Wednesday, 16 November 2011

What can we learn from mutants?

Elephants with one tusk are common, but not mutants. Tarangire NP, Aug 2011
One of the joys of working in the bush is that there's always something new to see, and every now and again we come across something very, very odd. Some times we see the disfiguring effects of a disease or accident – one tusked elephants are particularly common. But occasionally we find evidence of a much more fundamental accident – a genetic mutation. One of the commonest is albanism, or partial albanism (more properly called leucisism – technically, you can't have a partial albino). True albinos are very rare in nature and occur when all the genes that control colour are, somehow, switched off (even those involved in eye-colour). I don't know if this baboon is a true albino, as I couldn't see the eyes, but I wouldn't be too surprised if he was. (I'll find out one day I'm sure – he lives in Arusha NP and I first saw him as a tiny baby over a year ago. One day he'll come close enough to see!) More often you'll see animals that lack colour in just some parts of their body, or sometimes lack all the pigments of one type – lacking melanin (which gives the black colours) is relatively common, and often results in sandy looking creatures, as the orange and yellow pigments are still present. Still rarer than colour mutations are the really strange mutants you sometimes see, like the buffalo below – something completely mad has happened here!
Albino Baboon, Arusha NP, Aug 2011

Entertaining as it is to see such strange creatures, I think there's quite a lot we can learn from these animals. Look, for example, at this buffalo, and compare it with the normal animal in the same herd – it's not doing very well! That's not surprising – with horns like that I find it very hard to believe it can graze properly – more likely ir can only nibble the tallest grass everyone else leaves, or is forced to browse, which can hardly be good for a buffalo. As for the baboon, well, he seems healthy enough – but I was still rather surprised to see him still going strong now aged one year – there are so many crowned eagles, leopards and martial eagles around Arusha National Park, and he sticks out from the crowd so much I expected him to be the first to go. He's been lucky so far... Which gives us our first lesson - most mutations are bad for the health, which explains why we don't see many more mutants when we're out and about.
Mutant buffalo (probably cow), Tarangire NP, Sep 2011

Much more normal buffalo, same herd!

So what about evolution, I hear you ask? Evolution is works because mutations are passed on from one generation to the next, yet lesson one is that mutations are bad for the health! What's going on there? Now, whilst that is definitely true for big and obvious mutations (in fact, most of the really big mutations that occur are probably automatically aborted - miscarried - in the womb before birth), it doesn't mean there aren't lots of mutations happening that we don't obviously see. In fact, for every human it's estimated that there are NNN unique mutations we have that have occurred in the genes we inherited for our parents: we don't have perfect copies of our parents DNA at all. Happily, most of the mutations have no or very little impact – which is why we don't see them – but the good news is that a few might have small benefits. And so this is lesson two, that evolution normally happens in very, very small stages – the accumulation of lots of tiny little beneficial mutations that we generally never see. That's not to say that we can't see evolution in action with the mutations we do see – in fact, the elimination of 'bad' mutations from the population is just as much a part of the evolutionary process as the incremental development of new changes. So simply by looking at this skinny buffalo, we see natural selection working – whilst the animal might still be alive (and obviously has survived a number of years), I don't think it's in any condition to pass its genes on to the next generation. So that could be my third and final lesson that we can learn from these mutant animals – that natural selection results not only in the accumulation of beneficial traits, but also in the elimination of sub-optimal genes too. That might not sound so important right away, but maybe in time we'll look at why it does matter, particularly when animal populations are reduced and individuals start to breed with their own relatives.

And finally, let's just remember that accidents – like the one-tusked elephant – are completely different from mutations. The effect of an accident will never be passed on to future generations because it's got nothing to do with genes (though the propensity to have accidents, of course, might do!). Only mutations in the DNA will be passed on to future generations, if the animal concerned survives to breed.

Sunday, 13 November 2011

Woodpeckers as keystone species

It's been a while since I posted a birdy blog and since I got some nice pictures of a Cardinal Woodpecker at the weekend, I thought I'd use it as an opportunity to talk about woodpeckers in general, since they're surprisingly important in the habitats they occupy. As usual, we'll look to answer the three questions I use to prompt me when seeing wildlife – what is it? What's it doing? And what's it's role in the ecosystem.
Female Nubian Woodpecker, Kisima Ngeda, Aug 2011

So, for identification, woodpeckers are generally fairly easy. In most of northern Tanzania and Kenya, there are four common species of savannah woodpeckers – the commonly seen Nubian, spotted all over; Bearded, the largest and with a black throat and stripy face; the rather small and neat Cardinal, with spots on the back and streaks on the front, and the very colourful Grey. Away from the dry north of Tanzania the Nubian is replaced by a number of other options – Bennett's or Speckle-throated being the obvious ones. There are plenty of other species around, of course, but they're mainly associated with forest and we'll forget about them for now. Woodpeckers in general are rather widespread, obviously similar species occur on every continent except, rather strangely, Australia. They're relatively closely related to barbets and hornbills (note the zygodactyly – two toes forwards, two-toes backwards – they share with the barbets, easily seen in some of these photos). So, that's what they are. Now what do they do?

Male Cardinal Woodpecker, Manyara Ranch, Nov 2011

Normally you'll come across a woodpecker first when you hear it, either calling – Nubians in particular are noisy – or from hearing the 'tap-tap-tap' of their beak on a tree. Calling is often done by pairs, and we can safely assume it serves the dual purpose of strengthening a pair bond and communicating to neighbours that the territory is occupied. The tapping is where it gets more interesting – most of this is exploratory, trying to find hollow bits under the bark where tasty larvae may live, some is more obviously getting at the food once they've found it, and some it again a territorial statement like calling – though this purpose seems to be less common here in Africa than in northern regions. And the most interesting of all is the hard banging they use to excavate nest holes. I'm sure (unless you're Australian!) we've all seen the beautifully neat holes woodpeckers make for their nests, often several holes in a single stem. There's two things that are particularly interesting about this to me – the first is how they do it in the first place. The speed and pressure generated in order to dig into the wood is extraordinary – the deceleration from 6-7m/sec to stationary at impact isequivalent to 1000 times the pull of gravity – the effect on humans would be similar to Usain Bolt running head-first into a brick wall at the end of his 100m sprint. Not pretty, I should think! And the adaptations they have to avoid the problems we'd get from banging out head on a brick wall are also impressive – slightly differentlength upper and lower mandibles, extra thick skull, fluid-filledshock absorbers, unusual size and shape of brain, etc., etc. Quite remarkable really!
Grey Woodpecker, Near Arusha, March 2011

But the second thing about these holes is where they get really interesting. Woodpeckers mostly use theholes they excavate only once, after which the holes are available to anything else that likes to live in holes – birds, bats, other mammals and all. In fact, there's a huge array of animals that live in holes, but can't make them themselves (though some, like barbets, may make adjustments to get the hole right for them). Over time the holes get larger and larger, allowing a whole host of species to find homes. In some northern forests, woodpeckers have disappeared (for anumber of reasons we don't need to go into), and once the holes aregone, so too do all the other species that make use of them. Thus the loss of woodpeckers has much greater impacts on the whole ecology of a woodland than the simple direct effect – the consequences cascade down through other species too. Which is exactly why some people suggest woodpeckers may be seen as keystone species – a single species that holds together a whole load of other species and have disproportionate impacts on the ecosystem. Mighty important things, woodpeckers!
This Brown-breasted Barbet is probably uing an old woodpecker hole, Nr Boma Ng'ombe, March 2011


Saturday, 12 November 2011

Phenology – the timing of biological events.

First rains arriving over Manyara Ranch, Nov 2011
This is one of my favourite times to be in the bush, as the rains arrive and the savannah turns green. I love the excitement of the birds as they greet the rain, and the miracle of new grass appearing in just a few days and I probably get as excited by the first thunderstorms as my children! But as we all know, the timing of these events can change year to year. In fact, never more so that recently – one of the first impacts of global climate change that we see here in East Africa. Despite the changing season being such a profound event in the savannah, there's a surprising amount that we don't know about the patterns of seasonal change that we see.

For example, it's obvious that grass growth responds directly to rainfall – or at least to soilmoisture. If the rains are late, the grass stays dry, if the rains are early, it turns green early. But how does it know? To all intents and purposes the grass (or the seed) seems completely dead until something tells it the soil is moist and it's time to start growing again. In this case, actually, it's fairly straightforward – the moisture in the soil is in direct contact with the grass roots (or seed) and as that moisture is absorbed the cell cycles are started up again.
Pre-rains green flush in Brachystigia woodland, Kafue NP, Oct 2011 (pic. H. Frederick)

Other patterns are harder to understand, and the one that fascinates me most is the green flush that we see in miombo woodlands (and on Commiphora and several Combretum species too) before the rains. Not just immediately before the rains either, but some weeks before. How, and why, do they do that?

Let's remember first that savannah woodlands are deciduous (the trees loose their leaves) because during the dry season their leaves would loose too much water to allow the tree to survive. Add water, and there's no problem, so you'll see evergreens in the savannah only in riverine and kopjie habitats. So why, just when water is in shortest supply, do some species 'choose' to use some of their remaining stores of water and put out leaves before the rains come – and not just before, but a long time before? One of the things that we do know that might help us understand this is that once the leaves are out, the plants once again 'switch off' until the rains arrive. They've got leaves out, but they're not photosynthesising and respiration (plants respiretoo, of course) is pretty much dormant too. But then, once the rains do come, they're active within 24hrs. And another clue might come from the fact that we know there's a flush of nutrients (particularlynitrogen) associated with the first rains, that rapidly declines after the first few days of rain. So there's obviously a strong advantage if you can be ready and waiting for the rain – other trees that aren't ready will spend those first few nutrient rich days busy growing leaves and not be able to take advantage of the nutrient flush. So as long as you can minimise the costs of having leaves before the rains come (by essentially shutting down as much as possible), it seems plausible that the benefits could outweigh the costs (and clearly, for some species they do, or they wouldn't survive!). One thing that suggests this idea might be right is the fact that legumes – like Vachellia and Senegalia (I will get you toforget about Acacias!) - don't do it, they respond to soil moisture and, as we know, being legumes hey have no shortage of nitrogen, unlike other savannah species.
More pre-rain greening, Kafue NP (H. Frederick)

But why, then, be so early – why not just wait until the week before the rain before growing leaves and further minimise your costs that way? And here is where we really run out of hard facts and enter the realms of interesting scientific speculation – my guess is that because the date when the rains start is variable, you can't predict it that accurately. If you want to take advantage of that first nutrient flush, you've got to be ready for the earliest possible date the rains might fall – which (like this year) might be several weeks before the rains begin in normal years. I'm far from certain this is right – among other things, it requires that the benefits of being ready for that first flush are extremely strong, such that plants that catch it every year have a meaningful evolutionary advantage over plants that only catch it most years, which is testable but not guaranteed. But it's a good theory to work on for now.

The next part of the story that I'm interested in, of course, is how they do it? How do these plants 'know' when it's October and the rain is coming in a few weeks time? Unlike the grasses that simply detect water, these plants must keep track of the changing date directly. In the north where these processes have been studied in extraordinary detail, plants (andanimals) use changes in day length to keep track of the seasons – in spring and autumn in Aberdeen where I used to live from one day to the next day length could change by as much as five or ten minutes. But I find it hard to conceive that the same process is possible here where day length changes only by two minutes across the entire year – the difference from one day to the next can only be measured in seconds or fractions of seconds, and I find it hard to believe this can actually be the cue. But, amazingly, no-one's studied it so we just don't know.

There are other biological events that depend on precise seasonal timing, of course – like the millions of birds that spend months here until April, then head north to breed, but even here we don't always know the signals that the birds are using and why, for example, so many species seem to have been rather late arriving this year. But this has already been a long enough blog for one day, so that will have to wait for another time...

Thursday, 3 November 2011

Nutrients in the savannah biome

Of all the 'big four' processes that drive the ecology of the savannah, I think I've probably spent least time talking about nutrients. That might be surprising, because in some ways the cycling of nutrients is what helps us define an ecosystem. (Ecosystem is, in fact, a rather poorly defined term, but if there's anything that separates a habitat from an ecosystem, it's the fact that most nutrients and energy are well recycled within an ecosystem, whist habitats, once you ignore solar inputs, often have large in and out flows of nutrients and energy. So an ecosystem, such as Serengeti, can have lots of different habitats. On the other hand, a biome is a globally occuring set of similarly functioning ecosystems - savannah ecosystems around the world form the savannah biome. But let's get back to the point...) Nutrients are, however, extremely importand in shaping savannah habitats, both at large scales, and fine scale.
Termite mound, Mwiba Ranch, August 2011

The large scale patterns of nutrient availablity lead to different vegetation types in different areas, and drive large-scale migrations, both issues we've touched on already in this blog. So today I'm going to concentrate on the smaller-scale processes that act over just a few metres, but still have important roles to play in the ecology of the savannah. Let's start with what might well be the most important nutrient cyclers of the savannah - the termites. On the right is a typical termite mound in the middle of the dry season - note there's lots of uneaten grass in the foreground and background, but on and around the mound itself, there's nothing left but nibbled grass stems. Why? Because the termites have been busy working in the surrounding area to gather up bits of vegetation, and bring them to their mound. By gathering vegetation, then processing it in one spot, they concentrate nutrients at this spot, and the grass growing there is richer and better food than grass growing further away. The termite mounds become a nutrient hotspot, and animals know how nutrient rich their food is. Particularly in the dry season, when they only have dead matter to graze, small differences in nutrient content are very important. And even in the wet season these are preferred foraging areas and are often kept permanently short, as a grazing lawn. Once the process starts, in fact, it becomes self-perpetuating, as regularly grazed grass keeps growing new shoots and new shoots are always tastier (full of nutrients and low in the silica (a natural glass) grasses use as defence against grazing.), prompting more grazing and keeping the grass short, perhaps even spreading beyond the original termite mound as the additional benefits accrue - precicely the definition of a grazing lawn, and a very obvious example of how important the nutrient cycling carried out by termite really is at the large, observable scale we work at.

Impala Midden, Manyara Ranch, Nov 2010

Dikdik midden - what a lot of poo! Manyara Ranch, Nov 2010
The next process that's concentrating nutrients in the savannah is also so obvious that we often miss it - animals that use middens concentrate nutrients over several years in one spot. This nice impala midden shows another feature you often see about middens - again, the grass all around has been grazed to nearly nothing. And for exactly the same reason as before - the nutrient hotspots great lush grass that is heavily grazed, neighbouring grasses are also grazed and the impact spreads out to create a grazing lawn. (In fact, this one has suffered rather from cattle grazing too, but they respond to the same processes as the wildlife.) As you can see, the grazing is much more widespread than simply the focal nutrient spot, but it's quite possible that this midden and the others you can see around are the original cause of the heavy grazing over the whole of this little stretch. Always good things to point out when you're on a walk, especially if there are children about like my two...
Elephant diggings near Gibbs Farm, Dec 2010

See the tusk marks?! Elephant digging - there was also buffalo horn marks!

And for now I'll leave one final special case of nutrient hotspots impacting savannah ecology - the rare locations of mineral deposits utilised by a range of wildlife, but most famously by elephants. In some places, elephants have dug caves over 160m deep into mountainsides in search of nutrients (most famously on Mt Elgon where elephants have dug in search of calcium, sodium and magnesium). Other animals also come from far afield in search of the nutrient rich soil, leaving wide paths through the forest. These photos are from Ngorongoro, behind Gibbs Farm, where elephants are primarily searching for Molybdenum, selenium and cobalt. These micronutrients (nutrients required only in tiny amounts) are important for animal health and animals with deficiencies are generally rather unenergetic and not alert to the dangers of the world. So, rather a useful thing. And, of course, you don't have to head specially out to these caves (though it's worth a walk, and the birding around there is great!) to show this sort of activity to folk - several cuttings ont he main ascent road from Laodare gate to the crater viewpoint show obvious evidence of nocturnal mining by elephants, and the rock here is just as nutrient rich as in the caves at Gibbs.

So, next time your out and about, have a look for signs of nutrient concentrations, and see what's responding to it. And then remember the large-scale variations in nutrients too, that are so important for other processes!