A simple mutualism, not as simple as it sounds…

Uhusiano rahisi wa kutegemezana, sio rahisi inavyo sikika…

Are these Red-billed oxpeckers feeding on ticks or drinking giraffe blood?
Photo courtesy (and copyright) Paul Oliver.

We love talking about mutualisms in nature. If you’re not familiar with it, a mutualism is a relationship between two species that benefits both species. There are obligate mutualisms where the survival of one or both species is totally dependent on the other- like many figs and fig wasps. Or non-obligate where the species would still survive without the other. Mutualisms are great feel-good stories, but as we learn more about mutualisms we are learning that even though often both species are benefiting from the relationship it is sometimes not as innocent as it appears to be.

Let’s take the Red-billed oxpecker for example. Red-billed oxpeckers get nearly all of their food by eating parasites off of large mammals. They live so closely with animals that their beaks are adapted for combing through the fur, their toe structure is different from other birds and allows them to get a grip on animal’s fur at any angle.

Ticks suck animal’s blood causing anemia, condition loss, and can also carry diseases so our romantic view of the oxpecker-mammal relationship was that they were helping the large mammal by removing ticks, and in turn getting their food. Multiple different researchers have examined the stomach contents of oxpeckers and estimated that an individual oxpecker can eat about 100 adult ticks a day. Check out Dr. Colin Beale’s twitter thread on Red-billed oxpeckers here (you don’t have to be on Twitter, just click the link).

A researcher, suspicious of this relationship decided to run an experiment. He split a herd of cows into two groups. On one group of cows, they chased the Red-billed oxpeckers away, while on the other they let them do their thing. Then they counted the ticks. They also compared open wounds and the amount of earwax in the cow’s ears. What they found was that Red-billed oxpeckers didn’t affect the number of ticks on the cows, so the cows were actually not getting any benefit from the oxpeckers. Instead, the researcher also found that these birds were often enlarging the wounds that the ticks had made to get directly at the blood. The other thing they found out was that Red-billed oxpeckers reduced the amount of earwax in the cow’s ears, but we don’t know whether or not this is good for the cows.

Of course, this isn’t as simple as we might wish. It turns out that further studies have shown that the blood-feeding of oxpeckers is dependent on the abundance of ticks. When ticks are abundant, the birds tend to eat ticks but as the number of ticks reduces, they start to feed on wounds. There’s also some evidence that the relationship is more beneficial for different animals than others. Another amazing mutualism complication story coming up on Friday.

(Swahili version)

Tunapenda kuongea huhusu kutegemezana (‘mutualism’) katika ulimwengu. Kama huna uzoefu nayo, ‘mutualism’ ni uhusiano kati ya spishi mbili, kila mmoja akifaidika. Kuna spishi zenye ‘mutualism’ ya lazima, yani hawawezi kuishi bila uhusiano huo –  kwa mfano mikuyu na spishi ya nyigu. Ama inaweza kuwa sio ya lazima kutegemeana kwa zile spishi mbili. Kuongea kuhusu ‘mutualism’ ni jambo la kufurahisha, lakini tunavyojifunza zaidi kuhusu ‘mutualism’ tunagundua kuwa wanaotegemeana siyo mara zote kwa faida ya wenzio;  mara nyingi spishi moja inaleta madhara kwa wenzio, sio inavyoonekana.

Tuchukue mfano wa ndege Shashi domojekundu (‘Red-billed oxpecker’). Shashi domojekundu wanapata sehemu kubwa ya chakula chao kwa kula kupe na vimelea vingine vinavyonyonya damu ya mamalia wakubwa. Wanaishi karibu wanyama hawa kiasi cha kwamba midomo yao imebadilika kwa ajili ya kuchambua nywele ya ngozi yao, na miguu yao ina mpangilio wa vidole tofauti na ndege wengine inayowaruhusu kukamata ngozi ya hao wanyama kwa kila upande.

Kupe wananyonya damu ya wanyama na kusababisha upungufu wa damu, upungufu wa afya, na vilevile wanaambukiza magonjwa. Hivyo ilionekana kuwa uhusiano kati ya Shashi na mamalia ilikuwa kumsaidia mamalia na vilevile kumpatia oxpecker faida ya kupata chakula chake. Watafiti tofauti walichunguza tumbo za Shashi na kukadiria kwamba Shashi anaweza kula karibia kupe wakubwa mia moja kwa siku. Chunguza Twitter ya Dr. Colin Beale kuhusu Shashi domojekundu hapa (hauhitaji kusajiliwa na Twitter, bonyeza tu hapa).

Lakini mtafiti mmoja hakuamini kama uhusiano huu ulikua inavyoonekana. Alichukua kundi ya ng’ombe na kuwagawa kwenye vikundi viwili. Kwenye kundi moja walifukuza ‘oxpecker’ walipojaribu kutua juu ya wanyama, wakati kwenye kundi ya pili waliwaruhusu shashi kubaki na kuendelea na shughuli yao. Baadaye waliwahesabu kupe. Waliangalia vidonda na kiasi cha nta kwenye masikio. Waligundua kwamba kuwepo na Shashi haikupunguza kiasi cha kupe kwenye ng’ombe na kwa sababu hii, ng’ombe hawakupata faida. Waligundua kwamba mara nyingi, Shashi walikua wanapanua vidonda vilivyo sababishwa na midomo ya kupe na kunyonya damu ya ng’ombe. Kitu kingine walichokigundua ni kwamba Shashi walipunguza kiasi cha nta masikioni, lakini hatujui kama hiyo inawasaidia wanyama au la.

Uhusiano huo sio rahisi kutambua tulivyotegemea. Utafiti mwingine ulionyesha kwamba tabia ya Shashi kunywa damu inategemea na uwingi wa kupe. Kupe wakiwepo wengi, ndege wanakula kupe kwa uwingi, lakini uwingi wa kupe ukipungua wana anza kunywa damu ya mnyama. Kuna ushahidi zaidi ya kwamba wanyama wengine wanafaidika na Shashi zaidi kuliko wengine. Tegemea nakala ya kuhusu uhusiano kutegemezana ajabu ijumaa.

Sources:

Nunn, C. Ezenwa, V, Arnold, C. Koenig, W. 2010. Mutualism or Parasitism? Using a phylogenetic approach to characterize the oxpecker-ungulate relationship. Evolution.

Weeks, P. 2000. Red-billed oxpeckers: vampires or tickbirds? Behavioral Ecology.

Plantan, T. 2009. Feeding Behavior of Wild and Captive Oxpeckers (Buphagus spp.): A Case of Conditional Mutualism. Scholarly Repository.

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