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A chart of the amount of venom expended by different snakes for different prey sizes.
Amount of venom expended per hit by large and medium rattlesnakes on large and small prey. This figure shows that rattlesnakes inject different amounts of venom depending on prey size. It supports the idea that rattlesnakes have control of some sort on the lethality of a bite. (Recreated from Hayes, Lavin-Murcio&Kardong 1995)

A chart of the relationship between color and attack rate of different snakes.
Percent of time solid colored model snakes were attacked compared to models of the same base color with viper patterns. This shows that predators learn to avoid venomous snakes without bright warning colors, in support of the hypothesis that bright colors are not necessary for a Batesian mimicry system (Recreated from Wüster et al. 2004).

In both these cases, the non venomous snake increases its survivability by mimicking the venomous snake to the point that predators think they are the same species (and therefore themselves venomous). They are very good examples because the mimic does not gain any conspicuous colors that would hinder it from catching prey or make it more likely to be spotted by would be predators. There are two other forms of defensive mimicry that are subtypes of Batesian mimicry called Mullerian mimicry and Mertensian mimicry , which are forms of defensive mimicry between multiple venomous species and are explained with the coral snake mimics in [link] .

Olfactory mimicry

Another possible type of defensive mimicry is olfactory mimicry . It has been shown in the past that a few plants mimic the smell of dead meat or female insects to attract pollinators into the flowers. However, many scientists were skeptical of the possibility for defensive mimicry based solely on olfactory cues without any visual ones. In 1975, James A. Czalpicki and his colleagues performed an experiment that showed olfactory mimicry could theoretically serve as a form of defense (Czalpicki, Porter,&Wilcoxon 1975). During the experiment, several garter snakes were divided into two groups. Members of both groups were mostly fed minnows, but they were also fed night crawler worms on occasion. The experimental group was given a small dose of lithium chloride, which would make them mildly ill, while the control group was injected with saline solution. The results showed that the experimental group later rejected minnows that were dipped in “night crawler surface extract” so they smelled like the worms, but didn’t reject regular minnows. The control group did not reject the minnows that smelled like worms. Also, as a further experiment, they ran the same test but included some minnows made to smell like salamanders, without injection of LiCl, which neither group rejected. This showed that the experimental group rejected the earlier minnows because they smelled specifically like night crawlers and not because they smelled different from their normal meals for the previous several weeks. It seems though, that there have not been any cases found in the wild were olfactory mimicry is used by a species specifically for defensive purposes to date.

Death feigning

Another interesting defensive mimicry behavior in snakes is the well known death feigning of American hognose snakes, Heterodon spp . When threatened, a hognose snake flips onto its back and starts writhing around as if it is about to die from serious illness. This is followed by bloating and excretion (Munyer 1967). The snakes perform this display in water as well as on land, but tend to move more quickly to bloating and stillness when in water. When flipped back upright, the snake immediately flips onto its back again and continues the bluff. But what evolutionary purpose does death feigning really have? One possible explanation, and the most likely for hognose snakes, is that feigning death in such a dramatic and disturbing way will make the predator think that the snake has a disease or parasites and will not eat it for that reason (Milius 2006). It has also been discussed with insect and fish species that feigning death can provide defense by way of the bad odor emitted by many species displaying this behavior or can even be a form of aggressive mimicry that lures unsuspecting scavengers near the organism, which then 'comes back to life' and eats them. Some cases of similar behavior have been misinterpreted as death feigning, but are actually a form of defense where the bloating of the individual just makes it hard to swallow.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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