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Observations on the Nesting Ecology of the Mississippi Mud
Turtle, Kinosternon subrubrum hippocrepis Gray
Noah J. Anderson1,* and Brian D. Horne2
Abstract - Surface nesting is common among tropical turtles, but is an uncommon and poorly
documented event in temperate species. Here we report two separate instances of surface nesting
in Kinosternon subrubrum (Eastern Mud Turtle), in a Louisiana swamp in November, 2001 and
March, 2002. The clutches, each consisting of a single egg, were found on man-made earthen structures
within a Dwarf Palmetto-hardwood fl oodplain. The adaptive significance of ovipositing eggs
on the surface of compact clay soils is possibly related to the need for oxygen exchange between the
eggshell and embryonic membranes as well as avoidance of high water-table conditions. The two
nests were laid in winter months, unlike the typical May or June nesting of northern females.
Reproductive patterns of turtles are known to fit two broad types (with exceptions
and intermediates): those with few large clutches of small eggs laid during a distinct
nesting season (type I) and those with several small clutches of large eggs laid more
or less year-round (type II) (Moll 1979, Moll and Moll 2004). Moreover, turtles displaying
the type II reproductive pattern are found predominately in the tropics and
tend to have poorly constructed nests or none at all (Moll 1979). Kinosternid turtles
tend towards a type II pattern (Moll 1979). Kinosternon subrubrum (Lacépède)
(Eastern Mud Turtle), distributed in the southeastern portion of the United States, is
known to lay brittle-shelled eggs in a variety of substrates, such as sandy soils and
rotting logs, and locations, such as inside muskrat lodges and beaver dams, within
alligator nest vegetation, and under boards (Barkalow 1948, Burke et al. 1994, Carr
1952, Deitz and Jackson 1979, Dundee and Rossman 1989, Ernst et al. 1994, Mount
1975, Nichols 1947, Richmond 1945, and Tinkle 1959). Many of the Eastern Mud
Turtle nests found by Richmond (1945) were dug deep into the substratum, and the
eggs were covered with soil, but he remarked that little attempt was made to conceal
the opening of the nest. While poor nest concealment and shallow eggs have been
noted in two other kinosternid turtles, Sternotherus minor (Agassiz) (Loggerhead
Musk Turtle; Carr 1952, Ernst et al. 1994) and S. odoratus (Latreille in Sonnini and
Latreille) (Common Musk Turtle; Cagle 1937, Carr 1952), we have only been able
to locate one study that documented a single incidence of surface nesting in Eastern
Mud Turtle (Tinkle 1959). Here, we report of two incidences of surface nesting in K.
subrubrum hippocrepis Gray (Mississippi Mud Turtle) in a Louisiana swamp.
We found a single egg on 29 November 2001 and another on 9 March 2002 while
monitoring an area in the Jean Lafitte National Historical Park and Preserve, Barataria
Unit, Jefferson Parish, LA. Sites were visited every 24 hours for several days prior to discovery
of the eggs to check traps as part of a herptofaunal survey (N.J. Anderson, unpubl.
data); hence, we believe the eggs were found the day of oviposition. The first egg was
found partially exposed under leaf litter on a low dike adjacent to a water-filled ditch ca.
25 cm from the edge of the water and ca. 10 cm above the waterline. The egg was photographed,
but was not measured and left undisturbed. In the spring of 2002, another lone
egg showing no distinct banding and appearing weakly chalked (indicating its recent
oviposition) was discovered partially exposed under leaf litter on a low dike adjacent to a
man-made pond approximately 50 cm from both the edge of the water and above the waterline.
This egg measured 31.5 x 17.2 mm and was photographed for comparison to the
first egg. No evidence of any attempt at nest construction (i.e., scratch marks, disturbed
soil) was present at either site. The second egg was brought to the lab and later, after
several months of incubation, determined to be K. subrubrum hippocrepis. The neonate
was released at the nesting site shortly after hatching. From a visual comparison, the two
eggs appeared identical. Possibly the first egg was laid by a stinkpot, S. odoratus, but this
Notes of the Southeastern Nat u ral ist, Issue 8/3, 2009
564 Southeastern Naturalist Notes Vol. 8, No. 3
highly unlikely as this species is extremely rare at the preserve (i.e., no living specimens
were found after an 11-month intensive survey).
Kinosternon subrubrum was common in water bodies at both nesting sites. The
surrounding habitat was composed of Dwarf Palmetto-hardwood fl oodplains, which
frequently have surface water. Both of these nests were discovered on man-made dikes
that would rarely be subject to fl ooding; however, the soils are compact and clay laden,
which may interfere with excavation or underground development of eggs. As no
attempt was made at nest construction, it would seem to argue against the former possibility.
The last hypothesis is that the behavior observed was anomalous and that the
females abandoned attempts at nest construction, perhaps because of disturbance.
Our observations are different from many reports of “surface nesting” in other
kinosternid turtles. Most reports describe shallow nesting with the eggs laid in a depression
created by the female, resulting in portions of the eggs exposed (Carr 1952,
Richmond 1945, but see Cagle 1937 and Tinkle 1959). It is difficult to infer if this is
a common method of nesting at this site as no other natural nests (e.g., depredated,
intact, or successfully hatched) were discovered in the course of surveys from August
2001–June 2002. Ovipositing eggs on the surface may be a beneficial reproductive
strategy when no sandy soils are available. Sandy soils maintain high water potential
with low water content and the spaces between sand grains provide ample oxygen
for a developing embryo (Ackerman 1991, Packard and Packard 1988). Clay, with
its much smaller particle size, maintains a high water content thus decreasing or
eliminating oxygen between particles (Ackerman 1991). Furthermore, the small particles
of clay can clog eggshell pores that are important for gas exchange, especially
when embryos are near completion of morphogenesis and have the highest oxygen
consumption (Ewert 1979). Therefore, developing embryos in surface nests might be
less likely to asphyxiate during periods of high soil moisture.
The timing (March and November) of these two nests is also unusual. Most nesting
activity of K. subrubrum has been reported in May and June (Ernst et al. 1994, Gibbons
1983, Gibbons et al. 1982, Iverson 1979). The earliest reported nesting dates for this
species have been 15 Jan (in New Orleans; Dundee and Rossman 1989), February (one
observation in Louisiana; Tinkle 1959), mid-March (shelled eggs in a dissected female
from east Texas; Houseal and Carr 1983), late March (three nests unearthed by a plow
in Virginia; Richmond 1945), and mid-April (nesting female in Northern Virginia,
Ernst et al. 2001). Nesting in K. subrubrum is reported to occur as late as September
in Virginia ( Iverson 1979, Richmond 1945). Interestingly, Iverson (1977) suggested
K. subrubrum may nest continuously throughout the year in northern Florida (similar
latitude to our study site), but his conclusions were based on finding recently hatched
young in December and February. The November nest we discovered is of particular
developmental interest as nesting into periods of declining day length is possible evidence
for the expression of embryonic diapause (Ewert and Wilson 1996). It is perhaps
due to investigator bias (i.e., searching for nests during the summer months) that has
limited the observations of fall and spring nests, and it is possible that such nests are
more common than earlier perceived. However, the majority of studies on Kinosternid
reproductive condition suggest that spring and fall nesting is uncommon in this genus
of turtles (Gibbons 1983, Gibbons et al. 1982, Tinkle 1961). The exception appears to
be in Kinosternon baurii (Ewert and Jackson 2005, Ewert and Wilson 1996, Iverson,
1979b), which has been often reported nesting in spring and late summer. It is possible
that the southernmost populations of K. subrubrum have different reproductive strategies,
which are more typical of many tropical turtles, in that they are able to produce
viable eggs at times when it is biologically infeasible for northern populations.
Acknowledgments. We thank the National Park Service (David Muth) for
permits and financial support of the herpetological inventory. This manuscript was
greatly improved by comments from Dr. Peter Pritchard and an anonymous reviewer.
2009 Southeastern Naturalist Notes 565
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1Department of Biology, University of Wisconsin-Baraboo/Sauk County, 1006 Connie Road, Baraboo,
WI 53913. 2 Current address - San Diego Zoo, Institute for Conservation Research, 15600
San Pasqual Road, Escondido, CA 92027. *Corresponding author - firstname.lastname@example.org.