Wednesday, September 18, 2013

Mimicry - Survival through deception

Human–shark interactions have become more and more common over the last 10 years due to an increase in human population, and a higher number of ocean users. Fisherman, divers, swimmers and surfers all enter the realms of the shark when pursuing these activities and unfortunately although incredibly uncommon, shark-human conflicts do occur.

My research aims to investigate potential shark deterrents to help increase protection for surfers in “sharky” waters.  There have been a number of different shark deterrents investigated in the past to try and protect ocean users.  Electrical barriers, acoustic playbacks and chemical deterrents have all shown potential.  However none of these methods have been completely successful or sustainable and can have a negative effect on the environment and non-target species.

Shark nets were introduced in South Africa in 1952 in Kwa-Zulu Natal to protect bathers. However, the nets are not a species specific defense and kill huge numbers of sharks and other marine species such as whales, dolphins and sea turtles. Other deterrents such as the shark shield have been developed for individual safety of swimmers and surfers but when tested with large sharks such as the white shark it was less than successful (it was eaten).  Therefore it is important that more sustainable methods of protection are investigated in order to help conserve shark species and protect water users.

I am currently looking at whether or not the visual sign stimulus of orca whales (Orcinus orca) creates a natural fear and avoidance behaviour in white sharks (Carcharodon carcharias) and if this fear can be utilized as a shark deterrent to increase surfer protection.

The idea behind this theory originates from observations of orca whales feeding on elasmobranchs, including white sharks all over the world.A fascinating event occurred in the South Farallon Islands (a well-known white shark aggregation area) in 1999 where two orcas were seen eating a large white shark. Furthermore,white sharks were not seen in the area for almost 2 months after this event occurred. This reaction from the white sharks suggests that they may have a natural fear of orca whales and take precautionary measures to avoid them.

There are two possible explanations for this behavioural reaction to another species.  It could be an innate behavioural response where sharks have a“hard wired”genetic instinct to be afraid of orcas and react to their warning signals without any previous experience.  Alternatively it could be a learnt behaviour where the sharks have seen orca whales before and have experienced the specific visual, olfactory and acoustic cues of the orcas so they can avoid them as early as possible in the future. Either way it makes sense for white sharks to be able to recognize and respond to the threat of orca whales.

Orcas have a very striking black and white pattern on the ventral side of their body.  This unambiguous colouration and pattern could be a distinct warning signal to white sharks triggering a cautious response and signalling its threat. This could be valuable for juvenile orcas when they are still small and vulnerable to attack from large sharks if they get separated from their family pod.
Interestingly there is a small dolphin species found off the Southern African coast called the Heaviside dolphin (Cephalorhynchus heavisidii). These dolphins share this distinctive ventral pattern and also live in white shark occupied waters, but this dolphin poses no threat to sharks. However it could be possible that this dolphin species has evolved a similar pattern to the orca whale to increases its protection from becoming preyed on by large sharks such as great.  If the sharks recognise this pattern and it triggers a cautious or even a withdrawal response immediately, then the dolphin will be less likely to be attacked. This defense tactic these dolphins seem to have evolved is known as Batesian mimicry.

Source: interesting   


Batesian mimicry is a broad and somewhat complex area of evolutionary biology.  It was first brought to light by an English naturalist called Henry Walter Bates, whose work on butterflies in the Amazon rainforest in the 1800’s pioneered the breakthrough of this natural phenomenon.
Batesian mimicry is most commonly seen in insects, fish and snake species, but found in a number of terrestrial and marine species.  It is based on predators learning from past experience and used as a form of defense by less dangerous or harmless species in order to increase their survival rate.
The two main strategies of mimicry are Batesian and Mullerian mimicry.  There are three groups involved with this protective tactic. The species copying the signal is the “mimic”, the species being mimicked is known as the “model” and the predator the mimic is intending to deceive is the signal receiver.

Batesian mimicry works on the basis that the predator the species is trying to avoid has encountered the unpalatable or more dangerous model previously and learnt that the particular signal (i.e. colouration, smell or sound) means the animal is an unprofitable one and therefore is deterred from feeding on individuals that emit that particular signal, this could explain the similar colouration in the Heaviside dolphin.For this reason Batesian mimicry is most beneficial to mimics when the model species are in greater abundance than the mimic species. If there are more mimics than models the predators will not experience the negative reaction from the individual and therefore not learn to avoid the warning signs. This is known as negative frequency dependent selection.

One of the most famous and fascinating examples in Batesian mimicry in the marine world is the mimic octopus (Thaumoctopus mimicus). This highly intelligent cephalopod is capable of mimicking other marine animals for protection when moving across open areas of sand on the ocean floor. It has been known to mimic sea snakes, lion fish, sole fish and even sting rays. 

Source: Wikipedia
Mimicry is a very specific phenotypic evolutionary process, which once the initial transition from cryptic colouration to an aposematic one has been perfected the benefits for the mimic are obvious.  However the process from changing from a cryptic animal to an unambiguous one was confusing for biologists. If the process was a gradual change, such as in the case of most evolutionary processes, with each generation becoming slightly less cryptic and more unambiguous, then the species would suffer a significant fitness loss. 

One hypothesis made by evolutionary biologists Clarke and Shepherd (1960) was that this step consists of one large mutation using “super genes” where there is a sudden phenotypic change. This theory states that the first generation of this modification are not perfect mimics, but carry a remote likeness to the signal enough for survival and to gain selection for future evolution, where the mimicry is refined and perfected.

To test whether or not mimicking the visual signal of an orca whale will reduce attack rates on potential prey items at the surface I am towing foam decoys around Seal Island in Mossel Bay.  These decoys will simulate potential prey items at the surface and induce seal hunting behaviour from the white sharks.  There are 4 decoys towed in pairs, separated 15 m apart in order to give enough space between them to make them independent tests due to the low visibility of the water.

The two pairs will consist of a plain black decoy paired with an orca decoy, which will have the ventral black and white pattern of an orca whale, and a plain black decoy paired with a black and white chequered decoy as a control against the specific biological pattern.  If white sharks are afraid of the visual sign stimulus of orca whales it is expected that the orca decoy will be attacked less frequently than the other decoys. 

The chequered decoy will test if the sharks are avoiding the orca decoy because it views it as a threat i.e. an orca, in which case the chequered decoy will be attacked significantly more than the orca decoy, or purely for the reason that it doesn’t look like the shark’s intended target i.e. a Cape fur seal, due to the contrasting colours and therefore both the chequer and the orca decoy will have a similar attack rate.

Underwater activity will also be recorded using GoPro cameras to show any withdrawal at the decoys that cannot be seen from the surface.  If the orca pattern is seen as a deterrent by white sharks, it is expected that there will be a higher withdrawal rate on the orca decoy than the other decoys.

All four decoys towed at the same time to test the possible aposematic qualities of the ventral pattern of an orca whale in the same environmental conditions.

Environmental conditions also play a large part of white shark hunting behaviour.  It is assumed that sharks hunting in clear calm waters will be able to distinguish the difference between a real prey item such as a seal, and a foam decoy.  Therefore fewer attempts on the decoys in high visibility are anticipated.  However, when there is a disturbed sea surface due to wind chop and the water visibility is poor due to increased debris and wave movement, sharks appear to mistake a decoy for a prey item more regularly, as witnessed in previous research.  This is presumably down to a greater margin of error in identifying prey at the surface.  My study will investigate whether there is any correlation between the frequency of attacks on the decoys and the abiotic factors they are towed in, specifically to the Mossel Bay area.

The overall outcome of this research project is to investigate whether it is possible to reduce surfer’s risk of attack from white sharks by replicating the ventral pattern of an orca whale on the bottom of surfboards to act as a deterrent.  Furthermore, by recording the weather conditions and analysing correlations with attacks on the decoys, we can create better guidelines for swimmers and surfers specifically in the Mossel bay area (and also comparing it with other areas) making people more aware of higher risk times to be in the water.

Mike Barron  - Msc. Candidate

No comments: