Journal of Herpetology, Vol. 39, No. 4, pp. 649–652, 2005
Copyright 2005 Society for the Study of Amphibians and Reptiles
Rainfall and Depredation of Nests of the Painted Turtle, Chrysemys picta
KENNETH D. BOWEN1
AND
FREDRIC J. JANZEN
Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa 50011 USA
ABSTRACT.—It is commonly thought that predators use olfactory cues to find turtle nests and that these cues are
diminished by rainfall. We studied the relationship between rainfall on the date of oviposition and depredation
of nests of the Painted Turtle, Chrysemys picta, on a major nesting beach between 1996 and 2003. We analyzed
two scenarios: rainfall versus no rainfall on the date of oviposition; and no rainfall versus intense or weak
rainfall on the date of oviposition. For the first scenario, we found no consistent association between rainfall and
nest depredation before hatching. In 1996, rainfall on the date of oviposition appeared to increase the chance of
nest depredation; in 2000, it appeared to decrease the chance of nest depredation; and there was no statistically
significant relationship in the remaining years or overall. In the second analysis, the relative amount of rain was
associated with nest depredation before hatching. Nests constructed on days with a larger amount of rain were
less likely to be depredated than nests constructed on days with no rain or smaller amounts of rain. Nests
constructed on days with smaller amounts of rain were more likely to be depredated than nests constructed on
days with no rain.
Rates of nest depredation are high in many turtle
populations, which could result in intense selection
pressure on the behavior of nesting females (Spencer,
2002). If a subset of females reproduces successfully
while others do not, the genetic structure of the
population could be affected dramatically (Scribner
et al., 1993). Unfortunately, little is known about the
factors affecting nest depredation. A logical first step
toward understanding the ultimate consequences of
nest depredation is to determine its proximate cause(s).
Predators are commonly thought to use olfactory
cues to find turtle nests. Legler (1954) suggested that
the urine expelled by female Painted Turtles (Chrysemys
picta) during nest construction might aid predators
(including Raccoons, Procyon lotor) in locating nests.
Congdon et al. (1983) hypothesized that the scent trails
of female Blanding’s Turtles (Emydoidea blandingi)
leading to and from the nest, as well as odors associated
with the nest itself, were the major cues used by nest
predators (primarily Raccoons and Red Foxes, Vulpes
vulpes) to find nests. Spencer (2002) found that artificial
turtle nests containing eggs were more likely to be
depredated (by Red Foxes) than those without eggs.
Thus, turtle urine, general body scent, and egg odor all
may serve as olfactory cues for mammalian predators
of turtle nests.
Rainfall potentially interacts with the scent of nests
and nesting turtles, but there is confusion regarding
its effects. Legler (1954) hypothesized that nest odors
might be dissipated by rainfall and that nests surviving
to a rainfall event were less likely to be subsequently
destroyed. Burke et al. (1995) suggested that the
emergence of some turtles to nest during rainstorms
might be an antipredator mechanism for eliminating
olfactory cues associated with nests. In contrast,
Congdon et al. (1983, 1987) noted that the probability
of nest depredation declined with the age of the nest
but that the few nests that were depredated well after
1
Corresponding Author. Present address: 886 East
Blanchard Road, Shepherd, Michigan 48883, USA;
E-mail: hideneck@yahoo.com
oviposition were destroyed during or after rainfall. It
is possible that, in some cases, rainfall intensifies the
cues used by predators to find nests or that predator
activity increases following rainfall.
In this study, we tested for an association between
rainfall and the depredation of Painted Turtle nests.
Specifically, we determined whether rainfall on the
calendar date of oviposition had any effect on the
likelihood that a nest would be destroyed by predators
before hatching. We also tested for an association
between the relative amount of rainfall on the date of
oviposition and nest depredation. Therefore, we provide a test of the hypothesis that rainfall is associated
with the depredation of turtle nests.
MATERIALS AND METHODS
Data presented here represent a portion of a longterm study of the nesting ecology of C. picta ( Janzen,
1994) at the Thomson Causeway Recreation Area
(TCRA) near Thomson, Illinois. The Thomson Causeway is a ;450 3 900 m island near the eastern bank of
the Mississippi River, and it contains a ;1.5 ha nesting
area that is bordered on the east side by a 200 m wide
slough from which most turtles emerge to nest (for a
more complete site description, see Kolbe and Janzen,
2002). Raccoon, Striped Skunks (Mephitis mephitis), and
Fox Snakes (Elaphe vulpina) have all been observed
destroying turtle nests at TCRA. Our experience
suggests that these are the major nest predators at the
site, but we cannot rule out the influence of other
potential predators such as Red Fox and Virginia
Opossums (Didelphis virginiana).
Chrysemys picta nest from May to July in most areas
(Ernst et al., 1994). We monitored the nesting beach
at TCRA every nesting season from 1996 to 2003. Each
day, the nesting beach was searched intensively for
female turtles once every hour from dawn until dusk.
Once an observed turtle had finished nesting, we
mapped the location of the nest using nearby landmarks (e.g., trees). Each nest was then checked
periodically for evidence of depredation until late
September (the time by which hatching in all nests
is completed; FJJ, pers. obs.). We obtained daily
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SHORTER COMMUNICATIONS
TABLE 1. The relationship between rainfall on the date of oviposition and depredation before hatching of
Painted Turtle (Chrysemys picta) nests. When an odds ratio is one, the odds of depredation are equal regardless
of rainfall. Chi-square tests were interpreted at a 5 0.05 with one degree of freedom.
Year
Rainfall
# Nests
depredated
# Nests
surviving
%
depredated
Odds
ratio
v2
P
1996
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
16
21
3
19
16
34
27
103
19
71
21
170
43
113
51
124
196
655
18
67
36
96
26
84
6
47
26
46
0
14
48
162
37
110
197
626
47.0
23.9
7.7
16.5
38.1
28.9
81.8
68.7
42.2
60.7
100
92.4
47.2
41.1
57.9
53.0
49.9
51.1
2.83
6.2
0.01
0.42
1.9
0.17
1.52
1.2
0.27
2.05
2.3
0.13
0.47
4.5
0.03
0
1.7
0.19
1.28
1.1
0.3
1.22
0.6
0.43
0.95
0.2
0.66
1997
1998
1999
2000
2001
2002
2003
Total
precipitation data for the nesting season from the
United States Army Corp of Engineers Lock and Dam
13 approximately 12 km south of the study site,
although we were unable to determine the exact timing
of rainfall on any given day.
We tested for an association between rainfall on the
date of oviposition and nest depredation using contingency table analysis. We treated rainfall (vs. no rainfall)
on a given calendar date as the ‘‘explanatory’’ variable,
and the number of nests constructed on that date that
were depredated before hatching (vs. number of nests
surviving until hatching) as the ‘‘response’’ variable.
Each turtle nest was used only once in the analysis. We
also added year as a ‘‘control’’ variable, allowing us to
explore the possibility of differing effects of rainfall
among years (Agresti, 2002). Evidence for association
between rainfall on the date of oviposition and nest
depredation was based on chi-square tests of independence interpreted at a 5 0.05 with one degree of
freedom. The null hypothesis was that nest depredation
before hatching and rainfall on the date of oviposition
were independent. We interpreted the nature and
strength of association between these variables using
odds ratios (for calculation and discussion of odds
ratios, see Agresti, 2002).
We were also interested in the effect that large
differences in the amount of rain on the date of
oviposition might have on nest depredation. Using
the distribution of the amount of rainfall that fell during
the nesting season from 1996 to 2003, we compared the
number of turtle nests depredated before hatching
(response variable) among oviposition dates with
rainfall amounts in the lowest and highest quartiles
and dates with no rain (explanatory variables). We used
a Chi-square test of independence with two degrees of
freedom, and odds ratios were used to determine the
nature of any association. We also performed pairwise
Chi-square tests among the three categories. We
divided alpha by three (a 5 0.017) because each nest
was subjected to three statistical tests in this analysis.
Our null hypothesis was that nest depredation before
hatching and the relative amount of rainfall on the
date of oviposition were independent. We were unable
to perform a year-by-year analysis in this case as a result
of small and widely varying sample sizes in the rainfall
categories. Thus, we were restricted to analysis of data
combined across all years. We used only days with
rainfall when calculating quartiles. From 1996 to 2003,
there were 79 days with rainfall (out of 260 total days)
during the C. picta nesting season. The mean rainfall on
these days was 0.99 cm (1.37 SD; range 5 0.02–6.1 cm).
The upper (75%) quartile was located at 1.27 cm, and
the lower (25%) quartile was located at 0.2 cm.
RESULTS
The intensity of nest depredation varied greatly from
year to year, as did the relationship between rainfall
and nest depredation (Table 1). For example, the odds
ratios were close to one in the years 1998, 2002, 2003
and in all years combined, suggesting that nests were
equally likely to be depredated before hatching regardless of rainfall on the date of oviposition. In 1997 and
2000, the odds ratios were close to 0.5, suggesting that
nests were twice as likely to be depredated before
hatching when there was no rain on the date of
oviposition. However, in 1996 and 1999, the odds ratios
were greater than two, suggesting that nests constructed on days with rainfall were at least twice as
likely to be depredated before hatching. The odds ratio
was zero in 2001 because no nests constructed on days
with rainfall survived to hatching. Predation on nests
was particularly intense during 2001. The null hypothesis of independence was rejected for 1996 and 2000
(Table 1).
There was a statistically significant association
between the relative amount of rainfall and nest
depredation (Table 2). Odds ratios and pairwise
comparisons suggested that nests were less likely to
be depredated before hatching when rainfall on the
SHORTER COMMUNICATIONS
date of oviposition was in the upper quartile as
compared to days with little (v2 5 12.1; P 5 0.0005)
or no (v2 5 7.15; P 5 0.008) rain. In contrast, nests were
more likely to be depredated before hatching when
rainfall on the date of oviposition was in the lower
quartile than when there was no rain, although this
trend was not statistically significant at the corrected
alpha (v2 5 4.14; P 5 0.04).
DISCUSSION
Olfactory cues associated with nests and nesting
turtles are thought to be important clues for nest
predators, and rainfall may improve the probability of
nest survival by weakening these cues (Legler, 1954;
Whelan et al., 1994; Burke et al., 1995). However, there
is some evidence that olfactory cues do not affect nest
survival and that cues associated with nests do not
diminish with time. Using artificial turtle nests,
Hamilton et al. (2002) found no effect of the presence
of olfactory cues on nest survival. Wilhoft et al. (1979)
noted that depredation of artificial nests can occur
without secretions from female turtles or egg scent.
Snow (1982) found that Painted Turtle nests were
depredated regardless of age and suggested that the
gradual weakening of cues associated with nests, if it
occurs, may have little to do with nest survival.
There was a relationship between rainfall and nest
depredation at our site. This relationship was temporally inconsistent (and generally negligible) when
considered at the level of rainfall versus no rainfall.
Across all eight years of this study, the relative amount
of rainfall on the date of oviposition was associated
with nest depredation. Specifically, nests appeared less
likely to be depredated before hatching if they were
constructed on days with larger amounts of rain but
more likely to be depredated before hatching if
constructed on days with smaller amounts of rain.
The design of our study does not allow us to form
firm conclusions regarding the use of olfactory cues by
nest predators. However, Raccoons (Zeveloff, 2002),
Striped Skunks (Nams, 1997), and most snakes (Greene,
1997) are known to use chemical cues while foraging.
One potential explanation for our results is that heavy
rain might be effective in eliminating such cues
associated with nests but that light rain might intensify
nest odors when compared to days with no rain. Still
another possibility is that rain influences predator
activity. Fox Snakes, for example, are active during
summer rains (Ernst and Ernst, 2003). In this view,
small amounts of rain might increase nest depredation
by making predators more active, whereas large
amounts might decrease predator activity, or increase
predator activity but effectively diminish nest cues.
These hypotheses, although speculative, are testable
and merit future study.
There are several caveats to our study. Foremost, we
were unable to control for the timing of rainfall. For
example, rainfall during or after nesting is likely to have
a greater effect on nest survival than rainfall before
nesting. Future studies should attempt to control for
this variable. Also, we in effect defined survival as nests
remaining intact from oviposition to hatching. A
‘‘dead’’ nest used in our analysis might have been
depredated within hours of oviposition or several
weeks after oviposition. It is possible that destruction
651
TABLE 2. The relationship between the relative
amount of rainfall on the date of oviposition and
depredation before hatching of Painted Turtle
(Chrysemys picta) nests. Nest depredation was compared among oviposition dates on which the amount
of rainfall was in the upper quartile, days on which
rainfall was in the lower quartile, and days on which
there was no rain for the period 1996 to 2003 (nesting
season only). When an odds ratio is one, the odds of
depredation are equal regardless of rainfall. The Chisquare test was interpreted a 5 0.017 with two degrees
of freedom. The notation ‘‘UL’’ refers to the odds ratio
for upper quartile and lower quartile, ‘‘LN’’ refers to
lower quartile and no rain, and ‘‘UN’’ refers to upper
quartile and no rain.
# Nests
# Nests
%
Odds
Quartile depredated surviving depredated ratio
Upper
38
65
36.9
Lower
69
45
60.5
No rain
648
633
50.6
v2
P
0.38 12.2 0.002
(UL)
1.5
(LN)
0.57
(UN)
of a nest days or weeks after oviposition might have
little to do with rain on the date of oviposition, that is,
much can happen in the intervening period to affect
nest survival. Rain on the date of depredation, for
example, might play a larger role in such a case.
Consideration of the amount of time that elapses until
nest depredation and the inclusion of additional
explanatory variables should be part of future work
on this topic (Dinsmore et al., 2002). Furthermore, we
were unable to partition our analysis of the relationship
between nest depredation and relative amount of
rainfall by year. Thus, the cumulative analysis we
performed provides only a relatively rough idea of
what is transpiring at TCRA. Finally, it is important to
note that the results of this study may not be directly
applicable to other species of turtle, which can differ
substantively in nesting ecology (Ernst et al., 1994).
Turtles may emerge to nest during rainfall events for
reasons other than avoidance of nest depredation.
Wilson et al. (1999) suggested that Striped Mud Turtles
(Kinosternon baurii) move to and from nesting habitat
during rainfall to minimize evaporative water loss.
Rainfall may also facilitate nest construction by
moistening the substrate (Seabrook, 1989). Regardless,
the fact that many turtles emerge to nest on days with
no rainfall suggests that other factors affect the
decision-making process of gravid females (Spencer
and Thompson, 2003).
Nest predators may find turtle nests using nonolfactory cues such as visual disturbance (Spencer,
2002), nesting females on the beach (Congdon et al.,
1987; Eckrich and Owens, 1995), and the proximity of
other nests (i.e., high nest density; Valenzuela and
Janzen, 2001; Marchand and Litvaitis, 2004). Visual
cues could be affected by rainfall in much the same way
as olfactory cues. Future research should focus on
identifying the specific cues used by nest predators to
find turtle nests (Hamilton et al., 2002; Spencer, 2002)
652
SHORTER COMMUNICATIONS
and on determining how turtles (or managers) might
attempt to minimize these cues. Artificial turtle
nests (Marchand et al., 2002) will be useful tools in
these endeavors.
Finally, we wish to emphasize that analyzing data
from only one year at this site would likely have been
misleading, underscoring the importance of long-term
field studies (Tinkle, 1979). Temporal variance in
patterns of nest depredation is a potential reason for
discrepancies between previously published studies of
this phenomenon. Indeed, overall levels of nest
depredation and edge effects on nest depredation vary
yearly at TCRA (Kolbe and Janzen, 2002) and likely
elsewhere, and this variation should be taken into
account whenever possible.
Acknowledgments.—We thank numerous members of
the Janzen Lab and Turtle Camp crews for participating in field research at TCRA over the years and the
Army Corps of Engineers for continued permission to
work at the site. We also thank R. J. Spencer and three
anonymous reviewers for comments on the manuscript. Animals were observed under permits from the
Illinois DNR and the U.S. Fish and Wildlife Service and
in accordance with approved COAC protocols from
Iowa State University. KDB acknowledges the support
of a Graduate College Fellowship from Iowa State
University. The fieldwork in this long-term study was
largely supported by National Science Foundation
grants DEB-9629529 and DEB-0089680 to FJJ.
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Accepted: 7 July 2005.