Die invloed van tanniene in plante op die voedingsekologie van kameelperde (Giraffa camelopardalis)

Abstract

The project originated due to kudu mortalities on several Western Transvaal Bush veld farms. Conclusively was found that plants defend themselves against excessive browsing by means of chemical substances such as flavonoids and that neighbouring plants alarm each other upon utilization. Flavonoids are byproducts of photosynthesis. More than 3 000 flavonoid compounds have already been described of which condensed and hydrolyzable tannin is of most importance, in particular those steriochemical types with a molecular mass between 300 and 700. Tannin precipitates protein readily to the disadvantage of herbivores. The precipitate is undigestable to ruminants. Unprecipitated tannin inhibits enzyme activity within the digestive tract. Precipitation of muco-protein on the epithelium of the digestive tract decreases absorption of digestive products across the membrane. Tannin protein precipitation result an unpalatable taste (astringent) to the browser. Diversity of tannin composition within plants is species specific. Thirty to fourty steriochemical tannins may occur within the same plant. For each plant species a specific spectrum of tannin concentration values exists. Plant species can thus be divided into three categories according to dry mass tannin concentration threshold value (TCTV): species containing low TCTV (> o to 4 %), species containing high TCTV (> 4 to 15 %) and species containing extremely high TCTV (> 15 %). Within the canopy cover of Acacia nigrescens trees nine tannin concentration levels were distinguished. The levels resemble different degrees of zonized ruminant browsing. Tannin concentration differs with temperature variation and therefore follows a 24 hour cyclic oscillation curve. Active merystematic tissue of young leaf lets and sprout contain higher tannin concentrations than old mature leaves. Trees and shrubs respond to physical disturbance due to browsing. Tannin concentration of Acacia nigrescens increased by 70 % within two minutes after commencement of browsing (immediate response). A second reaction (belated response) followed within 30 to 100 min. after commencement (an increase of 120 %) . Normal equilibrium, corresponding to the 24 hour oscillation curve, was retrieved after 60 hours. The availability frequence of dietary plants within the habitat correlated poorly (r = 0,332; p < 0,001) with the feeding preference of giraffes. Feeding preference correlated acceptably (r = 0,730; p < 0,001) to tannin content of the diet. Giraffes positively selected low tannin content diets. Eighty three percent of the observed giraffe utilizations were of plant species containing low tannin concentration threshold values (less than 10 % TCTV). For 63 % of all utilizations on trees situated down wind from previously utilized trees, feeding duration was significantly shorter for the former trees than for the latter. Seventy four percent of all giraffe feeding movements were either upwind or across wind during a sequence of utilizations. Down wind utilizations were affected by previous upwind utilizations regarding increased tannin production within the tree leaves. It seems that ethyleen which is despersed by wind from disturbed/utilized leaf tissue, activate undisturbed tissue to increase its tannin biosynthesis activity. Giraffes sensed tannin increase by taste and tried to avoid it.