“Why do some animals live in groups?”- Semester 2 tutorial essay
Group living in the animal kingdom comes in many forms in terms of both numbers of individuals and the relationship between them- whether it be family or unrelated members of the population. This essay is a discussion of the reasons behind, and the structures of, animal groups with particular focus on examples of species that choose to collaborate in order to increase their chances of survival or to gain information that is useful to them. This includes discussion regarding how penguins huddle to survive harsh environments and how African wild dogs hunt as a pack to improve efficiency.
The main intention of all animals is to survive and reproduce to pass on their genes to the next generation. There are many animals that choose to live in groups to make this process easier through either gaining access to mates or through increasing the number of individuals who can care for the offspring, increasing their chances of survival. However, it is not always directly that the animals pass on their genes to the next generation. In honey bee (Apis mellifera) colonies, there is one queen who mates with hundreds of male drones. The queen is not the only female in the colony however as there are thousands of sterile female worker bees that do not reproduce themselves but instead work and defend the colony by stinging any individuals (human or non human) who try to enter or destroy the hive. Stinging intruders will ultimately lead to the death of the worker bee, this is the ultimate sacrifice and prevents the individual from passing on their genes to the next generation. While this may appear to be an act of altruism, it is in fact merely a mechanism by which they can ensure that their genes are passed on to future generations even without them producing offspring themselves. This is called inclusive fitness. This is because all female bees in the colony are related and so share genetic information. Essentially it is unimportant which of the individuals reproduce, as the end result is the same. This type of behaviour is also seen in naked mole rats (Heterocephalus glaber), which live in colonies made up of at least 75 members. The mating structure is very similar to that of bees in that there is a queen (the only female who reproduces) and around three males who are called kings. The other individuals do not reproduce but provide protection to the queen and kings from predators. This is another example of when members of a colony sacrifice themselves to protect the rest of their group. (Campbell et al. 2015, 1249-1250) Protection from threats is one of the key reasons why animals live in groups rather than living alone. This type of ‘safety in numbers’ approach is advantageous for all of the members of the group as having a large number of individuals may act as a deterrent for predators but also means that it is more likely that predators or intruders will be seen and an alert raised allowing any unaware animals to escape from the danger.
It is not always directly that other members of a group can provide protection. It is suggested that the motive for gregarious behaviour may be to avoid predation by a simple dilution effect. This means that when in a large group, the probability of you being killed by a predator is decreased, as there are many more individuals that could be caught instead. (W. A. Foster & J. E. Treherne, 1981) The likelihood of and individual being caught by a predator is one hundred times smaller if the group has one hundred members, providing that they are all of a similar fitness level. This means that either directly or indirectly, the animals in a social group may be competing with each other to be the fittest and therefore the least likely to be attacked by a predator. Hunting animals usually single out the weakest or slowest individual in a group, for example a weakened bison in a herd. All of the attention of either the individual or the pack will be on this particular animal and so the likelihood of it being killed is high, especially in pack hunting. This is a case of survival of the fittest in that those animals that are fit and healthy are unlikely to be selected by predators. This indicates that group behaviour may in fact only be beneficial for animals that are among the fittest of the herd, whereas the weakest of the herd are most likely to be captured even when they are part of the group.
Many carnivores have to hunt for food in order to survive; this is easier when hunting can be done in packs such as in wild dogs and dolphins (Delphinidae). African wild dog (Lycaon pictus) packs are usually made up of between three and twenty adults as well as their offspring. During a study into the effect that pack size has on the hunting success of African wild dogs (Figure 1), it was found that % hunting success (measured in kills per hunt) increased with pack size, with the highest success rate of around 70% being achieved when the pack contained 20 members. The hunting success of a pack made up of only four adults was 43%, which is significantly lower than the success of larger packs. This supports that it is beneficial to live in a group in terns of maximizing hunting success and the amount of food available per individual. Many other animals are pack hunters and so the benefits of living in a group must outweigh the costs, for example the decreased amount of food per animal if the hunt is not very successful. Although there is a positive correlation between pack size and hunting success, this is likely to plateau before reaching a 100% success rate as larger groups will be more conspicuous and more difficult to coordinate.
Figure 1- the relationship between pack size and hunting success of African wild dogs (Creel, S and Creel, N.M. 1995)
For those species that do not have to hunt for food, group living is still important- honey bees (Apis mellifera) use a complex behaviour called a waggle dance to provide information to the rest of the colony about where nectar, pollen, water or potential sites to locate a hive can be found. A worker bee carries out the behaviour when she returns from the location and observed by the other worker bees who remained at the hive. The first stage of the waggle dance is called the waggle run- this is when the bee walks in a straight line and waggles her abdomen. A bee may complete up to 100 waggle runs per dance, indicating the distance and quality of the feeding site, increasing in number for an increase in distance (von Frisch, 1967; Wilson, 1997). During the second stage (return phase) the bee turns left or right to get back to the starting position (Figure 2). This behaviour is carried out by all of the worker bees and acts as a huge source of information regarding the whereabouts of a point of interest. This means that not all of the worker bees have to leave the hive at once but they can all still collect the food/water/ pollen after the one who leaves to search returns. As they don’t all have to leave the hive they can continue their other roles and defend the colony from attack and ensure that it is not destroyed.
Figure 2 shows the stages of a waggle dance and the movements that the bee conducts to communicate with other members of the colony (Grüter, C. and Farina, W.M. 2009)
Not all groups are made up of individuals from the same species. Some species choose to take part in a symbiotic relationship in which each individual offers a particular trait or behaviour to the other in order to provide a ‘service’ to them and to get something back in return. This type of relationship is demonstrated between the pistol shrimp (Alpheus floridanus) and the goby fish (Nes longus and Ctenogobius saepepallens). One individual from each species lives together in a burrow on the bottom of a body of water. The burrow is built and maintained by the shrimp to provide shelter for both of the inhabitants, while the fish acts as a guard as it has superior vision. If a predator threatens the fish or shrimp, the fish communicates danger to the shrimp by diving quickly into the burrow, when this happens, the shrimp digs out the burrow deeper and both individuals hide deeper underground. This symbiotic relationship is beneficial to both species as it provides them both with protection from predators.(Randall, J.E., Lobel, P.S. and Kennedy, C.W. 2005) Despite it being an unlikely union, this pairing is very efficient and improves the survival of both species.
Huddling is a mechanism that provides warmth to endothermic animals such as penguins. This is especially important in situations where energy reserves are low or in extremely cold environments. (Gilbert et al., 2009) By aggregating close together, the penguins reduce the amount of surface area exposed to the surrounding environment to minimize the loss of heat (the amount of heat preserved is estimated to be between 6% and 53%). By reducing heat loss, the energy that would normally be used to keep warm can be used to grow or reproduce, ensuring the success of future generations and improving the survival rate of all individuals in the group even in the harshest of conditions. When huddling, infants are kept in the centre, as they are the most vulnerable to extreme conditions (Figure 3).
Figure 3 shows a group of penguins huddling in Antarctica. (Frederique Olivier for The Telegraph, 2014)
While the infants are kept in the centre to shelter them from the harsh environment, the adults periodically move from the outside of the huddle (the coldest part, shown in blue in Figure 4) towards the warmest part (shown in red). Two individuals are tracked throughout the diagrams and are outlined in dark blue. The rotation of the adults from the outside of the huddle to the inside means that the body temperature of the whole colony can be kept at a suitable level and all of the participants of the huddle benefit from the behaviour. This is an example of animals living together to conduct social behaviours that are advantageous of both themselves and others.
While some animals choose to remain solitary, for example bears (Ursidae) and black rhinos (Diceros bicornis), the mutual benefits of living in a group such as provision of food, shelter, warmth and protection lead many species to engage in social interactions and live together. Group behaviour, while having some drawbacks such as the sharing of food and resources, is seen to aid survival of animals and therefore it is favorable in many species such as penguins and bees.
References
Books
Campbell, N., Reece, J. and Urry, L. et al. (n.d.) Biology
Web pages
2014, P. (2016) Pictures of the day: 19 December 2014 [online]. Available from: http://www.telegraph.co.uk/news/picturegalleries/picturesoftheday/11303061/Pictures-of-the-day-19-December-2014.html [Accessed 8 March 2016]
Journal articles
Creel, S. and Creel, N. (1995) Communal hunting and pack size in African wild dogs, Lycaon pictus. Animal Behaviour, 50 (5): 1325-1339.
Foster, W. and Treherne, J. (1981) Evidence for the dilution effect in the selfish herd from fish predation on a marine insect. Nature, 293 (5832): 466-467.
Gilbert, C., McCafferty, D. and Le Maho, Y. et al. (2009) One for all and all for one: the energetic benefits of huddling in endotherms. Biological Reviews, p. no-no.
Grüter, C. and Farina, W. (2009) The honeybee waggle dance: can we follow the steps?. Trends in Ecology & Evolution, 24 (5): 242-247.
Randall, J., Lobel, P. and Kennedy, C. (2005) Comparative Ecology of the Gobies Nes longus and Ctenogobius saepepallens, Both Symbiotic with the Snapping Shrimp Alpheus floridanus. Environ Biol Fish, 74 (2): 119-127.
Seeley, T., Mikheyev, A. and Pagano, G. (2000) Dancing bees tune both duration and rate of waggle-run production in relation to nectar-source profitability. Journal of Comparative Physiology A: Sensory, Neural, and Behavioral Physiology, 186 (9): 813-819.
Spivak, M. (1996) The Wisdom of the Hive–the Social Physiology of Honey Bee Colonies. Annals of the Entomological Society of America, 89 (6): 907-908.
Seeley, T. (1995) The wisdom of the hive. Cambridge, Mass.: Harvard
University Press
von Frisc, K. The Dance Language and Orientation of Bees. Cambridge University Press, 1967. 566 P. $15.00. (1969). , 53 (2): 181-182.
Wilson, D. (1997) The Wisdom of the Hive: The Social Physiology of Honey Bee Colonies By Thomas D. Seeley. Perspectives in Biology and Medicine, 40 (2): 304-306.
Michelsen, A. (2003) Signals and flexibility in the dance communication of honey bees. J Comp Pysiol A 189:165-174