Many organisms walk a fine line, balancing their need for food with their need to avoid predation. Minnows in Texas are no exception to this rule. On a cold day (around 50 degrees Fahrenheit) with no precipitation and partial clouds, I observed several minnows on the edge of a local pond (I observed within a foot of the edge of the pond). The minnows ranged from about two to about four inches long and appeared to be of the same species and of the family Ciprinidae (Nelson, 2000). The pond had algae and debris lining its edge.
While I was watching the minnows, I noticed that they would continually flick around in a very excited manner. Flicking involved very short, fast movements, in which the minnows usually moved less than an inch in a second or less. Usually, this flicking would bring them to the top of the pond, or even closer to the shore. When the minnows moved, they also appeared to be nibbling on the algae in small bites – taking one nibble and then “flicking away”. Additionally, the minnows moved independently, but seemed to stay around the same group of other minnows. Finally, if I moved and a shadow passed over the water, the minnows would dart away. Their darting had them moving quickly, like with flicking, but in longer strokes – when darting the minnows usually moved out of eyesight, but definitely more than a few inches.
While watching the minnows, I developed several hypotheses regarding what evolutionary advantages some of their behaviors may have served. Firstly, I think that the minnows stayed closer to the shore because the area they were in was so shallow that a predatory fish would not be able to get to it and because this was where the most easily accessible food was. Previously while fishing in this pond, I had seen larger, predatory fish, including catfish, sunfish, and bass. On the other hand, by staying close to shore, the minnows would be more accessible to predatory birds. Although I did not see any birds during my observation, the fact that the minnows darted away when a shadow passed over them may suggest that they are used to predatory birds (having experienced them in their lifetime) or have evolved to have this trait. Secondly, I think that the minnows moved in a flicking fashion because, if a predator was near, the short, quick movements could not be matched. This relationship can be seen in tadpoles (who might have similar predators to minnows). Tadpoles exposed to predators were more likely to survive if they either had very limited movement, or made short, quick movements (Azevedo-Ramos et al., 1992). Additionally, the flicking of large numbers of shiny minnows may serve to distract or confuse a predator, similar to how large herds of zebras are hypothesized to use their stripes and movement to confuse their predators (Ruxton, 2002). Finally, I think that the minnows moved in a school because it would provide more protection from predation. This hypothesis is backed by a study on European minnows which found that those minnows which were from an area with high pike counts were more likely to have evolved schooling behaviors than those from areas without pike (Magurran, 1990). Lastly, the movement of the minnows away from the part of the pond that a shadow passed over probably came from the fact that predatory birds make moving shadows when they pass over the pond, so the minnows may have evolved to avoid an area which a shadow is moving over in order to avoid being eaten.
All of these behaviors can be drawn out to support the idea that a minnow will be more likely to pass on its genes if it survives for a longer period of time, thus increasing the proportion of its genes in the gene pool. The minnows which are most likely to have been favored by selection are the ones which have developed a system which will give them the highest reward (in this case, access to food) with the least risk (predation) to allow them a longer time to live and to have better reproductive success. To do so, they have developed behaviors such as swimming close to shore, staying in schools, and making short quick movements. Like many other animals, these minnows have continuously evolved (and are continuing to evolve) to one day reach the balance point (in this pond) which maximizes their reward and minimizes their risk.
References
Azevedo-Ramos, C., Van Sluys, M., Hero, J., & Magnusson, W. (1992). Influence of tadpole movement on predation by odonate naiads. Journal of Herpetology, 26(3), 335-338. Retrieved from http://www.jstor.org/stable/1564891
Magurran, A. (1990). The inheritance and development of minnow anti-predator behaviour. Animal Behaviour, 39(5), 834-842. Retrieved from http://www.sciencedirect.com/science/article/pii/S0003347205809479
Nelson, J. (2000). Family cyprinidae – minnows or carps. Retrieved from http://www.fishbase.org/summary/FamilySummary.php?ID=122. Accessed 2/3/2014
Ruxton, G. D. (2002), The possible fitness benefits of striped coat coloration for zebra. Mammal Review, 32: 237–244. doi: 10.1046/j.1365-2907.2002.00108.x
Tags: fish, group behavior, predation