Critical Analysis relating to article titled: Cost/benefit analysis of group and solitary resting in the cowtail stingray, (Pastinachus sephen).

Description, Habitat & Distribution

Cowtail stingrays (Pastinachus sephen) are widely distributed throughout the globe’s oceans. They are native to many countries such as Australia, Indonesia, Japan, Mozambique and South Africa (Union, Nature, and Resources, 2000). Cowtail stingrays are uniformly grey to brown in colour, medium sized, with their underside belly region being predominantly white. The tail area tends to be darker in colour.

Cowtail stingrays have a particularly thickened pectoral fin, resulting in straightened anterior margins with round apices. They have broad snouts which are blunt at the end in comparison to most other stingrays. Their small eyes are situated on top which are quite widely spaced apart. Their tail is very distinct in shape and appearance. It is long, extending into a leaf shape structure at the end of the tail. An upper fold on the tail is absent which is different when you observe the tail of a short-tail stingray or smooth stingray (Dasyatis brevicaudata). A single venomous spine is present, it is situated well behind the pelvic fins (Branch, Griffiths, and Branch, 2010). Overall the cowtail stingrays body tends to be wider than it is long.

Adults have denticles, in essence they are tooth like serrations or placoid scales on the surface of the disk which begin at the snout region continuing through to the tail. The denticles are excluded from margins of the disk plate.

Cowtail stingrays prefer to remain in shallow waters. As a result they are commonly found in intertidal lagoon areas, shallow reef systems (Michael,1993), sandy ecosystems and in certain environments river inlets (Compagno et al.1989).

Biology & Ecology

Cowtail stingrays mainly feed on bony fish species, various crustaceans and worms.

They are predominately solitary. Grouping is facultative, meaning the cowtail stingrays are able to group together as they feel, and when the benefits of grouping outweigh the benefits of remaining a solitary resting ray (Semeniuk, 2004).

Rays have shown a tendency  to group together when environmental conditions are not in their favour. Generally only grouping together when resting. Resting occurs on sandy shallow ocean beds or hideaway spots where they are less prone to be found by passing predators

Cowtail stingrays have been seen resting with other ray species, such as the Whipray. This may be an indication of trying to gain better predator detection mechanisms that the cowtail stingray may not posses (Semeniuk, 2004).

While studying this scientific paper, I found it pertinent to include some of my personal findings regarding the local Short Tail stingray (Dasyatis brevicaudata) population along the South Peninsula coastline upon which I live on, and compare it to the rays which are mentioned in the scientific paper.

Resting period of Cowtail Stingrays

The priority with any animal in the wild when resting, is to try and minimise the risk of being found by predators. Cowtail stingrays are most vulnerable to many shark species, particularly those of the Sphyrnidae and Carcharhinidae family (Hammerhead and Requiem sharks) as well as Bottlenose Dolphins belonging to the Delphinidae family. In order to achieve many of these, risk management strategies are put in place (Semeniuk, 2004) to avoid being caught out by their predators.

When cowtail stingrays rest, they do not take part in any other activity such as mating or feeding. Observation has seen them just in a resting phase. They tend to rest for a minimum of four hours per day (Semeniuk, 2004).

Irrespective of what animal I looked at, the choice in which the animal chooses to rest needs to be as safe as possible, and out of eye sight from passing predators. Secondly, animals may develop defence mechanisms or tactics to decrease their probability of being preyed on. Many rays have developed cryptic methods of hiding away from the predator, as such the cowtail stingray which is able bury itself underneath the sand thus reducing their chances of being noticed. Another technique is the stingrays electro receptor senses (ampullae of Lorenzini) facing towards any gap from which a predator may attack from. If the ampullae of Lorenzini are facing towards the direction at which an approaching predator may appear (Schmidt-Nielsen, 1997), the stingray will have a greater ability in detecting the approaching predator. Lastly, grouping as seen within the cowtail stingray population may also play a role in creating a “safe place” to rest (Semeniuk, 2004).

Grouping within the cowtail stingray population is not a necessity. However, they may prefer to group if they are experiencing a limitation with regard to detecting a nearby predator (Semeniuk, 2004). The detection abilities of a cowtail stingray will vary as it is related to many oceanic factors  such as visibility, incoming sunlight, tide state and condition. All of these factors are constantly changing on a regular and basis. Cowtail stingrays have only been observed in small group numbers ranging between 2-4 rays per group, of course bigger groups can occur.

Benefits of Grouping

The greatest and most obvious benefits of grouping whilst resting include the following: it creates bodily protection while resting, it increases an early warning system and create predator confusion, ideally reducing the potential of being consumed (Semeniuk, 2004).

It is advantageous for the cowtail stingray to congregate in a small group of minimal numbers to rest when water visibility conditions are low/poor. The lowered water quality can be caused by churned up sediment, low light levels, strong ground swell/ current and other variables. Grouping tended to take place during diminished water visibility when the rays senses of detecting a predator would be reduced. By grouping together, the cowtail stingrays are hoping to increase their detection skills, thereby having an early detection in place which would aid in their survival and possibly reduce the probability of being spotted.

Cowtail stingrays have created an advantageous spatial arrangement for protection against any threat. Most commonly the stingrays arrange themselves in a rosette formation when resting in a group. Rosette being with their heads facing inward of the group and their tails displayed outwards (Semeniuk,2004).Rosette design acts as a self defence mechanism against predators. It also enables the stingrays to have majority of their body protected, should they be attacked. Their tail acts as a sacrificial item versus putting their entire body at risk. As mentioned, their tails contain mechanoreceptors which contribute to an early warning system. With each stingray in the group possessing this feature, it is a combined benefit that would not necessarily been available if the cowtail stingray was resting on its own.  (Semeniuk,2004).

Spatial awareness within the article (Semeniuk,2004) is seen as being equally important with regard to approaching larger rays when settling to rest in a group.

In my experience, along the coastline in Simonstown, I  have found that its best done when the short tail stingray has had a clear and intended path of escape. In many situations I have encountered the possibility of getting very close to the stingray, and have even interacted with an individual without it fleeing. This is achieved because they are able to see a clear path of ‘flight’ should it be needed. Closing off this path in anyway may lead to panic and fleeing.

Grouped cowtail stingrays were measured as having a greater response distance when being encountered by a predator. This measurement was comparatively done with solitary cowtail stingrays in the same water conditions. The scientists used an analysis of covariance (ANCOVA) with a particular group size as the covariate. Perhaps, the greater response distance in grouped rays was due to resting in the rosette pattern, allowing their early warning system to react much quicker from all different angles compared to a solitary stingray which is just relying on itself to detect a predator from any direction. When cowtail stingrays group together there are two effects that can be observed within the group itself. Namely, the Trafalgar Effect and the Dilution Effect (Semeniuk, 2004).

Trafalgar Effect typically occurs when the stingrays group together with their pectoral fins overlapping another ray’s fins. When a predator approaches them, their communication will take place through touch (overlapping pectoral fins) which will get transmitted from one ray to another in the group. Once one cowtail stingray has been alerted to a possible threat there will be a “wave of alarm” which will travel among the group. This Trafalgar Effect can travel through individuals who are not even aware of the potential danger that may be approaching them, thus acting as an early warning system (Krause, 1993). It would therefore be beneficial to a stingray to rest in a group if they are continuously vigilant, or always being aware of their surroundings all the time.

I have noticed a constant analogy with the  short tail stingrays. When stingrays are resting in a group of two or more individuals, there are distinct levels of threat awareness. When I found  short tail stingrays resting in sandy open areas they portrayed a manner that was more relaxed despite the group not always resting in the typical ‘”rosette” pattern. More often they were grouped together in irregular patterns with their pectoral wings overlapping with one another. This observation along with Krause,1993 and Semeniuk, 2004 confirms the notion of cowtail stingrays communication of their surroundings can be carried out if they are in contact with one another.

When any threat is perceived (ahead of the animal/front on) while resting, a small wave-like pulse is flashed through the pectoral fins surface, thus alerting the neighbouring stingrays whose wings overlap, to make them aware of the predator. Should the threat remain within close and potential dangerous proximity, a further more vigorous pulse is sent through the whole wing’s surface. This “unglues” the ray from the sea bed, (at this stage I have actually seen sand particles being flushed through the gill plates), showing the intense inhalation and exhalation in preparation for fleeing the scene in order to escape the predator. If the threat is still present and the ray is determined to stand its ground and resist fleeing, then the tail sting will be raised and lowered in a threat display in an attempt to ward off the predator. If all else fails, with a burst the stingray will retreat and continue fleeing in order to escape to safety.

Grouped cowtail stingrays were observed to have escaped roughly 50m further away when compared to cowtail stingrays that were resting in solitude (Semeniuk,2004). I believe the reason for early escapement would be the combined effects of all the above mentioned factors enabling the stingrays to detect an approaching predator much quicker, unlike a solitary animal trying to guard all fronts in order to avoid being seen by a predator.

In conjunction with the Trafalgar Effect,  the Dilution Effect take places when there is a great number of individuals resting together. The greater the number in a group will therefore reduce the probability of a particular individual being predated on, due to their being a “choice” of prey (Krause, 2002). As a result, a stingray resting in a group could potentially get away with not being preyed upon if the dilution effect is great enough (Annenberg Foundation 2016).  Another benefit from grouping and dispersing upon the predator’s approach is that if the group disperses yet remains within good proximity to each other which could increase the confusion in selecting a target prey item for the predator, and as such may even result in the predator losing focus, spending too much energy on the prey without success and moving on without having preyed upon any cowtail stingray at all. Resting in a group brings down the success rate of successfully consuming prey, as capture is more difficult. This would stand the cowtail stingray population in good stead, rather than resting alone where the risks would be higher in defending oneself.

Over and above deciding to rest in a group, each individual stingray in the group needs to constantly be vigilant, use cooperative defence strategies in order to increase its probability of not being eaten. The resting group needs to remain and function as a cohesive whole, otherwise there will be a decline in the benefits of resting in a group.

Costs of Grouping

An obvious cost of grouping would be the increase in the group’s visibility to predators when resting. Due to their size, they would be able to be easily identified as opposed to resting individually and burying themselves beneath the sand (Semeniuk, 2004). Being caught off guard to any predator or threat would not be in favour to the cowtail stingray. The main objective of resting would be to know that you, as an individual, are as safe as possible, and that the resting individual is able to save a bit of energy while resting and not have to spend energy being on guard the whole time looking out for predators.

While the stingrays may remain well hidden in poor visibility conditions when solitary or grouped, this is would decrease when visibility improved. If a predator’s own visibility is not good, then grouped rays would possibly get away with grouping even in good water conditions. However, stingrays are not able to judge the visibility of a predator’s eyesight beforehand.

Secondly, the grouping may lead to an increase in competition with resources which could let to increased levels of aggression as a result of the competition, especially if the resources are particularly limited. This may prove detrimental to those stingrays that are unable to cope with resting in a group, with increased levels of stress, or if there is some sort of hierarchy that is present within the group of resting stingrays that prevent certain individuals to rest with others in the group, due to this competition or stress levels present.

Resting with other rays may increase the prospect of contracting diseases or parasites that could be passed onto one another. Whereas, if a cowtail stingray was resting on its own, it would reduce the possibility of contracting illnesses or disease. Illness or disease would be disadvantageous to a stingray that could reduce its population in generations to come.

There may also be an increased interference with commencing the initial flight within the group. However, there could possibly be a decrease in overall escape speed (Semeniuk, 2004).  Despite these actions, probably aiding in confusing the predator, it could also hinder a stingray’s escape efforts by being in a group as each stingray would be hampered by fellow resting cowtail stingrays all being contained in close proximity. As a result, it seems it would be better to be solitary when resting. This could increase the probability of a stingray being caught no matter how effective the Dilution Effect of the group is. Fleeing in a group is difficult when overlapping discs of rays occur when resting. When escape is needed to take place rapidly, it is decreased due to the fact that the cowtail stingrays congregating too close together. In some cases cowtail stingrays may struggle in escaping, and eventually falling prey to the predator (Semeniuk, 2004) due to being in a group which is too crowded or too close in proximity to one another. Switching is an action that has been observed with cowtail stingrays in groups (Semeniuk, 2004). It involves changing the escape trajectory in order to avoid a collision with another ray. This behaviour is commonly witnessed during the study. When switching takes place, it means the stingray has to decrease the distance between itself and oncoming predator, and in doing so, the initial escape speed is reduced becoming disadvantageous to the cowtail stingray involved.

Grouped rays displayed a slower speed than a solitary resting ray (Semeniuk, 2004). This illustrates that when in a group, one cannot simply leave when a predator approaches, as the group displayed a planned and concurrent escape with all members of the group. Signalling and communication takes place causing stingrays to disperse at the same time or as close to the same rate. The churning of sand when dispersing contributes to an individual having a slower dispersal speed which makes it harder for rays to actually disperse away from the predator. Instead, due to the lack of visibility many end up dispersing towards the predator. Even though a group may have a slower response speed, it does not change significantly with a group containing more cowtail stingrays. As these factors were present whether the group contained two cowtail stingrays or more individuals, they still form a group. Concluding that response speed would be slower when there is a group of 3 cowtail stingrays or more, nine or more rays were rarely seen. Grouping appears to be costly. In my view, if it wasn’t costly then surely there would be greater numbers of grouping cowtail stingrays? If group numbers were to rise to greater numbers, it would mean, that by the time the predator was in close range to the group (whilst all the cowtail stingrays were dispersing) it could be quite plausible that the predator would be extremely close to the last member of the group before it was able to flee itself (Lima,1994), resulting in a possible capture.

Being a member of a group means that in certain situations your escape trajectory route may get blocked or taken by another cowtail stingray in its own escape. Hence, the stingray would have to divert and change its escape direction, hampering, and slowing down, its own escape. In addition to escaping a particular pathway, one or more stingrays may follow the same route. The outcome is a crossing over pattern which also slows down and hinders the intended escape route for any stingray in the group. Worst case scenario is it could result in a collision between two stingrays, this would be detrimental to any stingray whilst trying to escape their predator.

Benefits of being Solitary

Cowtail stingrays who choose to rest on their own have shown to have a quicker response/escape time when fleeing away from an approaching predator. Proving advantageous as it reduces their chance of becoming the prey. It is possibly due to the lateral line that runs the length of the body which enables them to detect any water movements which potentially could have been caused by nearby predators (Marushka & Tricas,1998).

Some stingrays may continue to rest in solitary if the cost of resting in a group, in conjunction with any additional factors, is too high. As yet, there is no link with determining whether grouping is more beneficial in comparison to remaining a solitary animal.

Cowtail stingrays have a heightened lateral placement of their eyes, allowing them to have a greater field of vision (Bodznick,1990). This facilitates the rays when resting in solitary to continue to be on the lookout for predator from an array of angles. This continuously allows the stingray to have an escape route mapped out should a predator arrive.

It seems as though there were a few variables that influenced whether the cowtail stingray would remain solitary or would conform to resting in a group. Confirming that if the benefits outweighed the costs of grouping then it would do so. Incident light level and cloud cover were two major contributing factors in determining in which way a cowtail stingray would conform to resting.

The FID (Flight Initiation Distance) was dramatically affected relative to the visibility condition (Semeniuk, 2004). There was a definite correlation between FID and water visibility.  The FID was considerably greater when there were increased light levels. All indicating that the condition of the water and surrounding environmental factors play a crucial role in the escape of cowtail stingrays, and whether they can successfully avoid being hunted by their predator.

When I have been swimming along the coastline, I have witnessed Short Tail stingrays to be solitary while resting when the visibility is in excess of 8 metres.  Whereas, when the sand is churned up due to boat activity or weather conditions, I have noticed that the stingrays will congregate together in small groups of 2 -3 stingrays. It is interesting to note that despite them being different species, and situated in different parts of the world, they are perhaps seemingly using the similar predator detection methods as the cowtail stingrays in Australia. Whether this is in fact the case, further investigation would be required to fully determine whether this is the reason.

Costs of being Solitary

Semeniuk, 2004, found that the actual escape distance was considerably shorter when a cowtail stingray was resting in poor conditions on its own as opposed to resting on its own in good conditions.

While studying the cowtail stingrays there were noticeable delays in the response of a solitary cowtail stingray when a predator came close in conjunction with water conditions being reduced. Consequently it would be beneficial to group in poor condition in order to improve response times (Semeniuk, 2004) with the other detecting mechanisms that are in place. With these mechanisms in place it would create a concerted and coordinated escape which could be initiated sooner than if a cowtail stingray were on its own, despite being well camouflaged and hidden while resting alone.

Comparing Anti Predator Benefits of Mixed Species Groups of Cowtail Stingray and Whiprays at Rest

In addition to what has been observed with resting cowtail stingrays, another scientific study has been done looking at whether grouping of mixed species are beneficial rather than remaining in a single species group or solitary

(Semeniuk & Dill, 2006). It seems that the cowtail stingray enjoys grouping with the Reticulate Whipray (Himantura uarnak)( Semeniuk & Dill, 2006).

Cowtail stingrays preferred to group with heterospecifics, particularly Whiprays.  Cowtail stingrays have been seen settling next to them rather than passing by, and settling elsewhere or with other cowtail stingrays in the vicinity. This indicating that Whiprays seemed a preferred choice (Semeniuk & Dill, 2006) when deciding where to rest, the opposite effect when it was of their own species (Semeniuk, 2004).

Cowtail stingrays seemingly formed greater numbered heterospecific groups than when they remained in a monospecific group which generally were composed of smaller numbers. This is an incredible phenomenon given the lower density of Whipray population in the region.

Whiprays responded much quicker relative to cowtail stingrays when a predator, mocked by an approaching boat to a group of resting rays. Whiprays frequently were the first to respond when a predator approached the resting group. Therefore, it appears to be beneficial for the cowtail stingrays to stick with the Whiprays when resting in a group, as the Whiprays would receive an early warning due to them using secondary detector mechanisms situated in the length of their tail (Semeniuk & Dill, 2006). As a result, the cowtail settles preferably alongside a whipray when wanting to rest, assuming the whipray has a greater surrounding awareness of when a predator is near.  The relationship between the two species of stingrays must be a mutual and symbiotic one to allow this type of behaviour to take place. The tail of a Whipray is negatively allometric meaning that it is disproportionate in growth size to the ray as the ray changes in size. Semeniuk & Dill, 2006, suggests that perhaps as a young whipray, the tail plays a vital role being extremely long relatively to its size and decreasing in size as it grows. This ensures that it is successful in surviving the vulnerable early stages of life. An adult Whipray still has a longer tail compared to cowtail stingrays.

In the study there was a positive relationship between log₁₀ tail length and log₁₀ disc width for cowtail stingrays (r² = 0.622, F_1.28 = 78.99, p = 0.0001) (Semeniuk & Dill, 2006). In this particular relationship, the slope indicated a negative allometry. Yet the opposite gave close to identical results when the whole tail was visible. Relative to their disc width whiprays had longer tails when compared to cowtail stingrays, increasing the theory of being able to detect predators sooner than a cowtail stingray. Whiprays were able to respond first out of the two species when being interrogated by a predator, and detected a predator from a further distance than cowtail stingrays in a single specie group.

Heterospecific groups may form based on two primary reasons. Firstly, to try and reduce the predation of either species but resting in a group and reducing chance encounters (Waser, 1984). Another benefit for both species of ray when resting in a mixed specie group is there would be less resource competition between both species which would reduce the cost of resting in a group.

Grouping may not always be beneficial to both species resting within one group. The protector – species hypothesis states the being in a heterospecific group will provide benefits to at least one species that would not have been achievable if grouped solely with the same species (Pius & Leberg, 1998).

The cowtail stingray and whipray are quite closely related. For this reason it would make sense for them to group together. Both species of stingrays would intrinsically be in tune to one another’s responses to predators, and recognise each ones response to an approaching predator. Aiding in effective communication taking place despite them being different species.

It’s been studied that cowtail stingrays preferred to rest with whiprays compared to resting with fellow cowtail stingrays (Semeniuk & Dill, 2006). 18% of encountered cowtail stingrays were joined by passing cowtail stingrays as opposed to 62% of encountered whiprays. This study showed cowtail stingrays passing more fellow resting cowtail stingrays be it in groups or solitary. Settling more often with whiprays.

With everything mentioned above included, cowtail stingrays en masse settled with rays of similar or larger sizes than their own (Semeniuk & Dill, 2006) . One wonders if maybe size has any influence in deterring a predator as I have always seen similar sized stingrays grouping together.

Discussion

Against all benefits and costs, it is extremely interesting that grouping within the cowtail stingray population is facultative. This is an important feature as it means the stingrays continue to have a choice, but it is not necessary for survival. The observation of stingrays grouping together more frequently when the weather/water conditions were poor, contrary to when weather/water condition were good and in their favour, suggests they are continuously aware of their environment, and assessing their surrounding, using all mechanisms and techniques they posses to maximise their survival and ability to outwit and manoeuvre any predator.

Escape speeds of cowtail stingrays, both grouped and solitary, were measured by using a boat acting as a predator. The boat was considered to mimic that of an approaching predator (Semeniuk, 2004). With reference to the study using a boat to mock a predator, in my personal opinion, I found that the short tail stingray (Dasyatis brevicaudata)  displays various types of behaviour towards perceived threats i.e. predators in different localised areas.

I have observed that some small groups have evolved their behaviour similar to Pavlov’s Dog Theory (Classical Conditioning). Certain small groups of these rays have changed their resting times to correlate with an irregular human activity known as fishing. Commercial vessels are always under time constraints to obtain the best price at the market, so it’s not uncommon for the crew to clean a few fish while waiting to slip their boat. The waste is thrown over the side and eaten by the rays.  I have found that in these areas adults of both sexes will wait in deeper waters until the commercial fishing boats come into the slip way to retrieve their trailers to take the boats out the water, with the fish still on board the vessel, and as such, it’s an audible cue that draws them into shallower water, irrespective of  the time and/or tide state. This audible cue has become so acute that if a smaller leisure boat i.e. rubber duck comes to the slipway, no perceived reaction is noticed or made by the rays within the vicinity. However, should a heavier multi-engine commercial ski-boat arrive at the slip way, up to five rays will appear out of the depths to meet the vessels in the hopes of receiving fresh off cuts of fish / afval. I have tested this theory numerous times. We have surveyed the area prior to different vessels arrival, and have noticed a definite correlation between specific boats and ray activity. While in different areas we have noticed that rays are very shy whenever they are approached by incoming vessels.

I also noticed a distinct behaviour displayed when approaching resting short tail stingrays. I found that resting solitary animals will not react when swimming overhead even whilst resting in clear waters. The animal stays dormant, calm and still. Added to this particular resting behaviour, I likewise noted that a predator, in this case a human, must not approach the stingray from the rear as the animal almost certainly would flee in a burst. The best approach for majority of resting stingrays on the sea bed was slightly higher than the ray itself, and either  from just to the right, or left of centre, stopping for a few seconds, then closing the last little distance slowly. In doing this it is possible to make physical contact with the animal.

Approaching stingrays in this way, it is possible to get very close to observe, and leave without the animal being disturbed or fleeing the scene. Therefore, using a boat to represent a predator moving over the top of the stingray on the water’s surface to measure response flight time and distance in cowtail stingrays could result in very poor/inaccurate data results.

It is assumed that cowtail stingrays do not have excellent vision as they have been seen colliding with obstacles when experiencing poor visibility (Semeniuk, 2004). So it is a concern for them to always try and remain safe when visibility drops as their eyesight is compromised. On top of this they experience a decline in predator detection ability and an initial flight departure. This could explain the reason for optional grouping when poor conditions develop.

A stingray’s vision for predators will extend as far as its eye sight is able to go. So should a predator approach the stingray’s field of vision, it doesn’t immediately mean that the stingray will flee the area. The stingray will continuously evaluate the benefits of maintaining its ground relative to leaving. An animal will only leave when it is forced to do so, in order to avoid burning excess energy.  So, it will wait until the last minute when it is still able to escape successfully.

Conclusion

The fact that solitary cowtail stingrays are able to escape at much quicker speeds than those in groups indicates that despite being alone, it is easier to be solitary, if the stingray is needing to escape a predator nearby. A slower speed is a great cost to any stingray that is needing to escape rapidly if the predator were to be successful in its capture. Therefore, unless the conditions were not in favour to the cowtail stingray, it would continue to remain solitary where it is still able to escape at a distance that is safe enough to escape the predator, as having bursts of escaping energy available at all times costs energy.

When visibility is poor, it may prove beneficial for a stingray to group with fellow cowtail stingrays in order to create greater predator detection abilities through the many tails, eyes and electroreceptors that would collectively make up a group of resting cowtail stingrays. If a cowtail stingray chooses to remain single, then it may face the fact of not detecting the predator in sufficient time, therefore becoming prey.

The fastest escaping cowtail stingray within a group was not that much faster than a solitary resting stingray, suggesting that the amount of perceived risks a grouped stingray experienced, is relatively similar to that of a solitary stingray (Semeniuk, 2004). Each stingray is still able to act accordingly. However, not all stingrays will be able to escape as the fastest grouped stingray due to their being other variables that come with being in a group.

Cowtail stingrays may travel quite a distance before finding a safe resting spot regardless of the water conditions and visibility. It was noticed that stingrays even swam past fellow resting stingrays while in search of a resting spot. It would seem that despite the water conditions, travelling does not deter cowtail stingrays from finding the safest resting spot. Evidently stingrays were seen swimming past resting stingrays and still decided to rest on their own further away in a location where they felt safer. Demonstrating that travel is not seen as a risk to cowtail stingrays, when searching for the safest spot in which to rest (Semeniuk, 2004).

Overall a solitary cowtail stingray is just as successful on its own detecting predators, and being on guard and aware of its surrounds when the water conditions are good.  Therefore, it doesn’t warrant seeking a group to rest in as the benefits are reduced (Semeniuk, 2004). There has been no study in determining a predator’s success rate for capturing a prey in a group versus solitary resting stingray. Resting in a group may have severe consequences on the animal when water conditions are good. The opposite occurs when water visibility is bad and thus grouping becomes a preferred resting method of choice. Benefits must outweigh the costs of resting in a group, in order for species to continue the resting mechanism they currently take part in.

After studying this paper, it remains true for me to believe predator-species hypothesis remains constant when observing resting cowtail stingrays, whether it be in a monospecific or heterospecific group. Resting in a mixed species group means that the species who do not have a strong point for detecting predators can hitch onto the back of a species with greater sensory capabilities, taking advantage of their strengths. This, in the long run, could result in a potentially longer survival for the cowtail stingray.

Whiprays are quicker at recognising an oncoming predator relative to cowtail stingrays. Taking into account all the above, group numbers are still kept to a minimum. It may be advantageous to form bigger groups when grouping with whiprays, yet this has not been proven. Despite stingrays being in a bigger group which could be to the cowtail stingray’s detriment, given the various costs involved, it likewise could stand in good stead (Semeniuk & Dill, 2006). Social living or resting in this specific article comes with many benefits and costs (Alcock, 2009).

Predators, such as sharks, generally have quite good vision and detection senses themselves which, when all combined, aid to hunt down their prey. Thus it would be possible, regardless of group or solitary resting, to detect their prey, whether the prey are resting in low light or high light visibility conditions. The predator in question would use all its detection senses depending on weather conditions. Should water conditions be good, then it would allow the predator to easily indentify prey based on few mechanisms that it possesses. Sharks electro-receptor senses along its body have the ability to detect the bioelectric signals of any resting ray (Kajiura & Holland, 2002).

 

Thank you for reading my work. Fiona