Sagehen Creek Field Station

19 Aug, 02 AM - 24 Aug, 05 AM

BACKGROUND AND RATIONALE: Intraguild predation, or the killing of one species by competitor species, can greatly influence ecological communities and figures prominently in limiting local populations (Polis et al. 1989; Crooks and Soul? 1999). To mitigate these influences, subordinate species must have means to avoid or deter predator attacks. Potential strategies to evade intraguild predators include temporal and spatial variation in resource use (e.g. Schaller 1972; Tannerfeldt et al. 2002) and behavioral (i.e. vigilance; Lima and Dill 1990) and morphological (i.e. camouflage; Merilaita and Lind 2005) adaptations. Those species that are unable to avoid potential predators should have some anti-predator mechanism that deters predator attack. Visual interspecific signaling can communicate to potential predators that some prey species would be unwise to attack, either because the chance of success is low or because the chance of a counterattack is high (Ruxton et al. 2004). Aposematism, or the use of conspicuous contrasting coloration which warns potential predators that the bearer possesses some noxious quality, has been overlooked in studies of anti-predators adaptations in mammals. Accordingly, surprisingly little is known about the means and circumstances under which aposematism functions as an effective predator deterrent. Striped skunks (Mephitis mephitis) are perhaps the most emblematic of all aposematic mammals. They possess starkly contrasting coloration and when cornered spray a noxious anal secretion at their attackers. This defensive spray causes nausea, vomiting, contact with eyes can lead to temporary blindness and when inhaled it can cause death. Despite the well known aposematic coloration of skunks and the harsh consequences for ignoring their signal?s meaning, the current utility of skunk coloration as a predator deterrent has never been explored. OBJECTIVES: Here I propose to examine aposematism in mammalian carnivores using alternative presentations of taxidermy mounts. Specifically I will be addressing the following questions: 1.) Which skunk attribute, shape or color, is more influential in eliciting avoidance by other carnivores? and 2.) Does the local abundance of skunks influence the willingness of other carnivores to approach skunk and skunk-colored mounts? EXPERIMENTAL PROCEDURE: I will use de-scented taxidermy mounts of striped skunks and a similar sized, undefended carnivore (grey foxes, Urocyon cinereoargenteus) to evaluate the deterrent properties of black and white coloration, like that found in striped skunks. Mounts will be baited and monitored using motion detecting digital video cameras to capture any carnivore visits. I will use these visit to evaluate whether any mounts are avoided by other carnivores. Avoidance will be measured in terms of visitation frequency, latency to approach and approach distance. I expect the mounts predators perceive as being dangerous to be approached less frequently, approached with greater hesitancy and inspected from a greater distance, when compared to other mount presentations. The survivorship benefits conferred by the use of aposematic signals depends on the ease with which aposematic prey can be distinguished from alternate prey. I will use striped skunk and grey fox mounts to explore two cues that may aid predators? discrimination of defended (skunk) and undefended (fox) prey, namely body shape and coloration. I will present five camera stations baited with a mixture of fish oils and beaver castorium. Each station will be randomly assigned one of four mount (natural colored skunk, natural fox, grey-dyed skunk and black and white-dyed fox) or control (baited, but no mount) treatments. Mounts stations will be placed, in random order, at 250m intervals along road or trails. All visits at each mount will be recorded for 5 consecutive nights; each station will be re-baited nightly. Repeating visits at the same station by the same species within any 5 minute period will be excluded. After each sampling period mount stations will be moved to another transect within the study site, with the nearest station a minimum of 1 km from all previous transects. Stations will be relocated 4 times for a total of 20 station nights per site. Aposematism is thought to be most effective when aposematic species are found at high density so that the meaning of the signal is frequently reinforced (Guilford and Dawkins 1996). The vigorousness of carnivore responses to skunk and skunk-colored models may depend upon the frequency with which striped skunks are encountered, that is, a potential predator may be more likely to recognize and appreciate the warning aspects of skunk coloration if it has previously been, and perhaps frequently is, exposed to skunk defenses. To address whether predator behavior is influenced by the local abundance of striped skunks I will conduct a carnivore survey at each site. I will place 40 baited track stations at 250m intervals along roads or trails (human or wildlife) at each study site and I will record the number of visits from all carnivore species, including striped skunks and their predators. Track stations will baited with a mixture of fish oils and beaver castorium and will be run for 5 consecutive days, for a total of 200 station nights at each study site. At the end of my study period I will calculate the relative abundance of striped skunks at each site and explore any correlation between these measures and the behavior of species visiting mount stations. Data will be collected at seven northern California sites including five University of California reserves (Big Creek, Quail Ridge, McLaughlin, Valentine and Sagehen) as well as Fort Ord Public Lands (BLM) and Mitteldorf Preserve (Big Sur Land Trust). Data will be analyzed with each study area considered a single sampling unit, as the independence of sampling points within each study area cannot be guaranteed. Visitation rate, latency to approach and approach distance at each mount treatment type will be analyzed across study areas using 2-way analyses of covariance to evaluate whether these measures vary according to mount shape (skunk or fox) and/or mount color (skunk or fox). In addition, visitation rate, latency to approach and approach distance at skunk and skunk-colored mounts will be analyzed using analysis of variance to evaluate whether these measures vary with the relative abundance of striped skunks. PRELIMINARY RESULTS: Preliminary data were collect at two research sites in Northern California: Fort Ord Public Lands (hereafter, FO) and Landels-Hill Big Creek Natural Reserve (hereafter, BC). Carnivore species visiting track-stations and mount-stations included coyotes (Canis latrans), grey foxes, striped skunks, bobcats (Lynx rufus), long-tailed weasels (Mustela frenata), raccoons (Procyon lotor), mountain lions (Puma concolor), American badgers (Taxidea taxus) and western spotted skunks (Spilogale gracilis). Total visitation rate at mount stations was commensurate across study areas with a 0.43 visitation rate at BC and 0.48 visitation rate at FO. There was however a marked difference in visitation rate, latency to approach and approach distance according to mount coloration and shape. At both study sites the black and white colored mounts were visited less frequently, with greater hesitancy and examined from greater distance when compared to the grey mounts. The visitation rate of carnivores at track-stations was greater at BC than at FO with rates of 0.87 and 0.51, respectively. The proportion of species visiting also varied. Notably, striped skunks comprised less than 1% of track-station visits at BC and nearly 28% of station visits at FO. I used this disparity to examine between-site differences in visitation at striped skunk mount-stations. At BC, where striped skunks are comparably rare, carnivore visits to the black-and-white-skunk (BWS) mount stations made up almost 30% of all mount station visits. In contrast at FO, where skunks are abundant, BWS mounts were rarely visited (4% of all visits). ANTICIPATED SIGNFICANCE: For aposematic carnivores there is a near total reliance on the strength of their aposematic signal to avoid being killed by larger species (Hunter, in review); circumstances where signal quality or meaning is lost could be dire for population persistence. While striped skunks, the model species used in this study, are not of conservation concern, closely related spotted skunks (Spilogale spp.) have been declining precipitously throughout mainland North America for the last 60 years. The cause of this decline is wholly unknown and has been the subject of only one study (Gompper and Hacket 2005). A study by Cooks (2002) found that spotted skunks are unique amongst mammalian carnivores in that they require larger habitat fragments than would be predicted by their body size. In light of the results of my study to date, Crooks? (2002) findings may be interpreted as further evidence that spotted skunk populations must maintain a certain threshold density in order to persist; in those areas too small for spotted skunks to maintain this density their aposematic signal may lack sufficient reinforcement to continue to function effectively. In addition to the conservation implications, this study may shed light on the evolutionary origins of aposematism. The evolution of aposematic traits has been discussed in the literature although the predominant theories have only been tested using simulation experiments between predators and novel prey (i.e. Alatalo and Mappes 1996, Sherratt and Beatty 2003). Here, I present clear evidence for the density dependent effectiveness of aposematic signals between natural predator-prey dyads in their natural habitat. Furthermore my study challenges the assertion that there is an innate predator bias against specific colors and color combinations, as is often cited (see Ruxton et al. 2004). Instead the strong density dependent relationship I have observed thus far suggests just the opposite, that avoidance of aposematic prey is learned; in circumstances where interactions with skunks were rare, predators showed few, if any, avoidance behaviors. LITERATURE CITED: Alatalo, R. V. and J. Mappes. 1996. Tracking the evolution of warning signals. Nature 382: 708-710. Crooks, K. R. 2002. Relative sensitivities of mammalian carnivores to habitat fragmentation. Conservation Biology 16: 488-502. Crooks, K. R., and M. E. Soule. 1999. Mesopredator release and avifaunal extinction in a fragmented system. Nature 400:563-566. Gompper, M. E. and H. M. Hackett. 2005. The long-term, range-wide decline of a once abundant carnivore: the eastern spotted skunk (Spilogale putorius). Animal Conservation 8: 195-201. Guilford, T. and M. Dawkins, M. 1996. Receiver psychology and the evolution of animal signals. Animal Behavior 42:1-14. Lima, S. L., and L. M. Dill. 1990. Behavioral decisions made under the risk of predation: A review and prospectus. Canadian Journal of Zoology 68:619-640. Merilaita, S., and J. Lind. 2005. Back-ground matching and disruptive coloration, and the evolution of cryptic coloration. Proceedings of the Royal Society of London B 272 . Polis, G., C. Myers, and R. D. Holt. 1989. The ecology and evolution of intratuild predation: Potential competitors that eat each other. Annual Review of Ecology and Systematics 20:297-330. Ruxton, G. D., T. N. Sherratt and M. P. Speed. 2004. Avoiding attack: The evolutionary ecology of crypsis, warning coloration and mimicry. Oxford: Oxford University Press. Schaller, G. B. 1972. The Serengeti Lion. Chicago, University of Chicago Press. Sherratt , T. N. and C. D. Beatty. 2003. The evolution of warning signals as reliable indicators of prey defense. American Naturalist 162:377-389. Tannerfeldt, M., Elmhagen, B. and Angerbj?rn. 2002. Exclusion by interference competition? The relationship between red and arctic foxes. Oecologica 132: 213-220.

Visit #13135 @Sagehen Creek Field Station

Approved

Under Project # 8571 | Research

The current utility of aposematic coloration in striped skunks

graduate_student - University of California, Davis

Reservation Members(s)

Jennifer Hunter Aug 19 - 24, 2007 (6 days)

Reserve Resources(s) | Create Invoice

Classroom 1 Aug 19 - 24, 2007
Lab / Classroom 1 Aug 19 - 24, 2007
Treehouse(Lower Camp) 1 Aug 19 - 24, 2007