship between age t and age t+1. Fortunately, such precision is probably not practically important. Gibbons and Semlitsch (1982) have demonstrated that mortality in large emydid turtles remains approximately constant over time, and examination of Florida harvest sizeclass ratios suggests that the same may be true female alligators, at least over the first 10-15 years of maturity. In any case, the current alligator literature does not suggest important deviations from constant adult survivorship, so I shall simplify the demographic equations and replace L[t] with the single parameter L). Convincing estimates for L do not abound. Nichols et al. (1976a, b) suggest an approximate value of 0.89. Taylor and Neal (1984) believe that survivorship among adult male gators is about 0.775; these authors recognize that female mortality would be lower. Informal observations on radio-telemetered animals suggest to Wilkinson (pers. comm.) that adult female survivorship may be in the neighborhood of 0.95. Given this admittedly sketchy information, it may not be unreasonable to assume initially that adult survivorship is in the 0.85- 0.95 range among female alligators.

To calculate an alligator's potential reproductive years, one must consider not only mortality but also senescence. The time of onset doubtless varies across individuals, and in any case senescent effects are not necessarily sudden (Ferguson and Joanen 1983). Webb et al. (1983b) suggest that female alligator senescence occurs between 40 and 50 years of age. Table 1 gives expected reproductive lifetimes (Y in the equation above) for newly matured female alligators with various fixed survivorship, ages to maturity, and ages at senescence. From this table it is clear that unless annual survivorship is very high, the number of years between expected maturity and expected senescence is relatively much less important than mortality in determining Y. Furthermore, it also appears that Y is likely to lie between about 6 and 18 years.



Table 1. Expected Reproductive Lifetimes

Annual survivorship
0.85 0.85 0.85 0.85
0.90 0.90 0.90 0.90
0.95 0.95 0.95 0.95

Age at maturity
16 12 9 --
16 12 9 --
16 12 9 --

Age at senescence
40 45 50 infinity
40 45 50 infinity
40 45 50 infinity

Expected years as adult (Y)
6.03 6.12 6.15 6.15
8.73 9.20 9.36 9.49
13.80 15.91 17.21 19.50

N: Expected Annual Production of Hatchling Daughters per Mature Female

To avoid getting fancy, I shall express the complex parameter N as


N = (R) (E) (H) (F),

where the various equation components are as defined below.

R: Annual Nesting Probability. R expresses the probability that a reproductive-aged female nests in any given year. Field research in Louisiana suggests values ranging between 0.48 and 0.68 (Chabreck 1966, Joanen and McNease 1971, 1973, 1975, 1976). Working with animals in a thermally altered reservoir (Par Pond, South Carolina), Murphy (1981) believed the proportion of females nesting was less than 34%. Wilkinson (1983) reports about 27.5% for the South Carolina coastal plain. All these values are considerably lower than estimates reported for Crocodylus niloticus (87.6%; Graham 1968) and C. johnstoni (90%; Webb et al. 1983a). Perhaps this interspecific variation is a function of differing energy budgets and of more rigorous metabolic requirements in the alligator's temperate range. In that connection it would be particularly interesting to ascertain the percent of adult female gators that nest in certain subtropical Florida habitats. But for the present let us simply agree that, for alligators in general, the proportion of adult females nesting is probably between 0.2 and 0.7.

E: Probability of Nest Success. E is the probability that any given nest escapes predation, flooding, etc. and hatches. Again, field research presents a bewildering array of values. Metzen (1977) reports nest success of 10%. This occurred, however, in area of heavy black bear infestation and is probably about as unusual as the 90% success which can be observed some places, some



years, in Florida. Presumably more typical are the 48.3% and 74.2% success rates reported by Ruckel and Steele (1984) for two Georgia locations. Dietz and Hines (1980) give 67.9% for Orange Lake, Florida. The rate at Rockefeller Refuge, Louisiana, is about 68.3% (Joanen 1969), in South Carolina it is approximately 70% (Wilkinson 1983; this source reports the proportion of nests from which at least one egg hatched). Discounting the somewhat aberrant findings of Metzen (1977), one might conclude that values for E typically lie between 0.3 and 0.7.

H: Hatchlings per Nest. H is the expected number of living young that a nest will produce, given that the nest is not destroyed. Over the years, a great deal of information has been collected on alligator clutch size and fertility. Representative data on these factors are reported in Table 2 below. Where possible, information from geographically proximate areas was combined; I had to calculate some of the figures below from other types of statistics presented in the cited works.

Even in "successful" nests, there are various reasons that not all fertile eggs hatch, and therefore calculations based on percent fertility overestimate the number of actual hatchlings. On the other hand, difficulties in field observation usually mean that reports of hatchlings actually seen tend to underestimate production. I shall largely neglect these factors and assume that H, production per successful nest, is somewhere between 20 and 40.

F: Proportion of Hatch Female. F is the proportion of living hatchlings that are female. Most information on alligator sex ratios seems to focus on animals beyond the hatchling stage (Forbes 1940, Chabreck 1966, Nichols and Chabreck 1980, Murphy 1981, Murphy and Wilkinson 1982, Wilkinson 1983). Earlier, Ferguson and Joanen (1983) reported a reasonable sample of Louisiana marsh hatchling production as 80% female. On the other hand, Taylor (pers. comm.) believed the sex ratio in a north Louisiana system was close to 50-50. In Florida we have observed individual pods with nearly all imaginable sex ratios. My subjective evaluation is that our population-wide hatchling cohorts are no more than 60% female–and may be significantly less. Since alligator gender is determined by early incubation temperatures, it is entirely possible that hatchling sex ratios may differ substantially by geographical area. Nevertheless, by microhabitat nest-site selection, laying females can exercise some "choice" over the gender of their offspring, and arguments have been presented (Ferguson and Joanen 1983) for the likelihood of female-skewed hatchling production in numerous habitats. Therefore, despite field suggestions that gender ratios may not be so definitely skewed, I shall bow to Ferguson's greater expertise and state that F probably lies between 0.6 and 0.8.

P: Probability that a hatchling Daughter Survives to Reproductive Age.

Even in simplest form, this parameter must involve the growth and survival rates of immature animals. We shall model it as


P = S**M

where the equation components are as defined below.

M: Time of Maturity. M is the expected number of years between hatchling and attainment of reproductive maturity by female alligators. This parameter has already been briefly discussed above; indications are that in most alligators it lies between 8 and 16 years.

S: Average Immature Survival. S is the "average" (geometric mean) annual survival probability for immature female alligators between ages O and M years. (Recall that the geometric mean is necessarily equal to or less than the arithmetic mean.) Our field work in central Florida suggests that appropriate values probably lie between 0.55 and 0.70. This very rough range

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Crocodiles Their Ecology, Management, And Conservation