Monday, February 10, 2014

Mongooses, mambas and me

As soon as I arrived at the group I knew something was wrong.

Mongooses were darting everywhere. Small worried faces peered out from beneath a tumble of granite boulders, and the piercing chirps of distressed mongoose pups pulsed from all directions at once. I saw Black (the group’s alpha male) snatch up a youngster and race off, leaping from boulder to boulder, before disappearing into a thicket more than 50 m (165 ft) away. Meanwhile, close to my feet, Iorek, fluffed into the shape of a football, approached in a slow ninja crouch.

Then I saw the cause.

Coiled on a sheet of rock right in front of me was one metre (3 ft) of slate-grey snake. It coiled and writhed and twisted sinuously in an eye-catching way that made my skin creep. But that was just the beginning (or more accurately the end). From this squirming mass stretched another meter of totally inert snake which led (my eyes drawn inexorably on) to the final metre. This rose up vertically, placing the snake’s smooth grey head at waist height. The creature was not looking at me; its gaze was fixed firmly on the mongooses.

Ah, a black mamba.

Black mambas (Dendroaspis polylepis) are the world’s second longest venomous snake (piped at the post only by India’s king cobra). They reach 2 m (6 ft) by their first birthday and can grow to 4 m (13 ft). Reputedly also the globe’s speediest serpent (but who’s clocked them all?), they zip along at 5 m (15 ft) per second, through the branches or down on terra firma.
Photo by Michael Ransburg.

Absurdly, black mambas (Dendroaspis polylepis) aren’t black.
It’s the insides of their mouths that are ebony (oh, of course).
When antsy, a mamba will raise the first third of its body vertically off the ground, flash its sooty maw and hiss ferociously (note to self: avoid black gums).
Photo posted on Flickr by Viperskin.

Now black mambas have almost mythic status here in Africa. Preposterously long and super quick, they might have slithered straight from the novels of Rider Haggard or an Indiana Jones movie. Their bite (if untreated) is 100% fatal, and they’re said to attack without provocation, chase their victims and track them down using scent.

And all this is true ...if you’re a dwarf mongoose.

However, even if you’re a bipedal primate (and I’m assuming you are), black mambas are not to be messed with. Piss one off (by molesting or cornering it) and you’re in serious trouble. Retaliating mambas bite multiple times at a single strike (although normally too rapidly to see), injecting about 100 mg of toxin at each lightning-quick chomp (no inoffensive ‘dry bites’ here I’m afraid). 10-15 mg of the stuff will kill you (by paralysis and suffocation), so unless you can conjure up antivenin and life-support within an hour or so, you may as well start looking for that brightly-lit tunnel. 

Although all this is pleasantly titillating, it must be said that black mambas are masters at avoiding people. And they rarely nibble on humans. A study in the 1960s found that out of more than 1,000 snake-bite victims admitted to Durban hospitals over a seven year period, only eight had fallen foul of black mambas.
And whenever I meet one (usually only a few times a year), it’s always rocketing away from me.
So why was this snake at Bugbears just... er... standing there?
And what was with the whole tail-writhing thingey?

I edged a little closer but the snake still didn’t respond. Black mambas hunt by day, actively tracking warm-blooded critters by scent, so they pose a serious threat to dwarf mongooses.
But if mambas are quick so too are mongooses, and the snake will only make a killing if it can catch one by surprise. So the instant a mongoose detects a whiff of serpent it screams ‘SNAKE!’ and whole group comes running. Fur-fluffed, spitting and growling, they encircle the reptile, creeping forward on their tummies before hurtling backward whenever it stirs. I bite my fingernails and sweat a lot.

Once they’ve harassed it, the mongooses flee (unlike with other large snakes), hotfooting it at least 100-300 m/yds. Yet they always leave someone (usually the second-in-charge male) behind. This guard follows the mamba’s every move for 10-20 minutes (presumably to check it doesn’t pursue the group) before racing off to rejoin his family. (No, I don’t know how he knows where they’ve gone, but he does). This strategy seems to work well and mambas aren’t big mongoose-eaters.

But when the group has small pups the whole ballgame changes.
Pups are not quick. Pups are dim-witted and gullible.
Suddenly mambas appear out of the woodwork.

The mamba in front of me was being guarded by both Iorek and Bear; meanwhile the other adults were dashing back and forth, helter-skelter, snatching up pups from various hiding places and carting them off in different directions (the old eggs-in-one-basket issue). I watched Pooh carefully stow her small burden in a narrow rock crevice before hurrying back to watch the snake. However, the pup, used to being tucked up snugly with its littermates, freaked out at being left all alone. Its anguished chirps, combining with those of its four siblings, rose in a deafening chorus of distress.

The group’s matriarch Iorek (BF010) hastily redistributing a vulnerable pup.

The mamba still stood quite motionless, apart from its absurdly writhing  rear end. Is this how mambas hunt mongooses? Was the snake trying to hold the mongooses’ attention with its nether coils while ambushing them from above? (Is this why mambas are so bloody long??) I can’t find any mention of mambas hunting this way but behavioural studies of wild snakes are as rare as... er... snake’s legs. And doting snake-enthusiasts are unlikely to feed their pets mongooses (at least I hope not!). I became more convinced by this interpretation just a few days later when I came across another mamba doing exactly the same thing with Koppiekats (who also have little pups).

Black mamba venom is a potent concoction that locks down muscle cells in numerous devious ways. However, there’s a bright side: it also contains mambalign, a ‘better-than-morphine’ painkiller. Damaged and inflamed tissue becomes acidic (due to a build up of positive ions) and these ions trigger pain by flooding into nerve cells via special portals (‘acid-sensing ion channels’ or ASICs) on the cell’s surface. Mambalign locks these ASICs tightly closed, thus stopping pain. It’s not clear why mambas provide this unexpected boon to their victims, especially since the venom of many less-considerate snakes actually incites pain by locking ASICs open.You can read a popular account of mambalign research here.
Photo by Ian Turk.

Black mambas are easy to identify thanks to their Witch Weekly-winning smile. The guide books rarely mention this, favouring instead the sensationalist ‘coffin-shaped’ head.
Photo by David Bygott.

As I edged a little nearer to the mamba I dislodged a pebble which bounced down toward the snake. Instantly it shot off into the undergrowth. Iorek and Cricket raced after it, desperate to keep it in sight, but it was moving so fast it had gone in an instant.
Calling constantly to one another, the mongooses fanned out to search. They crept about tentatively on tip-toe and with their backs arched, sniffing and peering under boulders, into crevices and up into the overhead branches. After about 10 minutes, they seemed satisfied that the snake wasn’t loitering nearby and began their exodus. While Black kept guard from the top of a large boulder, the others hurriedly gathered up the five squawking pups and raced off with them toward a distant kopje.

Black (BM003) watching for the enemy while the group evacuates the pups.

Tick (BF038) carrying her little brother/sister out of harm’s way.

The mongooses regrouped about 150m/yrds away beneath a huge granite boulder. After Cricket and Bear had raced back for a last quick check of the area (to ensure no errant pup had been left behind), the group settled down in a huddle to groom one another consolingly. The pups, now happily together again, began to play wrestle.
But everyone was still unnerved.
How could I tell?
Each time I raised my hand unconsciously to shoo a fly, they’d all leap in the air!

Is it safe to come out yet?
 At three weeks old, dwarf mongoose pups are mobile but gormless. This is Arctos (BU061).

Thursday, January 23, 2014

Oh, the ingratitude

Everyone has days when they feel unappreciated.
You know how it is: those around you not only take your help for granted, they then go on to presume.
Well this is one of those days for me.

What’s caused this sad state of affairs?
Well, with the summer rain, my garden’s transformed into a jungle. The ‘lawn’ has reached head-height and the local avifauna have moved in.
This is not the problem.
I love watching the common waxbills clamber about the tussocks, scissoring off grass seeds with their redder than red bills. Down below the firefinches and blue waxbills hop about searching diligently for windfalls, and garrulous flocks of red-billed queleas swoop in to chatter and squabble among the trembling seed heads.

The joys of being lawn-mowerless
(Common waxbill, Estrilda astrild)

But these little seed eaters aren’t alone. In the wild mango, a pair of scarlet-chested sunbirds are canoodling, and tucked beneath my eaves, four families of white-rumped swifts noisily discuss the pleasures of the day.
But my grievances don’t rest with any of these critters.
It’s the garden’s tiniest feathered bug-eaters that have earned my ire.

Now tawny-flanked prinias aren’t a species you usually notice. They flit about in the undergrowth, pausing now again, with jauntily cocked tails, to peer around for ill-fated bugs. Only if something’s bothering them do you become aware of their loud and persistent complaints: przzt-przzt-przzt.

Tawny-flanked prinias (Prinia subflava) are Africa’s best attempt at a fairywren. However they’re traditionalists at heart, opting for monogamy and nuclear families. This juvenile was photographed by Neil Strickland.

Tawny-flanked prinias were once thought to flutter over much of Africa, India and east Asia (prinia is the Javan name for the species). However, the elegant inhabitants of the orient have recently gained taxonomic independence (becoming Prinia inornata) - presumably to their huge relief (my prinias could never look this refined).
Photo taken in Taiwan and posted on Flickr by John&Fish.

I was sitting on my veranda one morning, with my dogs and cat sprawled around me, when a prinia fluttered down to perch in a tangle of weeds almost at my elbow. Oh how charming, I thought, as I watched it throw out its chest excitedly and clarion its territorial przzts. It then zipped away, returning a moment or two later with a long, wavering grass stem.
Oh no, it couldn’t be...
Surely, not there...
But yes, it was determinedly twining the grass stem around a weed stalk.
I watched aghast: the creature was building a nest at perfect dog nose-height.

Now please bear in mind that while one of my dogs (Wizard) is a dyed-in-the-wool husky who sensibly eschews feathery aperitifs in favour of whale carcasses, the other is a husky-cross. I don’t know what her husky forebear coupled with, but it was certainly purpose-bred to exterminate rodents. Magic’s blood-lust for small defenceless critters is enough to turn even Wizard’s stomach.
I couldn’t decide whether the kamikaze homemakers, busily weaving a nursery just 1.2 m (4 ft) away from us, were the silliest birds to fly God’s airspace... or the most fiendishly cunning.
What snake, what monitor, what raptor would dare attack them there??

The pair dashed back and forth excitedly, hopping up and down on their toes and eagerly entwining leaf blades into their globe-shaped construction. Every now again one (the male?) would alight exuberantly on a weed top and przzt in triumph (just in case some predator hadn’t noticed them). I’m sure birds feel euphoric when they’re nest-building. These two just looked so, well... pleased with themselves.

Hmm, scissor truss joists here I think...

Both Ma and Pa prinia enjoy DIY but since they favour matching outfits (and both don their glad rags for the occasion (brighter with tail extensions)), I don’t know who this is.

The completed nursery.

I was convinced that sooner or later they’d realise the error of their ways and abandon the whole hare-brained scheme, but after a few days three tiny eggs – salmon-brown mottled with purple – appeared inside the rattan ball.

Having borne witness to this folly, I felt obliged to assist the venture. So, for the last month, I’ve been creeping about the garden, calling my dogs away dozens of times a day, dashing outside whenever a harrier hawk or coucal drops by and generally worrying over the hatchlings’ long-term prospects.

Now I do not expect accolades for this effort. I don’t expect the little creatures to sing my praises or bring me gifts (bugs?) or help with the housework. But their actual response left me speechless.

I was busily working on my computer (I do work sometimes) when I noticed a prinia hopping past the back door. I’d seen them flitting about there before and assumed they were hunting bugs. But this one was carrying something in its beak. It hopped on to the back door step, paused, examined the step with its head on one side, looking first to the left and then to the right, and then it carefully put down its burden – placing it meticulously, just so. It then flew off.

Curious, I went to see what it had been doing.
Bad move.
There, right before my door, was a little moist package of bird poop.
And when I looked closely I realised that the whole step was scattered with small white faecal sacs.They were using my door step as a waste dump for their nestlings’ poop!

Now I know that most birds remove (or swallow) their ankle-biters’ droppings so they don’t attract potential chick-munchers.
But why put the stuff on my doorstep?
Do they think my house stinks so badly it will mask the faeces’ odour?
Why not place the goo further along the veranda amid the piles of swift droppings? 
Why not put it, well... ANYWHERE else??

I’m just NOT appreciated.

Yeah, so?

Sunday, January 5, 2014

A festivity of stripes

Then the Ethiopian put his five fingers close together (there was plenty of black left on his new skin still) and pressed them all over the Leopard, and wherever the five fingers touched they left five little black marks, all close together. You can see them on any Leopard's skin...
How the Leopard got his spots’ from The Just So Stories.
Rudyard Kipling, 1902.

I’ve always disliked the Just So Stories.
As an animal-obsessed child I encountered them often but they inevitably left me feeling cheated and irritated. I really wanted to know why the leopard sported spots or how the elephant acquired its trunk and these silly tales could not enlighten me. (Maybe zoologists are born and not made??)
Fortunately neither Kipling, nor Aesop, tackled the tricky question of How the Zebra got its Stripes. A San (Bushman) legend, however, attributes the charcoal stripes to singe-marks...

But why am I pondering equine apparel?
Well, firstly it’s the festive season and what could be more festive than a zebra?
Romping, head-tossing and snorting, these roly-poly creatures are guaranteed to lift one’s spirit. They’re a dazzling chimera: part mythical beast, part fairground runaway, part my-first-pony.

Who can resist a zebra?

My second reason is that the local zebra mares are celebrating the season by dropping their foals. This has transformed my drive to work into a wonderful, ooh-ah experience. The new mums are cautious and they gallop off as I approach, their gangly-legged foals racing and weaving out ahead of them. The herd’s stallion trots a few paces before swinging around to confront me. Standing tall with nose lifted, ears pricked and nostrils flared, he paces forward a stride or two, doing his best to look fierce and intimidating. Once I’ve actually passed by, he wheels around and thunders off after his disappearing family.

Now before we start let’s get one thing straight: zebra is not a proper taxonomic term. In fact it’s a brazen act of colour discrimination.
Zebra simply means ‘striped horse’ and it’s like using one collective name for all spotted cats or all blue birds.

The great grandmamma of today’s six horse species galloped into existence on the North American plains about two million years ago, and then clip-clopped off to world domination. This equine granny is believed to have worn stripes and three of her great grand kids maintain traditional dress. However, these purists are no more related to one another than they are to the more individualistically attired horses and donkeys.
While the Grevy’s zebra (trotting about northern Kenya) is clearly an ass (no offense meant), the mountain zebra (counting down its days in the pointy bits of southern Africa) and the savannah-loving Burchell’s zebra are more closely allied to horses, enjoying the same free-wheeling lifestyle as mustangs and brumbies (i.e. itinerant harems).

The Grevy’s zebra (Equus grevyi) voices its dismay over its endangerment with a classic donkey’s ‘hee-haw’. As with the wild asses, stallions hold down real estate and then have their wicked way with mares that wander by. Photo by Steve Garvie.

The mountain zebra (Equus zebra) may look similar to the Burchell’s (give or take a dewlap and a bare tummy) but their differences run deep: the Burchell’s zebra boasts 44 chromosomes while the mountain zebra makes do with 32. Photo by Vince O’Sullivan.

The critically endangered Somali wild ass (Equus africanus) sired every donkey and mule on the planet; obviously a smart decision to hang on to those lewd stockings. Photo by Lisa Brown.

So why are some horses stripy?

I mean virtually every other grass-munching mammal favours demur browns or greys. What do zebra gain from sporting so much razzmatazz?
Well, sit back and enjoy some scientific just-so-stories.

Black and white stripes may dazzle and confuse leaping predators, or rouse in them a sense of danger (if skunks can do it...). Those stripes may deter blood-sucking flies, act as camouflage (at a distance) or break up a zebra’s outline. They may generate cooling air currents, enhance the wearer’s sex appeal or machismo, or allow individuals to recognise their nearest and dearest (every zebra is fingerprint-unique).

Of course ‘stories’ like these are the raw material of science. Whittled away by scientific endeavour, they’ll eventually yield a shabby effigy of reality. So I’m delighted that - after centuries of speculation - researchers are finally starting to test them.

The Burchell’s (or plains) zebra (Equus burchelli) looks like it’s been gift-wrapped by, er... someone like me. The stripes are aligned horizontally on the back half but vertically on the front.

The old razzle-dazzle, predator-befuddling fable has been given scientific credence by
Martin How and Johannes Zanker. Using piccies of Burchell’s zebras, they produced computer models which show that the zebra’s stripes create an optical illusion. Most animals (including you and me) have neural circuits in their brains that detect the direction an object is moving by monitoring how its contours appear. But when a zebra moves, its diagonal flank stripes, juxtaposed with the narrower vertical stripes on its neck and shoulders, produce confusing signals that flummox these motion detectors. Just like the classic barbers’ pole (whose spiral stripe appears to move upward when the pole spins), the zebra’s stripes make it appear as if the creature is moving in the wrong direction. In the pell-mell of a hunt, with multiple zebras leaping and jostling, these fickle signals may throw a lion off its stride.

Although it’s great to have this fable confirmed, it doesn’t fully explain why zebras are stripy. The problem is, not all species have the Burchell’s stripe pattern: Grevy’s zebras don’t have diagonal flank stripes, and the ill-starred quagga had no stripes at all on its back half (the bit most frequently viewed by blood-thirsty zebra-consumers).

Have I got them at the back too?

The brown quagga (a South African subspecies of the Burchell’s zebra) rashly lost many of its stripes. This sacrifice, however, failed to save it from being hunted to extinction for its skin during the 1800s. This scary spectre haunts a museum in Berlin (photo from Wikipedia).

The bug-proofing fable has also earned itself the scientific stamp of approval. In a horse paddock in distant Hungary, Adam Egric and a whole troop of researchers have been testing how colour influences the avarice of horseflies (tabanid flies). By leaving out coloured boards coated in sticky bug-glue, they’ve shown that dark colours (which reflect horizontally polarised light) are much more attractive to the little winged vampires than is white (which scatters light in all directions). Yet when the researchers put out a board with black and white stripes (which does both) they were startled to find that it lured fewer flies than just plain white. After trying various widths of stripe, they found that typical ‘zebra-width’ (i.e. Burchell’s) attracted the fewest flies of all. As a final confirmation, they created plastic horse models, smeared them with glue and left them outside for a two-week period. They then counted the number of ensnared horseflies and found that the black model harboured 562, the brown model 334, the white 22 and the zebra-striped model 8.

Sticky equids. Photo by Gabor Horvath (I’m unsure of the legality of using this image but I couldn’t resist it).

I must admit I’d be more convinced by this study if the researchers had done it with African bugs. Do the local tsetse flies show the same proclivities? Blood-thirsty insects can certainly pose a serious risk to their hoofed victims (because they transmit noxious illnesses) but if stripes are such an effective fly-repellent why don’t other succulent herbivores flaunt them too? And why is the Somali wild ass - who lives only a teeny bit further north than the very stripy Grevy’s zebra – unashamedly stripe-free?

Of course, the reason that ancestral horses opted for stripes in the first place may be different from the reason that stripes are a fashion item today. And different species may hang on to their stripes for different reasons... (oh, the trials of an evolutionary biologist).

To be honest, I’d put my money on a social function being the primary one (but maybe my gregarious little mongooses have biased my thinking?). Although the stripe pattern on the rear half of zebras varies greatly (both between and within species), everyone looks remarkably similar up front (i.e. the bit spied by other zebras during social encounters). And we all know that transverse stripes make one look fat. Since prancing about on tippy-toe with an arched neck is standard protocol for equids out to impress, it seems very likely that stripes function to make the wearer look bigger and heftier.
But of course this is just one more just-so-story until some dedicated soul heads out there with a little pot of hair dye...

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