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Chelonian Conservation and Biology, 2017, 16(2): 000–000
doi:10.2744/CCB-1256.1
!2017 Chelonian Research Foundation
Loggerhead Nesting in the Northern Gulf of
Mexico: Importance of Beach Slope to Nest
Site Selection in the Mississippi Barrier
Islands
ANDREW S. MAURER
1,*
AND
MATTHEW W. JOHNSON
2
1
Department of Applied Ecology, North Carolina State University,
Campus Box 7617, Raleigh, North Carolina 27695 USA
[asmaurer@ncsu.edu];
2
National Marine Fisheries Service, Southeast Fisheries Science
Center, NOAA, 75 Virginia Drive, Miami, Florida 33149 USA
[matthew.johnson@noaa.gov]
*Corresponding author
ABSTRACT. – Here we describe the effects of beach
morphological features on loggerhead (Caretta caretta)
nesting behavior on the barrier islands of the north-
central Gulf of Mexico. Our results show that
loggerhead crawl length decreases as beach slope
increases, and our data comparing nest crawls
(resulting in egg laying) versus false crawls (emergence
onto the beach without laying eggs) suggest that beach
slope and crawl length differ between the crawl types
but elevation does not. We infer that loggerheads cue
in to beach slope to reach a perceived elevation with
reduced risk of inundation, crawling longer distances
on flatter slopes compared with shorter distances on
steep slopes, but that after this elevation is reached,
other environmental variables may ultimately factor
into the decision to lay eggs.
Productive nesting beaches are vital to the recovery
prospects of sea turtle populations faced with rapid
environmental change and various anthropogenic pres-
sures. To manage for productive beaches into the future, it
is important to develop an understanding of the nest site
selection process. Data that describe the factors that benefit
or are preferred by nesting sea turtles can enhance the
ability to predict impacts of changes to nesting habitats
and can aid effective allocation of time and resources by
habitat managers. The future environmental uncertainty
associated with climate change and other anthropogenic
impacts (e.g., oil spills) underscores the importance of
these data. Managers will face increasingly variable
coastal habitats, tougher decisions, and greater operating
and restoration costs. Sea turtles already face a suite of
new conditions connected to climate change including sea
level rise and warming temperatures (Hawkes et al. 2007),
changing weather patterns (Knutson et al. 2010), and new
events such as coastal macroalgae influxes at nesting sites
(Maurer et al. 2015). This environmental variability is
expected to increase over time, resulting in more-frequent
changes to beach environments and making information
related to nesting behavior increasingly valuable.
The selection of a nest site can largely dictate egg
incubation conditions, which can affect hatch success and
hatchling gender and phenotype (Matsuzawa et al. 2002;
Booth 2006; Ditmer and Stapleton 2012). However, the
process whereby sea turtles select their nest sites is
difficult to determine (Miller et al. 2003). Previous work
has documented interspecific (Hays et al. 1995) and
intraspecific (Kamel and Mrosovsky 2006) differences and
has suggested that a multitude of environmental variables
may affect nest placement such as sand temperature,
salinity, moisture, particle size, and slope (Mortimer 1995;
Wood and Bjorndal 2000; Mazaris et al. 2006). In contrast,
other work suggests nest placement occurs unpredictably
(Hays et al. 1995). These varying conclusions may suggest
that region-, population-, or beach-specific data on nest site
selection are necessary.
Beach slope is a primary coastal characteristic that can
affect nest placement, as slope dictates the basic geometry
of a nesting site. Marine turtles often lay clutches in
locations that are inundated during storms and high tides,
resulting in loss of the egg clutch and reduced fitness (e.g.,
Mrosovsky 1983; Whitmore and Dutton 1985). Beach
slope may be an abiotic indicator to avoid clutch loss
through nest inundation (Horrocks and Scott 1991; Wood
and Bjorndal 2000). As beach geomorphology is altered
by climate-associated changes to storms and currents, as
well as by anthropogenic activities such as coastal
dredging operations, slope will likely become more
variable over time. As such, a better understanding of
slope’s role in nesting behavior would be beneficial to
management and helpful for predicting climate change
impacts. For example, Wood and Bjorndal (2000) showed
that loggerheads in Melbourne Beach, Florida, may use
changes in slope to find the back dune of the beach, giving
valuable information to future beach management in that
location.
Herein, we present results of a project elucidating the
role that slope has in loggerhead turtle (Caretta caretta)
nesting ecology on the Mississippi barrier islands in the
northern Gulf of Mexico. The objectives of our study were
to assess if loggerhead nesting behavior varies according
to beach morphology, describe nesting preferences, and
assess loggerheads’ ability to cope with morphological
change in this region. We hypothesized that there would
be a negative correlation between beach slope and
loggerhead crawl distance, with the result that nesting
turtles crawled farther on flatter sloping beaches to reach a
suitable elevation. Further, we hypothesized that beach
morphological characteristics (distance from water, eleva-
tion, and slope) affect the decision to nest or return to the
water without nesting. A secondary aim of this article is to
highlight sea turtle nesting in Mississippi and the north-
central Gulf of Mexico.
Methods.—DuringJulyof2012,wesurveyed14
loggerhead crawl sites on the southern-facing beaches of
Horn Island, Mississippi. This island is 1 of 5 managed in
Mississippi by the National Park Service’s Gulf Islands
National Seashore (hereafter GUIS). Historically, Horn
Island was a military outpost. However, it is now
undeveloped and uninhabited excluding a single housing
facility for Park Rangers. Horn Island is approximately
13 miles in length. The beachfront is patrolled several
times per week as part of routing Ranger activities. The
island is situated in a region routinely impacted by large
storms, heavy winds, and strong currents (Morton 2008),
resulting in highly dynamic barrier island habitats
(Claudino-Sales et al. 2008; Houser et al. 2008).
Additionally, this area was heavily impacted by the
Deepwater Horizon oil spill and was subjected to
extensive beach cleanup activities. During our study
there existed variation in beach topography along the
coastline, with variable distance from the high tide line to
the back dune, changes in slope, and the presence of
small berms. The entire beachfront is sandy habitat and
potentially suitable for nesting for the loggerhead, green
(Chelonia mydas), and Kemp’s ridley (Lepidochelys
kempii)seaturtlesthatnesthere.
Once a crawl site was located, we determined what
species made the crawl and if it resulted in a nest. A crawl
was determined to have resulted in a nest if there was
evidence of active egg covering and site ‘‘ camouflaging’’ .
We did not check for, or disturb, eggs during this study. If
the turtle returned to the water without laying, we
designated the location as a false crawl site. At the
locations of nests (n= 11) and false crawls (n= 3), we
measured the slope and length of a line perpendicular to
the high tide line (HTL). Slope, presented in this article as
a ratio (in units of m !m
"1
), was measured with standard
topographical survey equipment consisting of a leveled
tripod-mounted laser aimed at a measuring stick. Hori-
zontal distance from HTL was recorded with a Trimble
GeoExplorer GeoXT GPS (submeter accuracy). At nest
sites, we started measurements from the estimated location
of the clutch. At false crawl sites, we measured from the
point of the crawl path farthest from the water. Crawls
tended to be roughly parabolic, and at false crawls we
measured slope and distance to HTL from the vertex of
this parabola. Herein we do not focus on changes along the
length of the crawl such as berms (see Wood and Bjorndal
2000) and rather examine the overall changes in distance
travelled and elevation, comparing start point to endpoint.
For data analysis, we converted the GPS-derived
horizontal distance to a straight-line crawl distance using
basic trigonometry (Fig. 1). We then ran a linear regression
using slope as the predictor variable and crawl distance as
the response. We acknowledge concerns for the potential
for autocorrelation between the predictor and response
inherent in the geometric relationship between the two.
Specifically, the response can be thought of as cin the
Pythagorean Theorem a
2
þb
2
=c
2
, and the predictor
(slope) can be calculated using a/b. Thus, in a sense we are
regressing =(a
2
þb
2
) and a/b. However, the biological
underpinning of our hypotheses is that turtles are seeking
some imagined elevation threshold,a. If this hypothesis is
incorrect, then there would probably not be a significant
relationship between slope and crawl distance. Further, if
evidence for a target or threshold elevation is present, then
we should expect no significant association between
elevation and slope or elevation and crawl distance. We
ran regressions to evaluate those expectations.
To determine if there were morphological character-
istics that differed between beach sites that were chosen
for a nest versus actively rejected sites (false crawls), we
analyzed topographical data using 3 Welch’s t-tests to
compare slope, crawl distance, and elevation between the
two crawl types. Welch’s t-test is appropriate in this case
because it is robust to unequal sample sizes and variance
but gives results very similar to the Student’s t-test in the
event that those conditions are met (Ruxton 2006).
Results. — Topographical surveys revealed that the
loggerheads at our 14 sites crawled an average (6standard
error) perpendicular distance of 13.2 61.5 m from the
high tide line toward the dune, ranging from 6.20 to 24.8
Figure 1. Using straight-line distances, a 2-dimensional cross section of the beach at a sea turtle crawl breaks down into a right triangle.
On flatter slopes (a/b), turtles must crawl a farther distance (c) to reach the same elevation (a) compared with crawls on steeper slopes.
0CHELONIAN CONSERVATION AND BIOLOGY,Volume 16, Number 2 – 2017
m. Mean slope for all sites was 0.092 60.010 m !m
"1
.
The minimum slope observed was 0.027 m !m
"1
, while
the maximum was 0.16 m !m
"1
. Mean final elevation at
the crawl sites was 1.06 60.075 m and ranged from
0.533 to 1.64 m.
The linear regression of slope predicting straight-line
crawl distance showed a strong association between the
predictor and response (R
2
= 0.62; Fig. 2). Slope was a
statistically significant predictor of crawl distance in the
linear model fit ( p$0.05) and its estimated coefficient
was negative (Table 1). Thus, as slope increased, crawl
distance decreased.
Linear regressions with elevation as the response and
either slope or crawl distance as the respective predictors
suggested no relationship exists between variables. Slope
(R
2
=0.15, p= 0.18) and crawl distance (R
2
= 0.02,
p= 0.64) were not statistically significant predictors of
elevation and explained little of the variance in elevation.
Results comparing nest crawls to false crawls showed
that elevation did not differ significantly between the site
types (p.0.05; Table 2). Mean elevation for all 14 sites
was 1.06 m (SE = 0.075). Slope and crawl distance did
significantly differ between nest crawls and false crawls
(p$0.05; Table 2). On average, nest sites had an
approximately 5.5-m longer crawl distance and were
flatter, or less steep, by 0.04 m !m
"1
in slope.
Discussion. — Our results are consistent with the
hypothesis that sea turtles use beach slope as a nesting cue.
The strong negative correlation between beach slope and
crawl distance (R
2
=0.62) suggests that loggerheads
nesting on the barrier islands of Mississippi crawl shorter
distances on steeper slopes versus longer distances on
flatter slopes to reach a presumed elevation threshold. This
relationship is congruous with previous studies that
describe how nesting sea turtles interact with beach slope.
Hawksbills (Eretmochelys imbricata) in Barbados may
also use slope to reach a suitable elevation (Horrocks and
Scott 1991), although slope is likely just one of a number
of environmental factors affecting nest placement (Kamel
and Mrosovsky 2005). Loggerhead turtles in east Florida
likewise cue in to slope, though they may seek a beach
zone (i.e., the back dune) rather than an elevation threshold
(Wood and Bjorndal 2000). The exact nature of slope’s
role in nest placement may vary with beach-specific
morphology and configuration.
For Mississippi loggerheads, we cannot rule out that
factors not considered could influence crawl distance in
addition to slope, and we acknowledge the constraints of a
small sample size. However, our results provide some
evidence that slope is a central factor in nest placement.
The association between slope and crawl distance makes
sense, as it may suggest that loggerheads seek a safe
elevation and balance that goal against the energy costs of
crawling (i.e., turtles do not crawl longer than needed at
steep slopes; Fig. 2). The lack of correlations between
elevation and slope and between elevation and crawl
Figure 2. A linear regression shows that crawl distance is negatively correlated with beach slope (R
2
= 0.62; p,0.001) for 14
loggerhead (Caretta caretta) crawl sites. Eleven nest sites (filled circles) and 3 false crawl sites (open circles) were surveyed in July
2012 on Horn Island, Mississippi. We infer that slope is being used as a cue in nesting decision making; turtles adjust crawl distance
according to slope in order to reach some suitable elevation.
Table 1. Model fit results for the linear regression with beach
slope (m !m
"1
) as the predictor variable and loggerhead (Caretta
caretta) crawl distance (m) as the response. Coefficient estimates
are shown with associated standard error and p-values
(R
2
= 0.62). Data were collected from topographical surveys of
loggerhead crawls (n= 14) on Horn Island, Mississippi, in July
2012.
Coefficient Estimate SE p
Intercept 23.8 2.6 ,0.001
Slope "114 26 ,0.001
NOTES AND FIELD REPORTS 0
distance provides further evidence for this inference. The
presumed elevation threshold sought by loggerheads at our
sites appears to be approximately 1 m above sea level
(mean = 1.06 60.075). Only two sites had an elevation
below 0.9 m. The Mississippi Sound has a tidal range of
0.6 m (Moncreiff 2007), so the mean elevation we
observed may indeed safeguard against inundation. With
no significant difference in elevation between nest crawls
and false crawls, our results also suggest that once this
suitable elevation was reached, it is likely that other factors
ultimately drove a turtle’s decision to either nest or return
to the water without nesting (Miller et al. 2003). Two
factors that we analyzed that may influence this decision
are slope and distance crawled. We found that nest sites
had a significantly lower mean slope and longer mean
crawl distance than did false crawl sites.
A secondary objective of this article is to call attention
to sea turtle nesting in Mississippi and the northern Gulf of
Mexico. Our study is one of the first to present research on
nesting in Mississippi, highlighting the data deficiency for
the region. The barrier islands of the northern Gulf of
Mexico are habitats that could conceivably increase in
importance under projected climate change scenarios
described by the Intergovernmental Panel on Climate
Change (2014). These islands experience relatively low
human impact and represent some of the northernmost
nesting sites in the Gulf of Mexico at a time when climate
warming and temperature-dependent sex determination
threaten to further skew marine turtle sex ratios in the
southern United States (Hawkes et al. 2007).
Although regional nesting data exist because GUIS
staff opportunistically patrol the Mississippi island
beaches, no routine sampling program was in place at
the time of our study to document nest locations,
hatching success, etc. Data such as ours that specifically
describe the effects of beach morphology on regional
nesting behavior could be particularly valuable to post-
Hurricane Katrina and Deepwater Horizon island resto-
ration planning, especially if techniques such as sand
renourishment and topographical manipulation are used.
These barrier islands are the subject of a recently
approved restoration program expected to last 30–40
yrs (US Army Corps of Engineers 2016). This will be the
largest island restoration effort in the United States and
may result in significant rebuilding of beach habitats,
including the addition of sand to the coastal system.
Ultimately, this restoration effort will change the beach
habitats and impact sea turtle nesting. How the restora-
tion affects nesting populations will rely on the quality of
added sand, survival of nests, and the ability of hatchlings
to emerge and reach offshore currents for transport to
nursery areas.
Our finding that loggerhead nesting behavior in this
region is affected by beach morphology is encouraging
with regard to their ability to adapt to predicted changes in
weather patterns and the likely resultant increase in barrier
island geomorphological variability. If beach slope or
other morphological features consistently affect nesting in
a manner that helps to ensure nests are sufficiently elevated
to protect against inundation, physical changes that occur
between nesting seasons should result in less deleterious
effects on reproduction than if nest placement were more
random or spatially specific. However, we offer only
limited data, and we suggest that more research is needed
for these unique nesting habitats in the northern Gulf of
Mexico, especially to aid the restoration process. Aug-
menting scientific understanding of sea turtle nesting
behavior and nest placement can increase the capacity for
beneficial management and conservation of productive
nesting habitats in the face of increasing environmental
change.
Acknowledgments. — We would like to recognize the
in-kind support for this research from the Gulf Islands
National Seashore and specifically thank G. Hopkins, W.
Brewer, and all of the Law Enforcement Rangers
patrolling Horn Island. Funding for this project came
from the National Parks Service’s George Melendez-
Wright Climate Change Program. A.S.M. would like to
thank C. Layman for helpful comments on this manuscript.
An anonymous reviewer helped to improve this
manuscript.
LITERATURE CITED
BOOTH, D.T. 2006. Influence of incubation temperature on
hatchling phenotype in reptiles. Physiological and Biochem-
ical Zoology 79:274–281.
CLAUDINO-SALES, V., WANG, P., AND HORWITZ, M.H. 2008. Factors
controlling the survival of coastal dunes during multiple
hurricane impacts in 2004 and 2005: Santa Rosa barrier island,
Florida. Geomorphology 95:295–315.
DITMER, M.A. AND STAPLETON, S.P. 2012. Factors affecting hatch
success of hawksbill sea turtles on Long Island, Antigua, West
Indies. PLoS ONE 7(7):e38472.
HAWKES, L.A., BRODERICK, A.C., GODFREY, M.H., AND GODLEY,
B.J. 2007. Investigating the potential impacts of climate
Table 2. Results from t-tests comparing loggerhead (Caretta caretta) nest sites (n= 11) to false crawls (n= 3) using data collected via
topographical surveys on Horn Island, Mississippi, in July 2012. Group means are given with standard errors along with resulting t- and
p-values. Bold p-values are significant ( p$a= 0.05).
Beach variable Nest mean 6SE False mean 6SE tp
Slope (m !m
"1
) 0.083 60.011 0.13 60.012 2.7 0.037
Crawl distance (m) 14.4 61.7 8.94 60.56 3.0 0.011
Elevation (m) 1.05 60.094 1.11 60.086 0.52 0.62
0CHELONIAN CONSERVATION AND BIOLOGY,Volume 16, Number 2 – 2017
change on a marine turtle population. Global Change Biology
13:923–932.
HAYS, G.C., MACKAY, A., ADAMS, C.R., MORTIMER, J.A., SPEAK-
MAN, J.R., AND BOEREMA, M. 1995. Nest site selection by sea
turtles. Journal of the Marine Biological Association of the
United Kingdom 75:667–674.
HORROCKS, J.A. AND SCOTT, N.M. 1991. Nest site location and
nest success in the hawksbill turtle (Eretmochelys imbricata)
in Barbados, West Indies. Marine Ecology Progress Series 69:
1–8.
HOUSER, C., HAPKE, C., AND HAMILTON, S. 2008. Controls on
coastal dune morphology, shoreline erosion and barrier island
response to extreme storms. Geomorphology 100:223–240.
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC). 2014.
Climate Change 2014: Synthesis Report. Contribution of
Working Groups I, II and III to the Fifth Assessment Report of
the Intergovernmental Panel on Climate Change. Core Writing
Team, Pachauri, R.K. and Meyer, L.A. (Eds.). Geneva: IPCC,
151 pp.
KAMEL, S.J. AND MROSOVSKY, N. 2005. Repeatability of nesting
preferences in the hawksbill sea turtle, Eretmochelys imbri-
cata, and their fitness consequences. Animal Behaviour 70:
819–828.
KAMEL, S.J. AND MROSOVSKY, N. 2006. Inter-seasonal mainte-
nance of individual nest site preferences in hawksbill sea
turtles. Ecology 87:2947–2952.
KNUTSON, T.R., MCBRIDE, J.L., CHAN, J., EMANUEL, K., HOLLAND,
G., LANDSEA, C., HELD, I., KOSSIN, J.P., SRIVASTAVA, A.K., AND
SUGI, M. 2010. Tropical cyclones and climate change. Nature
Geoscience 3:157–163.
MATSUZAWA, Y., SATO, K., SAKAMOTO, W., AND BJORNDAL, K.
2002. Seasonal fluctuations in sand temperature: effects on the
incubation period and mortality of loggerhead sea turtle
(Caretta caretta) pre-emergent hatchlings in Minabe, Japan.
Marine Biology 140:639–646.
MAURER, A.S., DENEEF, E., AND STAPLETON, S. 2015. Sargassum
accumulation may spell trouble for nesting sea turtles.
Frontiers in Ecology and the Environment 13:394–395.
MAZARIS, A.D., MATSINOS, Y.G., AND MARGARITOULIS, D. 2006.
Nest site selection of loggerhead sea turtles: the case of the
island of Zakynthos, W Greece. Journal of Experimental
Marine Biology and Ecology 336:157–162.
MILLER, J.D., LIMPUS, C.J., AND GODFREY, M.H. 2003. Nest site
selection, oviposition, eggs, development, hatching, and
emergence of loggerhead turtles. In: Bolton, A.B. and
Witherington, B.E. (Eds.). Loggerhead Sea Turtles. Wash-
ington, DC: Smithsonian Institution, pp. 125–143.
MONCREIFF, C.A. 2007. Mississippi Sound and the Gulf Islands.
In: Handley, L., Altsman, D., and DeMay, R. (Eds.). Seagrass
Status and Trends in the Northern Gulf of Mexico: 1940–
2002. USGS Scientific Investigations Report 2006"5287, pp.
76–85.
MORTIMER, J.A. 1995. Factors influencing beach selection by
nesting sea turtles. In: Bjorndal, K.A. (Ed.). Biology and
Conservation of Sea Turtles. Washington, DC: Smithsonian
Institution Press, pp. 45–52.
MORTON, R.A. 2008. Historical changes in the Mississippi–
Alabama barrier-island chain and the roles of extreme storms,
sea level, and human activities. Journal of Coastal Research
24:1587–1600.
MROSOVSKY,N.1983.Ecologyandnest-siteselectionof
leatherback turtles Dermochelys coriacea. Biological Conser-
vation 26:47–56.
RUXTON, G.D. 2006. The unequal variance t-test is an underused
alternative to Student’s t-test and the Mann–Whitney U-test.
Behavioral Ecology 17:688–690.
US ARMY CORPS OF ENGINEERS (USACE). 2016. Mississippi
Coastal Improvements Program (MsCIP) comprehensive
barrier island restoration: Hancock, Harrison, and Jackson
counties, Mississippi final supplemental environmental impact
statement. US Army Corps of Engineers – Mobile District,
400 pp.
WHITMORE, C.P. AND DUTTON, P.H. 1985. Infertility, embryonic
mortality and nest-site selection in leatherback and green sea
turtles in Suriname. Biological Conservation 34:251–272.
WOOD, D.W. AND BJORNDAL, K.A. 2000. Relation of temperature,
moisture, salinity, and slope to nest site selection in
loggerhead sea turtles. Copeia 1:119–128.
Received: 30 March 2017
Revised and Accepted: 11 July 2017
Published Online: 20 September 2017
Handling Editor: Jeffrey A. Seminoff
NOTES AND FIELD REPORTS 0
