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. Author manuscript; available in PMC: 2021 May 1.
Published in final edited form as: Ethology. 2019 Dec 26;126(5):503–508. doi: 10.1111/eth.13000

Does Rattling Deter? The Case of Domestic Dogs

Nancy G Caine 1, Rita Muñoz 1, Michele M Mulholland 1,2
PMCID: PMC7938867  NIHMSID: NIHMS1063586  PMID: 33692603

Abstract

Although the rattling of rattlesnakes (Crotalus and Sistrurus) is widely accepted as being aposematic, the hypothesis that rattling deters approach from the snake’s potentially dangerous adversaries has not been well tested. In a controlled study using rattling recorded from captive rattlesnakes (C. oreganus helleri) and a variety of comparison sounds or no-sound controls, domestic dogs (Canis familiaris) showed no hesitation to approach camouflaged speakers projecting the recorded rattles. The dogs were equally likely to approach speakers projecting rattling as they were to approach speakers playing control sounds, or speakers that were silent. Furthermore, the dogs spent no less time in front of the speakers projecting the rattles than they did in front of speakers projecting control sounds or no sound. The dogs’ reactions may not be representative of other species with whom rattlesnakes come into contact, but the data suggest a need for some circumspection about the role of rattling in the rattlesnake’s defensive repertoire. Our results also suggest that dogs may be vulnerable to envenomation because they fail to react to the sound of rattling with avoidance.

Keywords: Crotalus, envenomation, Canis, aposematic, deimatic, rattlesnake, rattles

Introduction

The rattling sound made by snakes of the genera Crotalus and Sistrurus is widely assumed to be among the best examples of aposematism, whereby individuals warn potential predators of their ability to inflict harm, thereby discouraging the predator, or other source of danger, from attacking or otherwise injuring the snake. Surprisingly, however, direct evidence that rattling has this deterrent effect is scant. The only data about reactions to rattling that come from controlled studies involved California ground squirrels (Spermophilus beecheyi). Rowe and Owings (1978) found that squirrels initially react to rattling snakes (C. viridis oreganus) with withdrawal, but then switch to more offensive and harassing maneuvers directed at the snake. Swaisgood, Rowe, and Owings (1999) showed that squirrels will approach and eat in front of the sound of a rattling snake, but do so with vigilance and caution. However, ground squirrels may not be a good test case regarding the deterring effect of rattling, as adult squirrels engage with rattlesnakes in protection of their young and are themselves neither prey nor predators of rattlesnakes. Furthermore, most populations of California ground squirrels have substantial immunity to rattlesnake venom (Coss, Gusé, Poran, & Smith, 1993). There are other studies of rattlesnake interactions with mammalian and avian species but some of these focus on the snakes’ general reactions to disturbance (Parent & Weatherhead, 2000) while others look at reactions to snakes without providing details about the specific role of rattling in those interactions (Rogers et al., 2014, re: bears, Ursus americanus; Almeida-Santos, Antoniazzi, Sant’Anna, & Jared, 2000, re: opossums, Didelphis marsupialis; Sherbrooke & Westphal, 2006, re: greater roadrunners, Geococcyx californianus). In his classic two-volume work on rattlesnakes, Klauber (1956) presents dozens of anecdotes about reactions of a variety of species to rattlesnakes, but these are not systematic or experimental and usually, in fact, highlight remarkable variability of reactions.

According to Greene (1997), the source of threats to which rattlesnakes may be exposed, and for which rattling may have evolved, are less likely to be the large grazing omnivores who might step on snakes (Hay 1887), but crevice-probing carnivores such as coatis (Nasua narica) and bears. Canids are also among those species that might pose a threat to rattlesnakes. Rattlesnakes are limited to the New World and are found in habitats where canids, including wolves (Canis lupus) and coyotes (C. latrans), are or were common. It is unlikely that snakes ever formed a significant portion of the diet of these canids, but given the rather omnivorous nature of the canid diet (Bradshaw, 2006), it is also unlikely that snakes would ignore a canid’s approach. Domestic dogs, who evolved from grey wolves (Vilà, Maldonado, & Wayne, 1999) and have been found in the Americas as early as about 10,000 years ago (Perri et al., 2019), undoubtedly interacted with rattlesnakes from their earliest arrival here, and dogs continue to encounter rattlesnakes in rural and even suburban settings. Indeed, although statistics are hard to find, dogs are subjected to rattlesnake bites in significant numbers. In the United States, it is estimated that about 150,000 dogs and cats are victims of pit vipers annually (Peterson, 2006). In just a two-year period in Maricopa County, Arizona, there were 272 reported cases of dog envenomation by rattlesnakes (Witsil, Wells, Woods, & Rau, 2015). Most of the bites were to the head.

Based on the number of envenomations and the location of the bites, which suggest that dogs are moving toward the snake when bitten, it appears that dogs may approach rather than avoid rattlesnakes they encounter. Mulholland, Olivas, and Caine (2018) found that dogs showed no evidence of fear or avoidance in the presence of rattlesnake odors, nor did they avoid approaching and eating in front of realistic rattlesnake models (Muñoz, unpublished data). However, one might assume that rattling is the most successful way to deter a nearby dog. As rattlesnakes do not always rattle when disturbed (Greene, 1988; Almeida-Santos et al., 2000; Prior & Weatherhead, 1994), it might be that cases of dog envenomation occurred primarily when snakes did not rattle and thus failed to warn and deter the dog.

We set out to test the hypothesis that, as is generally assumed but remains mostly untested, rattling by snakes deters approach by animals that may prey upon or otherwise injure the snake. In doing so, we hoped to add to our understanding of why dogs are subject to high rates of envenomation by rattlesnakes by investigating the willingness of dogs to approach the sound of a rattling snake.

Methods

Subjects and Materials

Subjects included a total of 79 dogs, 40 males and 39 females, primarily of mixed breeds. They ranged from 2 – 41 kg (X¯=19.9 kg) and 4 months to 13 years old (X¯=3.3 years). All subjects were household pets and were recruited and tested at Mayflower Dog Park located in Escondido, California. Each dog participated in just one trial.

We made audio recordings of rattlesnake rattles and control sounds using a Marantz PMD-660 Digital Recorder with a Sennheiser M-67 directional microphone. Rattling was recorded from nine captive adult Southern Pacific rattlesnakes (Crotalus oreganus helleri; 6 male, 3 female) housed in Dr. Rulon Clark’s laboratory at San Diego State University (APF # 16-08-014C). All snakes were wild-caught adults collected from local populations in San Diego County. The snakes were placed one at a time in an open plastic 114 litre container and disturbed by gently probing the snake with snake catching tongs to induce rattling. The rattle sound produced by each snake was recorded for approximately 1 min at a distance of approximately 1.5 m. Control sounds were selected with a goal of including a wide variety of sounds, some of which were more like rattling than others: shaking a plastic maraca; shaking toothpicks in a plastic container; shaking hard candies in a glass jar; crumpling paper; striking a pencil against a hard surface; pouring water from one container to another; striking a gourd instrument. All control sounds were recorded for approximately 1 min from a distance of 1 m using the same recording equipment described above.

Sixteen stimuli, composed of 7 control sounds and 9 rattlesnake rattles, were created using Audacity(R) editing software (v.2.3.0; Audacity, 2018). The choice of stimulus was randomly determined for each trial. Each of the seven control sounds was used on average 6 times, with a range of 3–9 times, and each of the nine rattle stimuli was used on average 4 times, with a range of 1–6 times. Each of the 16 stimuli was 25 s long. Peak frequencies, measured in Raven Pro v. 1.5 (Bioacoustics Research Program, 2017), were similar for control sounds and rattlesnake rattles (X¯s=4590 Hz and 5053 Hz, respectively). Likewise, the peak amplitudes were similar between control sounds and rattles (Xs¯=82.8 dB and 86.6 dB, respectively). These measurements were taken indoors from 2.13 m away, which is the same distance as that between dogs and speakers at the start of a trial. The perceived loudness of the stimuli when played during an experimental trial of course varied in accordance with ambient noise in the environment, but it is highly unlikely that it would systematically vary by condition.

Bluetooth speakers (Tribit XSound Go, www.tribitaudio.com), with a frequency range of 85 Hz - 20 KHz and measuring 17 cm long x 5.7 cm wide, were used to play the sound files. For all trials there were two speakers, each perched on a 14 cm high x 15.3 cm diameter ceramic pot. Artificial ivy was used to camouflage the speakers. To prevent direct contact by dogs, the pots were surrounded by chicken wire. To encourage and assess the willingness of the dogs to approach the stimuli, we placed three dog treats on small white paper plates directly in front of the chicken wire. The method of using treats to encourage canines to participate in choice-type experiments has been successfully employed in the past (Siniscalchi, Sasso, Pepe, Vallortigara, & Quaranta, 2010). Each side had the same number, type, and arrangement of treats.

Procedure

The procedure was approved by the California State University IACUC (18–014) as well as the California State University Institutional Review Board (866896–4). Data collection took place between December 2018 and June 2019 between 10:00 am and 2:00 pm, typically for about 2–3 hr per session. Owners who appeared to be over 18 were approached upon leaving the dog park and asked if their dog(s) could participate in the study.

The two ceramic pots were placed 127 cm apart with a chalked line drawn midway between them to delineate the two sides. Another chalked line was drawn 60 cm in front of the pots. Dogs were scored as having approached the stimulus if any part of their body crossed that line. Stimulus order and side from which the stimulus sound was presented were randomized. Thus, subjects saw two potted plants, each containing a camouflaged wireless speaker. On any given trial one of the speakers played either a rattlesnake rattle or a control sound, controlled remotely. The other speaker was silent. The speakers were angled away from each other such that the speaker playing the sound projected away from the other (silent) pot, making it easier to localize the source of the sound. In pilot trials, human listeners reported that they were easily able to determine from which speaker the sound was emanating.

A research assistant instructed the owner to stand and remain inside a chalked circle 2.1 m away from the pots (Fig.1). The owner was told to hold his/her dog in front of him/her on a tight leash within the chalked circle until s/he saw a thumbs up sign given by another research assistant. The thumbs up sign corresponded with the initiation of the playback from the speaker. After seeing the thumbs up sign, the owner was instructed to loosen hold on the leash to allow his/her dog to approach the pots, or not, at will. Owners were asked not to direct or encourage their dog in any way. Owners wore ear plugs during the trial to prevent them from biasing the dog’s reactions.

Fig. 1.

Fig. 1.

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The experimental set-up. A speaker was placed across the top of each pot, angled in an outward direction, and obscured with foliage from an artificial plant. Three dog treats (not shown) were placed immediately in front of the pots. On any given trial, one speaker (randomly determined) projected either a control sound or the sound of a rattling snake. The speaker on the other side was silent.

Each trial was recorded using a Canon Vixia HF R600, placed on a tripod that was centered 2.48 m behind the pots. The trial ended after the 25 s audio recording was played twice. After the trial was completed, owners were asked about the dog’s age, sex, breed, and experience with snakes.

Videos were analyzed using QuickTime Player. The behaviors of interest were side of first approach (sound or silent side), the amount of time spent in proximity to each pot, and the latency to approach a pot. Scoring was done blind to condition, with the sound muted, such that the scorer did not know which side was presenting the sound, or if the sound was a control or a rattle.

Results

Of the 42 dogs in the control sound condition, 12 failed to move out of the circle and thus did not approach either pot while the stimulus was playing. Eight of the 37 dogs in the rattle condition failed to approach either pot. Dogs were no more likely to refuse approach in one condition than the other (Fisher’s exact test, p = .61) (Fig. 2). Most of the dogs who approached the silent side also approached the sound side in both the rattle (26 of 29 dogs) and the control sound (28 of 30 dogs) conditions. Next we examined which pot, sound or no sound, the dogs approached first, and if that was related to condition. In the control sound condition, 17 of the 30 dogs approached the sound side first; in the rattle condition 15 of the 29 dogs approached the sound side first (Fisher’s exact test, p = .79) (Fig. 3). Among dogs whose first approach was to the sound side, there was no difference in how quickly they left the circle to approach the pot in the control sound condition (n = 14, X¯=12.1 s, SD = 2.4) versus the rattle condition (n = 13, X¯=9.9 s, SD= 2.54) (t (25) = 0.64, p = .53) (Fig. 4). Similarly, when considering dogs who approached both sides during the trial and looking at time to approach the sound side regardless of which side was approached first, it took, on average, 13.54 s for these dogs (n=26) to approach the rattle side. An unpaired t-test confirmed that this was not significantly different (t (52) = 0.39, p= .69) than the time it took (X¯=14.68 s) for dogs in the control sound condition (n=28) to approach the control sound side. Finally, a mixed model ANOVA was used to look at the between subjects (rattle vs control sound condition) and within subjects (sound or silent side) variables, and any interaction between them, in terms of time spent in proximity to each pot. There were no main effects for silent side vs sound side (F[1,57] = 0.51, p = .48) or for rattle vs. control sound (F[1,57] = 0.94, p = .34), and no interaction (F[1,57] = 0.42, p = .52) (Fig. 5).

Fig. 2.

Fig. 2.

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Number of dogs who refused to approach either side, and number who approached at least one side, in the rattle and control sound conditions. (Fisher’s exact test, p >.05)

Fig. 3.

Fig. 3.

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Number of dogs whose first approach was to the side emitting the sound or the silent side in rattle and control sound trials. (Fisher’s exact test p >.05)

Fig. 4.

Fig. 4.

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In trials where dogs approached the sound side first, the elapsed seconds (mean and SD) before leaving the circle to approach the pot (t-test, p >.05).

Fig. 5.

Fig. 5.

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Number of seconds (mean and SD) the dogs spent in proximity to the sound and silent sides in the rattle and control sound conditions. Neither the main effects nor the interaction were significant (mixed model ANOVA, p >.05).

According to their owners, four of the 79 dogs had experience with live rattlesnakes. Two of these dogs were subjects in rattle trials. One had been bitten by a rattlesnake, and one had undergone rattlesnake aversion training, which involved protected exposure to live rattlesnakes. Both of these dogs approached the rattle side, and did so before they moved to the silent side. One dog spent 23 s at the rattle side and 12 s at the silent side. The other dog spent 7 s at the rattle side and 15 s at the silent side.

Discussion

Domestic dogs in our study failed to demonstrate any reluctance to approach a camouflaged speaker that was projecting a recording of rattlesnake rattles. The dogs were equally likely to approach and remain near speakers projecting rattling as they were to approach and remain near speakers playing control sounds, or speakers that were silent. To our knowledge, this is the first controlled study that tests the hypothesis that rattling deters the approach of a potentially dangerous adversary. In presenting these results, we shed light on the fact that domestic dogs are bitten by rattlesnakes with disturbing frequency, and raise questions about the general hypothesis that rattling by rattlesnakes deters potential sources of danger.

Williams (1966) proposed that rattling, which evolved from defensive tail vibrations (Greene, 1988; Allf, Durst, & Pfennig, 2016), functions not to warn but to direct threatening attention away from the snake’s head. However, the likelihood of an aposematic function of rattling is supported by a good deal of indirect evidence (Greene, 1992). For instance, rattling is observed only in defensive contexts (Greene, 1988), making it unlikely that rattling potentiates the caudal luring that has been proposed as the origin of rattling (Schuett, Clark, & Kraus, 1984; Sisk & Jackson, 1997; Rowe, Farrell, & May, 2002). Burrowing owls (Athene cunicularia) (Owings, Rowe, & Rundus, 2002) and gopher snakes (Pituophis melanoleucus) (Kardong, 1980; Sweet, 1985) both mimic the rattling of rattlesnakes, and it is hard to imagine that this mimicry would have evolved if it had not protected the mimics from attack. On the other hand, as described above, rattlesnakes do not always rattle when they are threatened or disturbed, raising questions about potential costs of rattling. In fact, the means for latency to approach and time spent in proximity to the pots in our study show a slight trend toward greater interest in the rattles than in control sounds. A cost of rattling might, therefore, include promotion of investigative behavior that is unwanted by the snake (Owings et al., 2002). Indeed, Klauber (1956) provides anecdotes about dog-rattlesnake interactions, many of which were quite aggressive and led to the death of the snake.

Fenton and Licht (1990) propose that the sound of the rattle is not necessarily aposematic but is deimatic, designed to gain the attention of and/or startle animals near the snake by virtue of its similarity to agonistic vocalizations typical of many species. A startle reaction buys the snake time to retreat, and gives the approaching animal a chance to more safely evaluate the threat. As mentioned above, Swaisgood et al. (1999) reported that the immediate reaction of foraging ground squirrels to rattle playbacks was to jump away from the speaker, but they often returned to resume eating, perhaps after reassessing the threat. Carefully constructed experiments that precisely examine the immediate reaction of individuals when first hearing a rattling sound are needed in order to tease aposematism and deimatism apart (Skelhorn, Holmes, & Rowe, 2016; Umbers, De Bona, White, Lehtonen, Mapes, & Endler, 2017; Holmes, Delferriere, Rowe, Troscianko, & Skelhorn, 2018).

The unfortunate absence of controlled studies that test the assumed deterrent effect of rattling makes it hard to know if domestic dogs are simply an outlier species when it comes to reactions to the sound of a rattle. Although domestication is unlikely to have protected the dogs from contact with dangerous snakes, the process of domestication may have somehow interfered with dogs’ interpretation of snake-related cues. On the other hand, it might be that the populations of grey wolves from which domestic dogs rather recently evolved (Botigué et al., 2017) were not subjected to significant snake-related selection pressures in their cold, northerly habitats (Mulholland et al., 2018). There is likely to be learning involved in recognition of the danger communicated by aposematic signals (Bates & Fenton, 1990), including rattling, but the two dogs in our sample who were reported to have had direct experience with rattlesnakes readily approached the speakers that were projecting the recorded rattles.

Our study benefitted from the use of multiple exemplars of both rattle and control sound stimuli to avoid pseudoreplication. Our conclusions are supported by consistency in the results across dependent measures (willingness to approach, latency to approach, time in proximity) and comparisons we made both within (sound vs silent side) and between (control sound vs rattling) experimental conditions. However, the setting in which we collected our data, while naturalistic, cannot be said to be fully natural. Dog parks have many distractions, including unfamiliar dogs and humans, and dogs can be quite excitable in these environments. We instructed the dog owners to stand quietly while the dogs approached the pots, and the calm demeanor of the owners might have communicated a lack of concern to the dogs. Furthermore, by isolating rattling from the other cues associated with rattlesnakes themselves, such as their distinctive heads, and from other aspects of rattlesnake behavior, including their seasonal patterns of activity, we might have reduced the salience of the acoustical signal. Perhaps, in a circumstance where all of the right cues are aligned, dogs would show more avoidance of the rattling sound.

In conclusion, based on the behavior of domestic dogs in our study, the hypothesis that rattling protects the snake by deterring the approach of potentially dangerous animals needs to undergo rigorous testing across a wide variety of species. In doing so it will be important to recognize that the deterrent effect of any signal does not operate in isolation from the context of the threat or the motivations, such as hunger, and past experiences of the individual it was designed to deter. A rattle’s deterrent effect is likely relative, and we need to understand the factors that make it so (Skelhorn, Halpin, & Rowe, 2016). Likewise, a better understanding of the particular defensive circumstances under which rattlesnakes engage in rattling is needed, as our current knowledge is mostly limited to published reports in which conditions and contexts of rattling are reported only incidentally (Owings et al., 2002, is an exception) or because the focus of the investigation was not specific to rattling (Duvall, King, & Gutzwiller, 1985). Finally, in showing that dogs readily approached the sound of rattling snakes, our data may help us understand the vulnerability of domestic dogs to rattlesnake attack.

Key Findings:

Although rattlesnake rattles are assumed to be aposematic, we found that domestic dogs are no less likely to approach the source of playbacks of rattlesnake rattles than they are to control sounds or silence.

Acknowledgements.

This work was supported by the Office for Training, Research, and Education in the Sciences, and the Research Initiative for Scientific Enhancement (RISE) programs at CSU San Marcos, funded by the National Institute of General Medical Sciences (NIH/NIGMS). We thank Cody Jensen for his help with data collection and Dr. Rulon Clark for access to rattlesnakes for recording rattling. Dr. Matt Rowe provided insightful comments on an earlier version of this paper. We have no conflict of interest to declare.

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