Relatively few studies investigating the sensory cues used by mammalian predators to locate freshwater turtle nests have provided definitive evidence of reliance on visual cues, although most studies to-date have focused largely on just raccoons and study designs sometimes limit interpretations (Table 2). For example, while experimental studies using artificial nests have suggested that nest-surface markings may be important cues for foraging raccoons, uniformity in surface markings among artificial nest treatments (e.g., all smoothed over or all made to resemble natural nest appearance) reduce the ability to test effects of different visual signal strengths or isolate them from other potentially co-occurring cues inherent in artificially constructed nest cavities: a limitation which is only sometimes implied/acknowledged (Burke et al., 2005; Bernstein et al., 2015; Czaja et al., 2018; Perazzo et al., 2018).

Burger (1977) was apparently the first to suggest that visual cues were not necessary for successful nest location by raccoons and red foxes (Vulpes vulpes), based on widespread predation of diamondback terrapin (Malaclemys terrapin) nests after visual cues had been eliminated on the sand-substrate nesting site by wind and rain. Results of several subsequent studies align with these results, including the experimental evidence provided by Hamilton et al. (2002) who found that 3-m wide areas of sand smoothed over artificial cavities containing quail eggs did not lower excavation rates by raccoons and other predators relative to unobscured nests (see also Edmunds et al., 2018; Table 3). Geller (2015) demonstrated that elimination of surface markings at both natural and artificial nests by broom sweeping did not reduce raccoon depredation rates between experimental and control nests. Additionally, artificial representations of the visual markings made by Ouachita map turtles (Graptemys ouachitensis) and snapping turtles (Chelydra serpentina) during nesting, but without underlying cavities, were excavated at low rates relative to artificial nests with cavities (≤26 vs. 94%, Geller, 2015; ca. 10 vs. ca. 50%, Oddie et al., 2015, from their Figure 3A; respectively). Finally, Voves et al. (2016) found that human evaluation of the degree of visual conspicuousness of natural painted turtle (Chrysemys picta) nests was not a consistent predictor of predation likelihood by raccoons.

In contrast, artificial nest cavities simulating surface disturbance by nesting female Australian Murray River turtles (Emydura macquarii) were excavated at greater rates by introduced red foxes than control nests with minimal surface disturbance, independent of whether artificial nests contained turtle eggs (80% depredation on disturbed treatment vs. 60% control), Coturnix quail eggs (70% disturbed vs. 57% undisturbed), or were without eggs (Spencer, 2002; his Figure 3). Similarly, Dawson et al. (2014) found that red foxes excavated more artificial cavities simulating oblong turtles Chelodina colliei nests initially scored as more obvious than those that were more cryptic.

Collectively, we found no papers that suggest visual cues at nests may be important to foraging raccoons provide unambiguous data in support (see Table 2), while several other papers present data to the contrary (Table 3). However, foxes may use visual cues associated with surface disturbance, likely in combination with other signals, to locate turtle nests.

Congdon et al. (1983) were the first to suggest that predators may use tracks of nesting females (which contain both visual and olfactory elements) to locate turtle nests. However, their statement was likely based on observation or anecdotal reports. The first experimental test of this hypothesis found that eliminating the tracks of nesting females by daily broom sweeping did not reduce depredation rates by raccoons on either natural or artificial nests (Geller, 2015). Camera data has also demonstrated that the foraging paths of nest depredating raccoons do not align with the paths of nesting Graptemys (Geller, 2012a,2015). Moreover, inducing raccoons to follow scent trails in experimental settings has proved difficult (S. Temple, pers. comm.). While these limited data suggest tracks may not be necessary, per se, for raccoons to find turtle nests, it is possible that nesting tracks are followed when more evident, such as in moist soils following rainfall or may be otherwise context dependent at the nest site or geographic location level.

For non-raccoons, however, Horváth et al. (2021), in Slovakia, EU, reported camera-based evidence of trail-following by both European wildcats (Felis silvestris) and European badgers (Meles meles); both of which followed artificially provided scent trails made of turtle-scented water to artificial European pond turtle (Emys orbicularis) nests. To our knowledge, this is the first well-documented report of trail following by a predator of freshwater turtle nests to appear in the literature.

In some of the earliest studies addressing the question of turtle nest predation, authors suggested that scents directly associated with oviposition could be used by predators to identify nest location. For example, Cagle (1950) suggested, based on anecdotal reports from professional egg collectors, that scent from gravid female turtle urine or oviductal fluids released during oviposition may function as a nest location cue for predators. Similarly, Legler (1954) speculated that “odoriferous fluid voided by the nesting female” may provide an olfactory cue for predators, although he did not provide data in support. In a subsequent study, however, Moll and Legler (1971) found that Ameiva lizards and armadillos (Dasypus novemcinctus) preferentially excavated artificial nests treated with female turtle urine compared to controls. Although that study was based on a small sample size (n = 1 treatment replicate), to our knowledge, this is the first experimental study to conclude that some vertebrate predators locate nests using scent cues directly produced by gravid female turtles during oviposition.

Subsequent experimental studies have continued to investigate the relative importance of olfactory cues directly associated with gravid female turtles during nesting. In these studies, authors typically applied water from nearby wetlands or from containers housing captive turtles to artificial nests and measured predation rates (Tables 2, 3). Most of this research has not provided evidence that scent from nesting female turtles increases the probability of nest predation by raccoons (Table 3). However, Burke et al. (2005) found that raccoons excavated a greater proportion of smoothed-over artificial nests with cavities refilled with Malaclemys terrapin-scented sand than when only unscented sand was used (means ca. 93%, n = 56 vs. 50%, n = 56, respectively; estimated from their Figure 1; difference statistically significant in one of two study years). In a follow-up study (Edmunds et al., 2018), 47% (n = 17) of artificial treatments consisting of 100 mL of terrapin-scented sand deposited on the surface were excavated by raccoons, suggesting the use turtle scent as a cue (not emphasized by authors). However, potentially informative controls of same-source sand without turtle scent were absent, and the excavation rate of artificial nests with cavities with terrapin scent (82.7%, n = 75) was similar to that of cavities without turtle scent (75%, n = 116) (2-tailed Fisher’s exact test p-value = 0.2831; analysis ours). Similarly, Oddie et al. (2015) found that raccoon excavation rates on their visually inconspicuous artificial nests with cavities supplemented with Chelydra musk (ca. 40%) or pond water (ca. 55%) were greater than those without these applied scent cues (ca. 20%; from their Figure 3A). However, locations where these same scents were applied to “artificial nests” that consisted only of visual surface markings were excavated at lower rates (<5% each) than when a cavity was present (overall mean ca. 47%), indicating that some factor associated with nest cavity presence was more influential in nest predation risk than odors directly associated with nesting turtles. Essentially the same conclusion was reached by Oddie et al. (2015), who suggested a synergistic effect of multiple cues in both nest detection and nest predation. However, they considered that cavity-related cue to be tactile, in contrast to some other explanations (such as soil odor, see below).

Related studies on non-raccoon predators are more limited, however Dawson et al. (2014) reported that artificial turtle nests with cavities sprayed with turtle pond water were excavated at higher rates by red foxes than those without pond water, both when they contained chicken eggs (53 vs. 48%) and when chicken eggs were absent (43 vs. 38%). Tuma (2006) noted that coyotes excavated adult yellow mud turtles (Kinosternon flavescens), including males, from their underground burrows while searching for nests. He attributed this to coyotes being attracted to the smell of the turtles, rather than to the smell of the eggs, although other cues, such as odors from disturbed soils, may also have been involved. And recently, Horváth et al. (2021) documented scent trail following by two European mesopredators and surmised that odors from nesting Emys orbicularis were likely the primary nest location cues.

Overall, although not without exception, most research on raccoons indicates little use of residual turtle scent at nests as nest location cues, while limited evidence suggests that canids and other nest predator taxa may use turtle scent cues to greater extents. More studies on the nest-foraging behaviors of a wider array of mammalian nest predators will be necessary to reveal potential interspecific differences in the reliance on scents associated with gravid female turtles.

Artificial nest studies comparing depredation rates of nests with turtle eggs or surrogates to those without eggs provide experimental tests of the importance of egg presence as a factor in nest predation dynamics. In an early study of this type, Wilhoft et al. (1979) found that raccoons actually excavated a greater percentage of the artificial nests depredated in their study (n = 83) when they contained only ping-pong balls (41%) than when they contained either turtle eggs (25%) or bird eggs (34%). Similarly, neither the presence nor absence of eggs appeared to influence artificial nest excavation rates in results obtained by Bernstein et al. (2015) in North America (predator species not reported), or by Perazzo et al. (2018) for two native fox species in southern Brazil.

A few raccoon-based studies (Geller, 2015; Oddie et al., 2015; Czaja et al., 2018) have compared concurrent nest depredation rates between natural nests and artificial nests with cavities lacking both turtle scent cues and eggs (i.e., just refilled cavities). With the exception of Oddie et al. (2015), which found predation rates on natural nests (57% within 4 days of oviposition) to be higher than those of artificial nests without applied scent cues, eggs, or visual disturbance (20%; from their Figure 3A), these studies found that raccoon depredation rates of artificial nests lacking these olfactory signals were similar to those of natural nests, where these potential cues were present.

However, these results contrast with those obtained both by Spencer (2002) and Dawson et al. (2014) for red fox depredation of simulated nests in Australia. Spencer (2002) found higher excavation rates of artificial nests with either turtle or quail eggs than those without eggs, both when surface disturbance was reported as more evident (ca. 80% for turtle eggs and 70% for quail eggs vs. 60 and 45%, respectively) and when efforts were made to minimize disturbance (ca. 60% for turtle eggs and 57% for quail eggs vs. 20 and 15%, respectively; from his Figure 3). Dawson et al. (2014) also reported that artificial nests with chicken eggs were excavated by foxes at higher rates than those without eggs both when sprayed with pond water (53 vs. 43%, respectively) and when not sprayed (48 vs. 38%, respectively).

Overall, there is limited definitive evidence to date suggesting that olfactory cues from nesting turtles or turtle eggs are important cues to raccoons foraging for newly constructed freshwater turtle nests. However, as noted regarding visual cues, olfactory cues from eggs or nesting turtles may be used to a greater extent by nest-foraging red foxes and certain other nest predators (Tables 2, 3).

Efforts to resolve which factors associated with soil disturbance function as nest location cues to predators have been complicated by the confounding presence of co-occurring visual, olfactory, and tactile signals inherent in nest cavity construction, resulting in differences in proposed nest detection mechanisms across studies (Table 2). For example, in the earliest experimental demonstration of the role of disturbed soil as a scent cue, Wilhoft et al. (1979) speculated that olfactory differences caused by the variation in soil moisture between disturbed and intact soils were responsible for increased raccoon predation of artificial nests. Alternatively, Spencer (2002) suggested that disturbed soils, in addition to increasing the potential visual evidence, may enhance the odors of nesting females or their eggs. Several studies have reported on predator excavation of artificial nests constructed by removing soil and replacing it into the nest cavity without surrogate eggs or the application of additional olfactory cues (Tables 2, 3). The elimination of experimental additions of olfactory or visual cues from artificial nests presumably limits predators to two options for locating nests: either olfactory cues originating directly from soil disturbance, or tactile cues resulting from differences in soil density between the nest and the surrounding area.

Geller (2015) and Buzuleciu et al. (2016) attempted to decouple the co-occurring sensory cues present at newly constructed turtle nests, each concluding that the odor of disturbed soils is the primary cue used by raccoons when foraging for turtle nests. In Geller (2015), high predation rates on broom-swept, natural G. ouachitensis nests and artificial nest cavities without eggs, showed that neither visual evidence of nesting turtles, nor olfactory cues from nesting turtles or their eggs were necessary for raccoons to locate nests. Although surface hardness differences between natural and artificial nests and the surrounding substrates were not tested, camera data consistently documented raccoons foraging for turtle eggs with noses close to the ground as they searched turtle nesting areas, strongly suggesting a reliance on olfactory, rather than tactile nest cues (Geller, 2015; Figure 1). Based on these observations, scents derived directly from soils disinterred during nest construction were considered primary nest location cues.

Similarly, Buzuleciu et al. (2016) demonstrated that artificial nests with cavities were associated with three to four times higher raccoon predation than those consisting only of surface-applied scent (gravid female M. terrapin scent, neutral scent, and no-scent control). This study also tested the hypothesis that scent of recently disinterred soil was the primary olfactory cue used by raccoons to locate terrapin nests by comparing the predation rate of unprotected, recently constructed artificial nests to that of artificial nests “aged” for 48 h within raccoon exclusion cages. They found that depredation rates of newly created artificial nests (all with 10–12 cm deep, refilled cavities) were about five times higher than that for 48-h-old nests (84 vs. 16%, respectively). Caging artificial nests for 48 h presumably allowed volatile organic compounds to dissipate, resulting in reduced olfactory cues for the raccoons. Given that artificial nests were caged for a relatively brief duration (48 h) and no rainfall occurred during the experimental period, it is unlikely that soil compaction in the caged treatments prevented raccoons from using tactile searching to locate the opening of the artificial nest chamber.

Both Geller (2015) and Buzuleciu et al. (2016) independently proposed that geosmin, an odiferous metabolite from the soil microbe Actinomycetes aerosolized from disturbed soil profiles and recognized by humans as the smell of disturbed soil (Lindbo et al., 2012), may be one of the underlying olfactory cues produced during nest cavity construction. Based on this proposition, the odor of geosmin alone, or in combination with other related microbial hydrocarbons released by soil disturbance during nest cavity construction, serves as a point-source signal identifying locations of turtle nests to raccoons and possibly other nest predators.

Edmunds et al. (2018), in the first direct test of geosmin as a nest location cue, found that locations where small amounts of geosmin were shallowly injected under soil substrates were, indeed, excavated by raccoons. They further noted that greater administered volumes of geosmin at the same concentration (0.5 mg geosmin/1 mL methanol) resulted in higher excavation rates (25% excavation rate at 0.1 mL, n = 20 vs. 37% at 0.2 mL, n = 19). However, because excavation rates of geosmin treatments were lower than those of both natural nests (67%; n = 42) and standard artificial nest cavities without eggs (overall 82.7%, n = 75, from Treatment 1 in their Table 1), Edmunds et al. (2018) instead concluded that raccoons primarily use tactile cues associated with differences in soil density between nests and surrounding substrates to identify nest locations, although they did not experimentally test anything related to substrate density. Moreover, they did not explore how additional increases in amounts or concentrations of geosmin in their experimental trials may have influenced nest predation rates or discus how the volatilization timelines of the small amounts of injected geosmin in their study may differ from that within the volumes of soil disturbed during natural nest construction.

The only study purporting to provide data supporting the use of surface hardness as a tactile cue for foraging raccoons is Oddie et al. (2015; Table 2). In that study, artificial nests with manufactured cavities (presenting a less-compacted soil surface relative to the surrounding substrate) experienced a higher depredation frequency compared to artificial nests without cavities, independent of whether visual or additional olfactory cues were present. While Oddie et al. (2015) concluded that raccoons probably use more than one sensory cue to locate nests, they proposed that tactile cues resulting from cavity presence were used to make a final determination as to whether to excavate a nest. However, because all treatment combinations used to test a “tactile” (cavity present) predator response (i.e., applied turtle scent, pond water scent, no-scent, or visual treatment) shared hand-excavated and refilled cavities, their results are confounded by scent cues originating from soil disturbance and therefore make the role of tactile cues difficult, if not impossible, to resolve.

With few exceptions, studies indicate that newly constructed turtle nests experience most predation within a few days of construction (Table 4). Nonetheless, a few studies (e.g., Wilhoft et al., 1979) have suggested that the risks of predation on freshwater turtle nests can extend much later into reproductive periods (Table 5). However, one caveat to interpreting the scent cue longevity observations in Wilhoft et al. (1979) is that their raccoon-depredated artificial nests were made in late July, weeks after natural Chelydra serpentina nesting activity had ended. Thus, their results only show that raccoons were still foraging at their study sites and would depredate newly constructed artificial nests during these later dates, not that the aging, natural turtle nests made during the just-completed nesting season were still vulnerable to predators. As a result, some authors have used this paper as support for claims of late-season nest cue retention and vulnerability to depredation, without recognizing this important distinction.

Further examination of papers demonstrating long-term nest predation risk reveals two main commonalities which may explain divergence from studies that report shorter nest vulnerability timelines: the inclusion of data from hatchling emergence periods in predation risk assessments or having nesting areas with more diverse predator communities, rather than just raccoons (Table 5). Both of these factors have introduced complexity, as well as some confusion, into the literature on turtle nest predation dynamics, and are discussed below.

Beginning with Burger (1977), many authors have suggested that new nest location cues arise at the onset of, or during, the hatching period as hatchlings fracture eggshells and absorb residual yolk sacs, yet remain in or above the nest chamber prior to emergence. In studies surveying entire reproductive periods, these renewed cues result in a secondary nest predation peak and extend the reported age of depredated nests relative to studies with shorter timelines. Pre-emergence cues of nest location may include olfactory signals from embryonic fluids and disturbed soils created by subterranean hatchling movements, or hatchling vocalizations (Ferrara et al., 2012; Riley and Litzgus, 2014; Geller and Casper, 2019). Upon surface emergence, new nest location cues potentially include odors arising from nest cavities via exit holes (Congdon et al., 1983; Christens and Bider, 1987), and visual cues provided by hatchling tracks (Congdon et al., 1983; S. D. Gillingwater, pers. comm., in Riley and Litzgus, 2014). However, the importance of potentially confounding, olfactory cues co-occurring with visible tracks has not been investigated, nor have any published studies attempted to resolve the relative importance of these possible cues on predation frequency during the hatchling emergence period.

A less often recognized source of variance in reported nest survival timelines concerns the composition of the involved predator community. To date, Galois (1996) represents the only experimental work that has attempted to directly assess the differential sensory capabilities of common turtle nest predators, finding that olfactory cues likely play a predominant role in turtle nest detection by both striped skunks (Mephitis mephitis) and raccoons, based on food-conditioning and discrimination learning trials. Galois (1996) further determined that striped skunks are likely more narrowly reliant on olfaction than raccoons, in accord with previous research demonstrating the importance of olfaction to foraging skunks (Langley, 1979; Nams, 1991). Raccoons, though also characterized by a well-developed olfactory sense, were found to use both tactile and visual cues to a greater degree than skunks.

Some inferences regarding the role of differential olfactory sense development on nest depredation dynamics in field settings can perhaps be gained by comparing depredation timelines in studies of both natural and artificial turtle nests. For example, in Snow (1982), often cited in support of long nest vulnerability timelines, striped skunks were strongly predominant predators, with little on-site presence by raccoons. Although that study did not examine nor discuss the potential implications of differential sensory capabilities among predator species, it is possible to deduce from Figures 2, 3 in that study that the oldest nests destroyed by skunks and foxes were 22 days old and 8 days old, respectively, while the oldest nest depredated by raccoons was likely ≤ 5 days old. Similarly, Congdon et al. (1987), proposing a differential, olfactory basis, found that red foxes destroyed older Chelydra serpentina nests than did raccoons (mean = 9.4 days, n = 32 vs. mean = 1.9 days, n = 13, respectively). Dawson et al. (2014) also documented long predation timelines by red foxes, with ongoing excavation of artificial nests up to 60 d old occurring throughout monitoring periods. The mid-season nest predation exhibited by skunks and canids likely reflects reliance on turtle egg scent cues themselves, as these odors are presumably the only ones remaining after potential visual, tactile, and other olfactory cues have faded.

In a recent review of nest predation timelines, Riley and Litzgus (2014) presented age-related nest predation data both from their own as well as previously published research. They concluded that nest location cues persist during the several-week period following nest construction and suggested that additional nest predation peaks after week one for the turtle nests in their study were associated with increased numbers of canids at these later dates. They also noted that interspecific differences in cues used by predators, in addition to predator densities and individual behavioral differences, may be partially responsible for reported variation in nest predation timelines. However, while Riley and Litzgus (2014) thus noted the potential for a predator species effect on nest depredation timelines, they did not discuss how differential sensory capabilities among predator species, per se, may have impacted their findings or largely explain the results of other research [e.g., the long vs. short predation risk timelines of Snow (1982) and Burke et al. (2005), respectively; see their Table 1]. In fact, red foxes, able to detect and depredate relatively old nests (see above), were strongly predominant in their study and may have been responsible for the somewhat atypical nest predation patterns they observed.

Available evidence thus indicates that the age-related risk of predation is, at least in part, dependent on the species composition of the local predator community due to differential sensory abilities among species.