The vital role of hippos

Umuhimu wa viboko. (Scroll down for swahili)

Hippos are nocturnal grass-eaters (grazers). Because they are most active at night we actually don’t know much about what they do at night. Most of the time they come out of the water at night and follow well-worn paths to grass. They’ll eat 40-60kg of grass per night, then return to the water where they spend the day. 40-60kg is not a lot of food compared to their weight, but hippos have a fairly efficient digestive system.

Hippopotamus are fore-gut fermenters similar to ruminants. Depending on how you want to define and count chambers they have a 3 or 4 chambered stomach. When hippos eat, they chew the grass and then swallow. The food enters two chambers that are similar to the rumen called parietal sacs (some people consider the two sacs to be one chamber because they are basically the same). The big difference between hippo digestion and rumination is that hippos don’t re-chew their food. The grass just sits in these chambers mixing with bacteria and protozoans that produce the cellulase to break down the cellulose in the grass. From there it moves into foregut where it continues to ferment. Eventually it gets into the glandular stomach where it mixes with gastric juices or acids for chemical digestion before entering the intestines where the nutrients are then absorbed. It is a slow but efficient process in hippos and depending on the quality of the grass they are eating can take from 48hrs during the wet season to 120hrs during the dry season.

Simple diagram of hippo digestive stomachs.

Hippos spend at least 50-70% of the day in the water, so we can assume that 50-70% of what they eat at night gets excreted in the water. In 2006, a census of hippos in the Mara & Talek rivers in the greater Kenyan Maasai Mara found over 4,000 hippos or a density of about 34 hippos/km of river. Based on what hippos eat, they estimated that hippos are excreting more than 36 metric tons into the river every day. That is 36 tons of nutrients. Just looking at nutrients, they calculated 1,277 tons of Carbon, 180 tons of Nitrogen and 18 tons of Phosphorous every year.

When water levels drop the amounts of nutrients can get too high and cause fish to die.

The same researchers then looked at another nutrient that most of us don’t think about called Silica. Plants take up Silica from the soil and turn it into an important biological building block called a phytolith. When plants die this biological building blocks accumulate in the soil. Diatoms, tiny microorganism in the soil and in aquatic system need silica to make their skeletons. Diatoms are the base of many aquatic food chains especially in the East African lakes, so we must not underestimate the importance of hippos in our riverine systems. If you want to read the scientific papers they’re linked at the bottom.

Viboka ni walanyasi (grazers) wanaotembea usiku. Kwa vile wanatembea usiku kuna mengi hatuyafahamu kuhusu wanachokifanya usiku. Kawaida viboko wanatoka kwenye maji usiku na kufuata njia walizozoea kutafuta nyasi. Huwa wanakula kilo 40-60 za nyasi kwa usiku mmoja, halafu hurudi kwenye maji kupumzika mchana. Ukifanananisha uzito wa kiboko na kiasi cha mlo wake siyo kubwa lakini kiboka anayeyusha majani kwa ufanisi.

Kiboko ni mnyama mwenye mfumo wa utumbo unaofanana na wanyama wanaocheua chakula chao (ruminants). Kutegemea na namna unavyotaka kuhesabu vyumba vya tumbo unaweza kusema kuwa ana vyumba vitatu au vinne. Kiboko anavyo kula, huwa anatafuna nyasi na kuimeza. Chakula kinaingia kwenye vyumba viwili vya kwanza vinanvyo fanana na tumbo la nne ya ng’ombe ‘rumen’ lakini vyumba hivi vinaitwa ‘parietal sac’s (watu wengine huhesabu kama chumba kimoja kwa sababu vinafanana). Tofauti kubwa ya mfumo wa kuyeyusha chakula ‘digestion’ ya viboko na wanyame wengine wanaocheua (ruminants) ni kwamba viboko hawacheui chakula na kukitafuna tena. Nyasi inakaa kwenye vile vyumba na kuchanganyikana na bakteria na protozoa zenye uwezo wa kuyeyusha kwa kutengeneza ‘cellulase’ ambayo inayeyusha kambakamba za ‘cellulose’ kwenye nyasi. Baada ya hapo, chakula kinasogea kwenye tumbo lingine la mbele na kuendelea kuyeyuka ‘ferment’. Baada ya hapo, chakula kinasogea kwenye chumba cha mwisho, tumbo lenye tindi kali ‘acid’ ya kuweza kumaliza kuyeyusha zaidi kabla hakijaendelea kwenye utumbo mdogo na mkubwa ‘intestine’. Huu mfumo wa digestion unachukua muda mrefu lakini ni fanisi. Kutegemeana na ubora wa malisho, inaweza kuchukua masaa 48 kipindi cha masika hadi masaa 120 kiangazi.

Viboko wanatulia kwenye maji kwa asilimia 50-70 ya siku, na tunaweza kukadiria kuwa asilimia 50-70 ya mlo wa usiku hutolew kwenye maji kama kinyesi. Mwaka 2006, sensa ya viboko kwenye mito ya Mara na Talek kwenye hifadhi ya Masai Mara huko Kenya ilikuta zaidi ya viboko 4,000 na wastani ya viboko 34 kwa kila kilometa ya mto. Kwa kupiga hesabu ya kiasi cha mlo wa kiboko, walikadiria kuwa viboko wanatoa kinyesi zaidi ya tani 36 kwenye mto kila siku. Kwa kumaanisha tani 36 za virutubisho. Walivyo hesabu virutubisho, walikadiria tani 1,277 ya kaboni, tani 180 ya naitrojeni, na tani 18 ya ‘phosphorous’ kila mwaka.

Baadaye, watafiti hao hao walichunguza kirutubisho kingine kinacho itwa ‘silica’. Mimea hutoa silica kwenye udongo na kuibadilisha kuwa kitu muhimu kwenye mifumo ya kibiologia kinachoitwa ‘phytolith’. Endapo mmea inakufa, hii silica huongezeka kwenye udongo. Kuna viumbe vidogo kwenye maji na udongo ambavyo vinaitwa ‘diatoms’. Hizi diatoms zinahitaji silica kwa ajili ya kujenga muundo yao. Hizi diatoms pia ni msingi wa mzunguko wa chakula kwenye mito na ziwa za Afrika Mashariki na ni muhimu. Tusipuuze umuhimu wa viboko kwenye mito yetu. Soma nakala za kisayansi kwenye ‘links’ hapo chini.

Main sources: The hippopotamus conveyor belt: vectors of carbon and nutrients from terrestrial grasslands to aquatic systems in sub-Saharan Africa. Amanda L. Subalusky, Christopher L. Dutton et al. Freshwater Biology (2014)

Evidence from the digestive tract on phylogenetic relationships in ungulates and whales. P. LANGER 2009

Hippos (Hippopotamus amphibius): The animal silicon pump. Jonas Schoelynck & Amanda L. Subalusky.

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