Growth and Condition of American Alligators (Alligator
mississippiensis) in an Inland Wetland of East Texas
David T. Saalfeld, Kevin K. Webb, Warren C. Conway,
Gary E. Calkins, and Jeffrey P. Duguay
Southeastern Naturalist, Volume 7, Number 3 (2008): 541–550
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2008 SOUTHEASTERN NATURALIST 7(3):541–550
Growth and Condition of American Alligators (Alligator
mississippiensis) in an Inland Wetland of East Texas
David T. Saalfeld1,*, Kevin K. Webb1,2, Warren C. Conway1,
Gary E. Calkins3, and Jeffrey P. Duguay1,4
Abstract - Since removal from the endangered species list, Alligator mississippiensis
(American Alligator) populations have recovered to allow regulated harvest
throughout most of their range. However, harvest/population management is complicated
since alligators are long-lived, reach sexual maturity at a minimum size rather
than age, and experience differential growth rates depending on geographic location,
growing season length, local environmental conditions, habitat, and population
density. To date, few data exist on age, sex, growth, and size structure of inland alligator
populations. In this study, alligator growth rate and condition were quantified
through an intensive mark-recapture study conducted at Angelina-Neches/Dam B
Wildlife Management Area. Between May 2003 and October 2004, 279 alligators
ranging in size from 29.7 cm to 348.0 cm (total length [TL]) were captured, and 48
subadult alligators were recaptured (<125 cm TL). As recaptured individuals were
biased towards smaller individuals, recaptured subadult alligators were divided into
two size classes: size class 1 (<50 cm) and size class 2 (50–125 cm). Mean growth
rates for size class 1 were 32.4 cm/year and for size class 2 were 27.6 cm/year. For
both size classes, mean body condition was 1.8. Overall, subadult alligators within
our inland study area exhibited faster growth rates and lower body condition than
most other populations studied throughout their range.
Alligator mississippiensis Daudin (American Alligators) were listed as
endangered in 1967 under the US Endangered Species Preservation Act;
however, populations have recovered sufficiently to allow regulated harvest
throughout most of their range (Groombridge 1987). Despite a tremendous
volume of research on American Alligators, few long-term data exist on
age and sex structure, growth rates, and size throughout their range (see
Wilkinson and Rhodes 1997). Additionally, American Alligators are longlived
(i.e., up to 80 years), reach sexual maturity at a minimum size rather
than age, and experience differential growth rates (Brandt 1991, Dalrymple
1996, Deitz 1979, Hines et al. 1968, Wilkinson and Rhodes 1997). Although
current alligator management strategies are suitable on short time scales, the
additive or compensatory impacts of harvests upon alligator population age,
size, and sex structure on longer time scales remain unknown. As opposed to
other game species, where gender- and age-specific harvest regulations are
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University,
Nacogdoches, TX 75965. 2Advanced Ecology, Ltd., Center, TX 75633. 3Texas
Parks and Wildlife Department, Jasper, TX 75951. 4Division of Biological and Physical
Sciences, Delta State University, Cleveland, MS 38751. *Corresponding author
542 Southeastern Naturalist Vol.7, No. 3
adjusted annually, alligator harvest is less selective due to lack of sexual dimorphism
and available hunting techniques. Therefore implementing habitat
and/or harvest management strategies is likely more complicated than for
other shorter-lived, rapidly growing species.
Growth rates and morphological variability (e.g., condition) of American
Alligators have been studied in wild populations in South Carolina (Brandt
1991, Wilkinson and Rhodes 1997), Louisiana (Chabreck and Joanen 1979,
Elsey et al. 1992), and Florida (Dalrymple 1996, Deitz 1979, Hines et al.
1968, Jacobsen and Kushlan 1989, Temsiripong 1999). In general, alligator
growth rates vary according to size class, gender, and geographic location
(Brandt 1991, Dalrymple 1996, Deitz 1979, Hines et al. 1968, Wilkinson
and Rhodes 1997). Specifically, growth-rate variability, even within similar
size classes and genders from different geographic locations, results primarily
from differences in resource availability [(Brandt 1991, Roots et at.
1991, Wilkenson and Rohodes 1997), habitat suitability (Brandt 1991, Dalrymple
1996, Deitz 1979, Jacobsen and Kushlan 1989, Rootes et al. 1991),]
population density (Brandt 1991), growing season length (Brandt 1991), and
salinity (Chabreck 1971, Rootes et al. 1991).
Variability in any or all of these factors can impact alligator ecology
and management, particularly if management schemes in one region rely
upon data generated from areas or populations unrelated and geographically
disjunct from populations of interest, where habitat and growth rates may
not be similar. For example, in inland Texas, alligator harvest management
strategies are based upon assumptions that inland and coastal alligators exist
at similar densities and exhibit similar growth rates (Webb 2005). However,
inland wetlands are more heterogeneous and less saline than coastal wetlands
and are often dominated by bottomland hardwood forested wetlands,
river and creek drainages, emergent wetlands, deep and shallow open water,
and fl oating vegetation (Webb 2005). As resource availability, alligator densities,
growing season length, and salinity generally vary between coastal
and inland wetlands (Webb 2005), we hypothesized that growth rates and
condition would also differ between these populations. Thus, the objectives
of this study were to quantify and compare growth rates and body condition
of inland alligators within east Texas to previous studies.
Field Site Description
This research was conducted within east Texas at the 5113-ha Angelina-
Neches/Dam B Wildlife Management Area (Dam B WMA) in Jasper and Tyler
counties, located at the confl uence of the Angelina River, Neches River,
and B.A. Steinhagen Reservoir. A variety of habitats occur at Dam B WMA,
including shallow open lake-emergent marsh, creek channels, river channels,
deep open-water, and swamps/sloughs (Webb 2005). Dominant aquatic
plants observed at Dam B WMA included Eichhornia crassipes, (Mart.)
Solms (Common Water Hyacinth), Salvinia minima, Baker (Common Salvinia),
Alternanthera philoxeroides, (Mart.) Griseb. (Alligator Weed), Hydrilla
verticellata, (L. f.) Royle (hydrilla), Polygonum spp., L. (Smartweed),
2008 D.T. Saalfeld, K.K. Webb, W.C. Conway, G.E. Calkins, and J.P. Duguay 543
and Nelumbo lutea, Willd. (American Lotus). Dominant woody species
included: Taxodium distichum, (L.) Rich. (Bald Cypress), Cephalanthus occidentalis,
L. (Buttonbush), Salix nigra, Marshall (Black Willow), Triadica
sebifera, (L.) Small (Chinese Tallow), Quercus nigra, L. (Water Oak), Quercus
lyrata, Walter (Overcup Oak), Nyssa aquatica, L. (Water Tupelo), and
Pinus spp. (pine species) (Godfrey and Wooten 1981).
Capture and handling
During May–September 2003 and 2004, we captured, marked, and
released alligators at Dam B WMA using several capture techniques (i.e.,
snake tongs, pole snares, hands, and swim in live traps; see Webb 2005
for complete description). Upon capture, alligators were restrained with
duct tape, and each individual >50 cm in total length was sexed by cloacal
examination (Chabreck 1963, Joanen and McNease 1978). Although
Allsteadt and Lang (1995) developed techniques to sex alligators <50 cm,
this technique was not used due to logistical constraints (i.e., minimization
of handling time, poor lighting conditions due to all captures occurring at
night, and small numbers of hatchlings captured). We measured the following
morphological features for each individual: total length (TL, cm;
ventral tip of snout to tip of tail), snout–vent length (SVL, cm; ventral tip
of snout to proximal tip of vent), eye to nare length (cm), total head length
(cm; dorsal tip of snout to distal part of head scute), tail girth (cm, circumference
of tail directly behind rear legs), and mass (g). All length measurements
were taken with a flexible tape measure, and masses were obtained
from Pesola® hanging scales (Baar, Switzerland). We uniquely marked all
captured alligators by at least two of the following: dorsal tail-scute removal,
numbered Monel tags (#681 for alligators ≥152 cm and #1 Monel tags
for alligators <152 cm), or passive integrated transponder (PIT) tags. We
measured all the aforementioned morphological features for all recaptured
alligators. We excluded all individuals recaptured within 12 days of initial
capture from subsequent analyses in order to eliminate any measurement
error resulting in negative growth.
Recapture, growth rate, and body condition estimation
We estimated growth rates using TL; SVL was not used since no significant tail loss was documented. As alligator growth rates are not constant
(i.e., feeding and growth stops or slows during winter; Chabreck and Joanen
1979; Rootes et al. 1991), annual growth rates were adjusted according to
growing season duration as indicated by air and water temperatures. To
estimate alligator growing season length for our study sites, we collected
average daily air temperature data from Jasper, TX (Webb 2005). Assuming
that alligators at Dam B WMA grew after water temperatures rose above
20–23 °C (Brisbin et al. 1982, Coulson and Hernandez 1983), we estimated
alligator growth days at Dam B WMA to be from 1 April–31 October, or
214 days (Webb 2005). Therefore, we calculated daily growth rates for each
recaptured individual by dividing the change in TL by the number of growth
544 Southeastern Naturalist Vol.7, No. 3
days between captures. Daily growth rates were then extrapolated out to
annual growth rates (cm/yr) by multiplying them by growing season length,
or 214 days. Additionally, intrinsic growth rate variable (k), maximum attainable
length (L∞), and age at maturity (assumed to be 1.83 m; Giles and
Childs 1949, McIlhenny 1934, Joanen and McNease 1975, Klause 1984)
were estimated through the construction of von Bertalanffy, Logistic, and
Gompertz growth curves (Chabreck and Joanen 1979, Elsey et al. 1992,
Jacobsen and Kushlan 1989). We fitted each growth curve similar to Fabens’
(1965) modification of a von Bertalanffy growth curve for mark/recapture
data without known ages. We estimated values for k and L∞ by iterated least
squares methods using nonlinear regression (PROC NLIN; SAS Institute
1999). We used Akaike’s Information Criterion (AIC) to select the best,
parsimonious growth curve to fit our data (Akaike 1973).
Condition (K; Le Cren 1951), an index of the relative fatness of an
animal and also an indicator of its well being/health (Taylor 1979), was
estimated from the relationship between length and mass using the equation:
K = M * L-b, where M = mass (g), L = total length (cm) and b = slope
of the regression of ln (TL) and ln (M). If growth is isometric, b would be
approximately equal to 3.
We used analysis of variance (ANOVA; PROC GLM; SAS Institute
1999) to examine differences in growth rates and body condition among
size classes (size class 1 = <50 cm, size class 2 = 50–125 cm, size class 3 =
125.1–160 cm, and size class 4 = >160 cm) and between sexes, where sufficient sample sizes of recaptured individuals were available. For growth
rates analysis, only size class 1 and size class 2 were used because only one
individual from a larger size class was recaptured. An alpha level of 0.05
was used for this analysis, and least squared means separation was used to
examine differences (P < 0.05).
We captured, measured, marked, and released 279 alligators ranging in
size from 29.7 cm to 348.0 cm (TL; Fig. 1) at Dam B WMA from 12 May–
18 August, 2003, and 15 April–9 September, 2004. We captured alligators
using tongs (n = 116), hand grabbing (n = 67), walk-in cage traps (n = 57),
pole snares (n = 35), and other methods (i.e., dowel sets, n = 4). During
this time, we recaptured 49 individuals, 48 of which were sub-adults (<125
cm TL). Only one adult (>183 cm in TL) was recaptured and was excluded
from further analyses.
Mean growth rate for recaptured alligators <125 cm was 29.39 cm/yr (SE
= 2.5), irrespective of size class and sex. Overall growth rates decreased as
size increased for alligators <125 cm (y = -0.259x + 42.516, r2 = 0.594; Fig. 2).
Growth rates were similar (F1, 46 = 0.83, P = 0.368) between size class 1 (mean
= 32.38 cm/yr, SE = 3.0; n = 18) and size class 2 (mean= 27.59 cm/yr; SE =
3.7; n = 30). Additionally, growth rates were similar (F1, 44 = 1.00, P = 0.322)
between sexes. Based on AIC, the best growth model for our pooled data was
2008 D.T. Saalfeld, K.K. Webb, W.C. Conway, G.E. Calkins, and J.P. Duguay 545
von Bertalanffy (Table 1), therefore all further analyses used this model. The
modified von Bertalanffy growth curve (Fig. 3) fitted to our mark/recapture
data provided an estimate of 258.9 cm for L∞ and 0.00606 for k, where we
estimated time to maturity for alligators in our study area to be 10 years.
Overall mean condition for all size classes and sexes combined was 1.84
(SE = 0.06; Fig. 4). Condition ranged from 1.46 to 2.97, depending upon size
class (Table 2), and were similar between sexes (F1, 104 = 0.34, P = 0.710).
However, condition for size class 4 individuals was (marginally) higher
(F3, 104 = 2.78, P = 0.045) than any other size class, and condition was similar
among individuals within size classes 1–3 (P > 0.05).
Figure 1. Length
released at Angelina-
Dam B Wildlife
M a n a g e m e n t
Area, TX, May–
Figure 2. Mean
(cm/year) of recaptured
total length (10-
cm size classes)
caught from Angelina-
Dam B Wildlife
M a n a g e m e n t
Area, TX, May–
546 Southeastern Naturalist Vol.7, No. 3
Subadult alligators (i.e., individuals <125 cm in TL) grew faster in this
study (29.39 cm/yr) than subadults in the Shark Valley region of Florida
(13.3 cm/yr in Jacobsen and Kushlan , 13.6 cm/yr in Dalrymple
), north Florida (11.9–21.1 cm/yr in Deitz , 24.0 cm/yr in
Temsiripong ), South Carolina (14.6 cm/yr in Bara , 23.5 cm/
yr in Brandt , 18.0–20.2 cm/yr in Wilkinson and Rhodes ), and
Louisiana (22.0 cm/yr in Chabreck and Joanen ), but grew at rates
similar to alligators north of Shark Slough, FL (31.0 cm/yr in Hines et al.
Table 1. Akaike’s Information Criterion (AIC), intrinsic growth rate variable (k), and maximum
attainable length (L∞) for each growth curve fitted to American Alligator (Alligator mississippiensis)
mark/capture data from Angelina-Neches/Dam B Wildlife Management Area, TX, during
May–September 2003 and 2004.
Model AIC L∞ k
von Bertalanffy 1154 258.9 0.006
Logistic 1363 140.0 0.004
Gompertz 13,844 116.8 -0.084
Figure 3. Length-at-age relationships (± standard error) derived from fitting a von
Bertalanffy growth curve to mark/recapture data of American Alligators (Alligator
mississippiensis) from Angelina-Neches/Dam B Wildlife Management Area, TX,
collected from May–September 2003 and 2004.
2008 D.T. Saalfeld, K.K. Webb, W.C. Conway, G.E. Calkins, and J.P. Duguay 547
). Additionally, subadult alligators at Dam B WMA grew faster (29.39
cm/yr) than subadults at Mad Island WMA in coastal Texas (14.0–16.0 cm/
yr; M.T. Merendino, Texas Parks and Wildlife, Austin, TX, pers comm.).
Although subadult alligators grew faster in this study, overall condition for
subadults (mean = 1.79) and all size classes combined (mean = 1.84) were
lower than estimated alligator condition for all size classes in Florida (2.5 in
Temsiripong , 2.7 in Rice ).
Although sample size of recaptured individuals was small for this study,
our results corroborate past studies suggesting that geographic variation has
Figure 4. Condition factors of American Alligators (Alligator mississippiensis) captured,
marked, and released from Angelina-Neches/Dam B Wildlife Management
Area, TX, during May–September 2003 and 2004 by total length.
Table 2. Means and standard errors for body condition of (K) American Alligators (Alligator
mississippiensis) captured, marked, and released from Angelina-Neches/Dam B Wildlife Management
Area, TX, during May–September 2003 and 2004 by size class.
Size class Size range (cm) Mean K Standard error
1 <50 1.76 0.25
2 50–125 1.82 0.07
3 125.1–160 1.73 0.05
4 >160 2.84 0.68
548 Southeastern Naturalist Vol.7, No. 3
an important infl uence on both growth rates and condition. Some causes of
geographic variability have been attributed to resource (food) availability
(Brandt 1991, Dalrymple 1996, Deitz 1979, Jacobsen and Kushlan 1989,
Rootes et al. 1991), habitat (Brandt 1991, Rootes et al. 1991, Wilkinson
and Rhodes 1997), growing season length (Brandt 1991) and population
densities (Brandt 1991; M.T. Merendino, pers comm.). Although resource
availability was not examined during this study, current studies are examining
prey selection and densities to directly estimate their infl uence on
growth rates and condition within inland wetlands of east Texas. As suggested
from previous studies (Rootes et al. 1991, Webb 2005), substantially
different habitat (i.e., shallow open lake-emergent marsh, creek channels,
river channels, deep open-water, and swamps/sloughs) combined with lower
alligator densities at Dam B as compared to coastal habitats likely contributed
to faster growth. Alligator density at Dam B WMA was estimated to
be approximately 7.5 ha/alligator (1.52–2.35 alligators/km; K.K. Webb et
al., unpubl. data), lower than the 3.2–5.7 ha/alligator reported in coastal
Louisiana (McNease and Joanen 1978) and the 2.56–9.02 alligators/km reported
in Florida (Wood et al. 1985). Although no studies have specifically
tested the infl uence of population densities on growth rates in alligators,
others have speculated that growth rates are density dependent (Brandt 1991,
Webb 2005). For example, if alligators exist at relatively high densities, and
food resources are limiting, competition for food may increase competition
among alligators leading to decreased growth rates. Schoener and Schoener
(1978) documented density-dependant growth in Anolis lizards, with growth
rates being directly correlated with population density in their study.
Jacobsen and Kushlan (1989) suggest slower growth could affect an alligator’s
age to sexual maturity and increase its susceptibility to predation,
disease, and cannibalism. Due to the higher growth rates documented at Dam
B WMA, time to sexual maturity (10 years) was shorter than the estimated
13–17 years for South Carolina (Murphy and Fuller 1982) and 13–18 years
for the Everglades (Dalrymple 1996, Jacobsen and Kushlan 1989), but similar
to the estimated 8–10 years in Louisiana (Joanen and McNease 1975,
1987). Despite this shorter time to sexual maturity in east Texas, if alligators
are in poor condition when they reach maturity, they may be unable to
reproduce and compete for limited resources (e.g., optimum nesting sites and
prey). This consideration could have important management implications
in terms of long-term viability of alligator populations, particularly those
exposed to regulated hunting pressures.
To date, few studies have focused on inland populations of American
Alligators, especially in Texas. This study indicates there may be important
geographic differences in age at maturity, condition, and growth rates
within subadult alligators between inland and coastal populations. Such
differences could have dramatic effects on alligator population parameters
such as recruitment, survival, and overall population size and age
characteristics. Thus, it may be necessary to modify current management
strategies between inland and coastal populations as such variability in
basic life-history parameters likely requires geographically or regionally
specific management guidelines.
2008 D.T. Saalfeld, K.K. Webb, W.C. Conway, G.E. Calkins, and J.P. Duguay 549
Financial, logistical, and technical support was provided in part by the Texas Parks
and Wildlife Department and McIntire-Stennis funds through the Arthur Temple
College of Forestry and Agriculture, Stephen F. Austin State University. Special appreciation
to the staff of Martin Dies Jr. State Park, R. McFarlane, T. Anderson, and
K.J. Lodrigue for additional logistical, financial, and technical support. Thanks to
B. Koerth, V. Dowden, D. Cantu, S. Crook, C. Anderson, and A. Webb and all others
who assisted with this field work. Thanks to T.P. Wilson and the two anonymous
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