In this review we looked at the effect what sodium has on different animals, and how they adapted to these conditions. We looked at the distribution of Loxodonta africana africana and how sodium influenced these distributions. This helped to indicate that sodium was in fact a driving force for the distribution and selection of sodium rich salt licks. In the case of Gorilla beringei beringei the strange behaviour of decaying wood consumption had scientists wondering until the link between sodium and the selected pieces of wood were made. These two are just a drop in the water bucket of how the quest for sodium shaped the ecology of the animals. As for physiological attributes that needed to change we saw that the preservation of sodium and salts had to play and important role as the absence thereof would lead in most likeliness to the death of the non adapted individuals. So animals started changing their behaviour towards the consumption of sodium to balance their physiological attributes which needs the sodium to work efficiently. Being a very important mineral to these physiological systems sodium deficiency had to be over come without being to costly. The biggest problem in acquiring this deficiency would be that the diets that are followed are low in sodium, as to suppose most carnivores have no problem with sodium deficiencies experienced by herbivores and in some regions omnivores. So herbivores and omnivores had adapted to these diets and are very successful in their respective environments.
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The quest for sodium comes into the open from the diets that are consumed by most herbivores and omnivores as these diets are very low in sodium which fulfil very important physiological systems. A deficiency in sodium can lead to very detrimental effects in all animals that are deprived from this mineral; here is a few aliments that can cause fatal disruptions; sodium is the source for cations in the blood and tissue fluids, then leading to blood volume loss and this in turn leads to the weakening of the functional capacity of the animal, sodium also balances the acid-base and pH balance, the regulation of muscle contractions, the nerve impulse transfer to the nervous system, can lead to weakness, incoordination and softening of the bones (Denton 1973, Klaus et al. 1998, Mutanga et al. 2004, Rothman et al. 2006, Ruggiero & Fay 1994, Woolfenden & Millar 1997). Poor growth can be a result of sodium deficiency (Woolfenden & Millar 1997), this also leads to the reduction of speed and endurance during a chase or flight and as well as the range, this sums up to a decrease in the grazing capacity (Denton 1973). This shows how essential sodium is to animals. As their diets consist mostly out of grass, shrubs and fruits (Denton 1973). These plants have no need to accumulate a lot of sodium, thus leading to areas where plant species will not contain the necessary amounts of sodium to sustain the animals physiological systems which depend on the plants as a food source alone (Mutanga et al. 2004). A good example would be the plains of the northern regions in the Kruger National Park (Mutanga et al. 2004). Here they tested the sodium concentration in the plants via field spectrometry (Mutanga et al. 2004). So this gave rise to very good physiological adaptions that had to be made by the animals to withstand these selection pressures that arises from the salt deficiency. These physiological mechanisms should be able to increase the conservation of the sodium (Denton 1973).
So most big mammals get sodium from sodium rich water or where there is low soluble sodium they get it from natural salt licks (Denton 1973). These natural salt licks are usually excavations in soil were there is a high concentration of sodium (Denton 1973), I will only refer here to it as salt licks. The eating of these soils is called geophagy. Geophagy is recognized under a lot of different animals and is common under large herbivores and omnivores (Ruggiero & Fay 1994) for example the Cape Mountain Zebras, Equus Zebra Zebra (Penzhorn 1982), African Plains Elephant, Loxodonta africana africana (Weir 1973, Weir 1972, Weir 1969) and Mountain Gorilla, Gorilla beringei beringei (Rothman et al. 2006). By analysing the salt licks it showed that there was a significant difference in the sodium concentration from the soil samples from salt licks and the immediate soil samples next to it and all though many nutrients are present in the soil samples, sodium is the element that the animals are looking for in most of the cases(Denton 1973, Mutanga et al. 2004, Ruggiero & Fay 1994, Stark 1986, Weir 1969, Weir 1972, Weir 1973).
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The behaviour to supplement minerals seems to be happening when these minerals lack in the natural food, this leads in the case of sodium to a appetite for sodium (Denton 1973, Stark 1986). It has been shown that the hormone aldosterone is influential in this longing of sodium in animals (Denton 1973), however some soils are suggested to might have detoxifying properties and is consumed not just for the sodium in the soil (Ruggiero & Fay 1994). It has been revealed that some animals are severely sodium deficient with a desire for sodium which relates too a very high level of aldosterone in the blood (Denton 1973). It is also shown that population densities are influenced by the presence of sodium rich salt licks (Denton 1973, Weir 1969, Weir 1972, Weir 1973) esspecialy elephants in Wankie National Park in Central Afric.
So there are positive and negative points to the consumption of these salt licks. Firstly the positives are the up take of minerals that are deficient in forage; maintaining an equilibrium in the major minerals of the mammalian body especially it the dry season; now able to satisfy the demand of seasonal needs examples are lactation, calving, growth of antlers and bone growth; may help against faecal loss thus limiting diarrhoea; help absorb tannins and alkaloids and other intestinal ailments (Klaus et al. 1998). Some negative aspects involve the energetic costs to reach and utilize the salt licks; reducing the time to forage; higher chance for predation at salt licks; increase poaching by humans; increase transmission of disease and parasites because of the high concentration of animals in the area; the soil can increase tooth wear (Klaus et al. 1998). Taking all in to account we could see that animals still find it more beneficial then detrimental to utilize the salt licks as they would not be able to cope with their environments if there were no salt licks or sodium of any kind freely available. Elephant and gorillas are excellent examples as a lot of research work has already been done on them.
Elephant distribution due to sodium availability
Looking at studies done by J.S. Weir especially on Loxodonta africana africana we can see how this quest to replenish the sodium levels have changed behaviour in the distribution of elephants in their habitats. By looking at the distribution around pans with high or low sodium concentration it is clear that elephants gather in the areas around the pans with a higher concentration of sodium either as soluble sodium or in the form of salt licks (Weir 1972, Weir 1973).
There was a major influence on distribution especially in the dry season were elephants stayed close to the salt licks (Weir 1972, Weir 1973). Sites from other areas such as Chobe showed the same result as to the fact that sodium was the main attractant during the dry season especially were there was a high water soluble sodium concentration (Weir 1973). This points to the conclusion that sodium is the main attractant to the area in which the animals resides themselves as some animals will even walk past other pans or salt lick to reach the sources with a higher sodium concentration (Weir 1969, Weir 1973). It is also believed that the brown coloration in the soil near salt licks had higher concentration of sodium, this is believed to help the elephants locate the salt licks year round by how it smelled (Weir 1969). The elephants seem to spend a lot of time around these salt licks as there is a very high concentration of urine and faeces in the areas around the salt licks and are believed to help with the increasing concentration of minerals in the area (Weir 1969).
Al this shows us the importance of sodium and the acquisition of this mineral from the soil and water. Without this elephants would have had to spend hours foraging to gain enough sodium to balance their need for it (Weir 1972). So by utilizing salt licks they can spend this time they don not need to forage in a more productive manner.
Gorillas replenishing sodium by means of wood eating
The discovery of this odd behaviour leads to a lot of questions to as why they were doing this. Work done by Rothman et al. had the answer that was looked for. The eating of decaying wood was very selective as to which pieces of wood was fed apon (Rothman et al. 2006). The wood chosen always had a higher concentration of sodium (Rothman et al. 2006). This again took a lot of time but essentialy saved time in the sence that they did not need to forage. This could be said as the decaying wood was only 3.9% in wet weight of their daily consuption but represented 95% of the daily sodium up take (Rothman et al. 2006).
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It is well known that many primate in forest ingest or lick decaying wood including chimpanzees, ring-tailed lemurs and mountain monkeys (Rothman et al. 2006). Indicating that sodium deficiency in primates does exist and that they neeed to suplement this deficiency to some extent in the wild. Sodium deficiency provokes the appitite for sodium as the aldosterone levels increase (Denton 1973) and the animal start to seek sodium. The wood pieces that were consumed contained a significantly higher amount of sodium than those that were left un touched (Rothman et al. 2006). This suggests that this is not just a random event and that the consumption of decaying wood is indeed a quest for sodium.
Influencial properties of sodium deficiency before and after weaning
Sodium deficiency in young animals can delay the maturation of these animals and as previously stated that it can lead to poor growth (Woolfenden & Millar 1997). The results of depreviating animals of sodium showed that there was a significant diffrence in the growth in the individuals that were given salt at libitum pre- and post-weaning, than those that were not given salt at any or one of the stages (Woolfenden & Millar 1997). The individuals that were only depreviated pre-weaning grew significantly farster than the individuals that were sodium dificient in pre- and post-weaning or just post-weaning (Woolfenden & Millar 1997). Thus leaving the conclussion that the most important part of growth in respect to sodium deficiency is the post-weining time period. It also indicates the importance of sodium during the mammalian growth (Woolfenden & Millar 1997).
Soil and decomposing wood eating in elephants and gorillas respectively have shown us how important the sodium concentration is in considering the salt lick or piece of wood respectively. This behaviour is shared with many other animals on earth and there is a vast amount of studies thst have been done on this in the world. Some with slightly different scenarios but essentially all the same answer as to what the determining factor was that intended seclection for or against the use.
This again emphisized the results that the sodium concentration in analyzed soils from salt licks and from the wood pieces that were used all had one thing in common and that is the higher concentration in sodium in respect to what was availible in the surrounding area (Denton, 1973) (Klaus, et al., 1998) (Mutanga, et al., 2004) (Rothman, et al., 2006) (Ruggiero & Fay, 1994) (Stark, 1986) (Weir, 1969) (Weir, 1972) (Weir, 1973). Even though there was traces of other elements in the soil samples, this had no contributing effect to the selection there of as there always was a high sodium concentration present in the salt licks and wood pieces selected.
This echoed the fact that animals know were and when to seek for this scares mineral in their diet. By adapting methodes to consume sodium from salt licks or decaying wood it ilustrates the different ways they have gone to achieve this feat. Another way of aquiring enough sodium but not reviewed here is the consumption of burned wood (Weir, 1969) (Weir, 1972). These methodes are extrodanary if we take in account that only the drive for sodium and the lust there of through aldosterone has led these animals to these unique strategies. Sodium being so important in the physiological aspects of the working body as to reproduction, movement and regulation it is also clear why animals would have needed to adapt or die in the literal sence. Soil being a very important benefit to animals were it contains enough sodium to suplement their dietary need, no extra suplementation needs to be given in the wild. On the other hand on game farms, farms and national parks were these soils are lacking it would be sensible to place extra salt in the field so that the animals can replenish their deficiency if desired. By introducing suplements in the enviroment the animals quality would increase and they will be much healthier.
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