A few things you (probably) didn't know about weaver ants

 Ants aren't usually the first things people look at when on safari, but they are fascinating beasts when looked at up close. We briefly featured siafu here once before, but that's not enough for a really important group of invertebrates, and it's time to rectify that. Finding I had some nice pictures of Weaver Ants Oecophylla longinoda (right) I thought they might make a good start as they're not only fairly common in some areas (particularly near the coast), but they're pretty interesting too. In fact, on starting a bit of research I discovered they're even more interesting than I first thought! There are actually two species in this genus, the African species, and a closely related species that occurs across Asia and into Australia. There being (I suspect) rather more myrmecologists in Australia than Africa, a lot of the relevant research comes from there, but it seems highly likely 'our' species do the same, so here are a few things you might not have known about weaver ants before.

Why are female raptors usually bigger than males?

 A friend of ours foolishly sent me a message yesterday saying he'd got both spotted eagle owls and african wood owls in his garden at the moment. As I'm sure anyone sensible would have realised, that immediately resulted in my inviting the whole family over to his house for lunch today with the join aim of avoiding washing up and seeing some nice owls up close. I'm pleased to report success on both fronts! After the initial surprise of seeing the two species in practically neighbouring trees (eagle owls are well known predators of other owl species, making over 10% of the diet in one study), the thing that struck us most was the extraordinary degree of dimorphism exhibited by the pair of wood owls. In most birds of prey - the owls, hawks, eagles, etc - the female is the larger bird, the male being smaller which is exactly the opposite of what is normally the case in birds. Why this should be is a fairly interesting question which comes in two parts - firstly, when should there be such a large difference in body size in (particularly) birds of prey? And secondly, why should the females specifically be bigger than the males (why is it the opposite of most other species)?

How to survive the Serengeti: predation, food and body size

Serengeti Lions eat a diversity of mammal species.

Another paper describing one of my favourite talks from the TAWIRI conference back in December has just been published, this time in the Journal of Animal Ecology (available to some here). Grant Hopcraft and colleagues are interested in why different grazing animals use different parts of Serengeti in different ways when they all eat grass. Why, for example, do you usually find buffalo around riverine areas, whereas gazelles tend to be on the open plains? At some level it's obvious that where you find an animal is the place that it survives best and there are two aspects to survival that most ecologists would agree are crucially important: where will you eat? And where will you be eaten? Although both buffalo and gazelles eat grass, and both are eaten by large cats, it's quite possible that grazers of different size will be affected differently by these same two processes. And that's what Grant and colleagues set out to test.

Nitrogen in the savanna biome

Nutrients, along with fire, water and herbivory, are one of savanna ecology's big 4 and we've covered quite a bit about nutrients in the savanna biome in general already on the blog. So far we've mostly covered the issue in a general sense, not focussing on specific nutrients but a new review by Corlie Coetsee and others of the nitrogen cycling in Kruger National Park (available,but sadly not free to view here) made me think it was time we tackled the issue slightly more specifically.

Middens are amazingly rich patches!

Nitrogen, although the most abundant gas in the atmosphere (nearly 80% of air is nitrogen), is one of the three commonest elements limiting plant growth (the others are the P - phosphorus - and K - potassium - of traditional NPK fertilisers). It's vitally important to life, because it's a major component of protein, but most of that nitrogen int he world is in fact useless for plants or animals being what we call inorganic. Before plants or animals can make use of it a chemical transformation needs to occur from the inorganic form, to a form that is bound to hydrogen or oxygen atoms and can be used by plants. The cycling of nitrogen from inert inorganic forms, to useful organic forms is referred to as the nitrogen cycle, and the rate at which conversions happen are can be critical at determining the fertility of soils. The most important process that converts inorganic to organic nitrogen is driven by various types of bacteria, in particular there's a group called Archaea that we now think probably aren't bacteria at all that play an absolutely critical role in this process.

Life spans of tropical birds

Admirng a White-browed Scrub Robin!

Pangani Longclaws are very impressive up close

This morning I took my children with me to go out and do some bird ringing (banding, if you're from the US or Australia!) with some friends just out of town. As is often the case when I'm ringing, I get asked lots of questions about why we ring birds, today's best was why we keep catching immature birds? There are lots of answers to this question and some of them focus on the fact that young birds are just less careful and are generally more stupid than their parents - but this bias aside it is a really good question and had me thinking again about the differences between bird lifestyles here in Africa, and those in higher latitude areas like Europe and North America. It just happened that the question came up after we'd caught two migrant birds (from Europe) and a very new baby White-browed Scrub Robin - all birds hatched within the last year. What I said at the time (and now I can look back over all the birds we caught this morning it's even clearer) was that whilst we often catch more immature than adult migrants, that's not true for the resident African birds we were catching. In fact, we only caught about 25 birds this morning, and only three of them were migrants (we were hoping for more, but this year has been surprisingly poor so far down here, the talk of all the local birds at the moment, as conditions seem good to us!), but all the migrants were young, whilst of the remaining resident African birds, only three were immature. This sample alone, of course, is of limited use: as a scientist I'd want much larger sample sizes - and I'd also want to know it wasn't just a seasonal thing - many of our local birds are only just begining to breed, so there simply aren't many babies around yet. But put our results together with many others, and start to look at the data gathered in more detail and it is indeed clear that African birds do things rather differently to their higher latitude counterparts - as described in a nice paper by Wiersma et al "Tropical birds have a slow pace of life" (get it free here!). Not unlike a lot of people living here either!

Lovebirds can live up to 20 years - they can also bite hard, so be careful extracting!

What is the savanna biome?

Savannas around the world are often open woodland like this Serengeti pic.

Remember a few months ago I reported on some talks I'd enjoyed at the ATBC/SCA-Africa meeting here in Arusha? Well, the research behind one the the talks I enjoyed most was published late last year and I think it really helps explain what makes the savannah biome (which is probably why it was published in Science - great job Carla!).  However, before going into too much detail about this research, I think it's important to make sure we know exactly what I mean by a biome in the first place...

When I talk of the savanna biome (or, indeed, the savannah biome, since I am English) I'm not referring to a habitat like grassland, or Acacia woodland; nor am I referring to a specific ecosystem like the Serengeti Ecosystem. Rather, I'm refering to the set of habitats that make up the savannah biome globally - the collection of grassland and woodland types that all have the same main processes operating on and in them, the savannah big four are, of course, nutrients, water availablity, herivory and fire. They might (and often do) contain remarkably different species, but the same processes are at work and, to remarkable degree, they show largely similar vegetation forms and structures. A biome may therefore be thought of as a set of habitats that share similar ecological processes wherever they occur across the world. By contrast, I tend to define an ecosystem as a single geographical area (like the Serengeti Ecosystem), within which nutrients are cycled with relatively little input or output from neighbouring ecosystems. Hope that's clear...

Fire is crucial to maintain savanna, particularly in wetter areas. Tarangire July 11

So, if we're going to understand what the savanna biome is, we need to look at the processes that are the dominant forces within it. If we can understand these, then we can predict where savannahs will be found throughout the world - and we can also understand what might happen to savannahs if we humans mess around with these processes through, for example, the effects of climate change. And this is exactly what Carla Staver and her co-authors have done in their paper (which I'm afraid is probably hiding behing the paywall here). They were particularly interested in discovering what determines the boundary between forest and favanna biomes (the boundary with desert is a much more obviously rainfall driven boundary), and decided that the most likely factors were rainfall and fire. Whilst that might well be true, there's plenty of evidence that herbivory is a major player too (at least here in Africa) and nutrients have been proposed as major players too (though there's also plenty of evidence to suggest this really isn't the main thing). However, they didn't look at these - herbivory they mention but exclude from their analysis with the very reasonable excuse that we just don't have a good idea how much herbivory there is around the world, but I think it's a bit of a shame they didn't have a go at nutrients too. Still, enough of what wasn't looked at - what did they find?

How colourful are birds?

Lilac-breasted Roller, eveyone's safari favourite! Indigo & Violet

One of the best things about birding in the tropics is the sheer brilliance of many of our bird's plumage. After a day in the field you can easily think you've seen birds of every colour of the rainbow - and you're probably right! In a paper published last year by Mary Stoddard and Richard Prum (available free to all here) they demonstrate nicely that whilst they do cover the whole rainbow (and more besides), they still don't cover even the majority of the potential colours available to them - only about 26-30% of the potential options, it seems.

Red: Scarlet-chested Sunbirds use structural and pigmented colours

To understand how anyone can ever assess exactly how many colours are availalbe to birds, you need to start by understanding that many birds (though not all) actually see more colours than us - a lot can see ultraviolet light, as well as the usual combination or red, green and blue that we humans (and most other primates) see. (By contrast, most other mammals lack even our ability to see red, so they must live in a rather dull-looking world!) So by knowing the entire set of colours that can be generated from red, blue, green and ultraviolet the authors identified the potential range of colours available to birds, and they then sat down and looked at nearly 1000 (965 to be precise) sets of feathers and precicesly measured their colour, then plotted it in the red/green/blue/uv colour space. And they discovered that despite their efforts to find feathers covering as many different colour types as they could, they only found colours in about 1/3rd of the available space. In particular, birds seem to be missing a lot of the different options of green and purple. Now, there certainly are green and purple birds out there, but not all the possible forms of green, and not all the possible sorts of purple. (They're not particularly good at pure UV either - but that might in part reflect the author's inability to identify strongly UV feather groups in their initial search - we can't see it after all!)

Ecology of broad-leaved woodlands in the savannah

Terminalia Woodland, Sasakwa Hill, Grumeti Reserves, July 2009

We've covered a number of savannah habitats so far, but one we haven't touched on so far are the broad-leaved woodlands. On the one had that might seem surprising - broad-leaved woodlands of one type or another are the dominant vegetation of much of the savannah biome, but on the other hand and despite the area they cover, they're not the areas most safaris spend too much time in. The reasons for this are obvious if you do start driving around them - wildlife densities are much lower in the broad-leaved woodlands than in the grasslands and Acacia woodlands that form the other major habitats of the biome. Despite this overall pattern, however, some species are actually commoner in this type of habitat than elsewhere. So why is this, and what is special about the broad-leaved woodlands?

Our Nov training camp was in broad-leaved woodland nr Tarangire!

As usual, we'll answer these questions by reference to the savannah big four: water, nutrients, fire and herbivory. The most immediately obvious thing about the broad-leaved woodlands in savannahs is that they're usually found on the higher ridges of an ecosystem - as you move off a ridge you come through the broad-leaved woodlands (typically Combretum - Terminalia woodland in much of East Africa, though a lot of Brachystegia in the southern part of the region) and gradually enter a belt of Acacia woodlands and grasslands on the lower areas. As we should know by now, these ridges are likely to have very poor nutrient loads in their soils - the ridges are often of very old rocks, 550 Million years old or more and have been well and truly washed by rain for much of that time, with the nutrients that were once present now washed down hills to the lower areas where they get used by Vachellia/Senegalias and grasses. So life is pretty tough in these areas, whether your a plant or an animal that feeds on the plants. Nutrients are particular hard to come by, so growth rates tend to be lower and there's consequently less nutritious food around to browse, explaining the relative lack of wildlife in these areas. The lack of nutrients also explains why the leaves of broad-leaved woodland species turn yellow before falling, whilst those of Vachellia and Senegalia do not: plants with nutrient shortages will try and recover as many nutrients as possible from their leaves before they drop them, and as they withdraw nutrients, so the leaves change colour. By contrast Vachellia and Senegalia ('Acacias') are legumes and have a have an ample supply of nutrients so don't need to do this withdrawing so much before loosing their leaves at the start of the dry season. The comparative lack of browsing in these areas, of course, also explains why broad-leaved woodlands aren't as thorny as other woodland types - there's little nutrient, so there's little browsing, so there's little need to defend yourself from browsing in these areas.

Lesser Kudu are often in broad-leaved woodlands. Tarangire Aug 2011

Herbivory then, is reduced. It's far from absent, but it's definitely reduced - and mainly done by some of those animals that are less frequently seen in other areas: Greater and Lesser Kudu are fans of broad-leaved woodland, so too are eland, grey duikers and the like. Why is it that these animals actually seem to like spending time in the nutrient-poor broad-leaved woodlands? Well, on the one hand you could suggest that if they didn't eat there, nothing would and even though it's nutrient poor it would still be a wasted resource, and that's certainly true to a point. But also these animals are all mammals that aren't the best at dealing with predators - they'll usually run a bit, then freeze, which works well enough when predators are at low densities, but isn't going to be so effective on a plain, or where there are large numbers of predators. So it might be predator avoidance that drives these animals to the nutrient poor hillsides that othe animals avoid - though of course we've no way of saying if in fact it might be the other way around, that once you specialise on nutrient poor food you don't need to be so good at avoiding predation!

Tabora (long-tailed) Cisticola is often common in broad-leaved woodland

What of the other two processes, water and fire? Well, they don't tend to differ so much between broad-leaved woodlands and the 'Acacia' woodlands. Maybe a little less fire (the grass doesn't grow so well), and a little less water remaining on the shallower soils, but these differences are tiny compared to the major differences in nutrient availablity and herbivory. So, in the interests of simplicity, let's leave the broad-leaved woodlands there for today - interesting places to visit for specific animals and plants (and some nice birds too!) that specialise in these habitats, but not the main focus of many game drives.

How old is a baobab?

Baobabs even make Giraffe look small! Tarangire, April 2010

I was lucky enough to see the New Year in, at Tarangire NP where, as always, I was impressed by the immensity of baobabs Adansonia digitata. When you see elephants dwarfed by a tree, you know it's impressive, and Tarangire is the place to spot both elephants and baobabs. But the two questions I'm often asked about baobabs are firstly, how old are they? And then, why don't we see baby baobabs? As they're perhaps both related issues, and because I found a nice paper that explains how we determine the age of baobabs (available, but not free I'm afraid, here), I thought it would make a nice post.

Room for one more? Take samples from the cavity...

The age of many trees is easy to estimate by simply counting annual rings: in temperate climates growth happens during the summer period, and slows during the winter, depositing a dark ring each year. In the tropics many trees have growth rings formed during the dry season. The problem with baobabs is that they have a succulent trunk that (a) gets stripped by elephants for water, (b) doesn't really have clear growth rings and (c) is often hollow. So a standard method of counting rings won't tell us how old the tree is. Instead we have to turn to radiocarbon dating. This is a method commonly used to age archaeological remains and replies on the fact that when a plant grows to 'fixes' CO2 from the atmosphere in its woody matter. Now, the carbon (C) in the CO2 of the atmosphere occurs in two forms which we call ¹⁴C and ¹²C. Now ¹⁴C is radioactive and changes ('decays') at a constant rate to Nitrogen, whilst ¹²C is stable. So if we compare the proportion of ¹⁴C and ¹²C in a sample with the proportion present in the atmosphere, we can calculate how much ¹⁴C has decayed, and therefore how old our sample is. So what Patrut and others have done, is to take samples of wood from with the hollow cavities within one particularly large baobab, and use radiocarbon dating methods to estimate the age of the tree. And they find some interesting results even for this one tree - the tree in question has two stems, one much larger than the other. We might suspect that the larger stem is, of course, the older one. But this isn't the case at all - the smaller trunk is estimated to be over 1060 years old, whilst the much fatter trunk is around 300 years newer. So once the tree is already pretty big, the size of the trunk is no useful guide to the age of the tree - and the authors also note several other similar studies they've done of other trees which confirm this pattern. They suggest that what matters is the initial conditions over the first 100 years of a stem's life - if it's particularly favourable the stem grows very quickly, and then keeps growing quickly for the rest of it's life (early life conditions are often very important for later growth rates in a range of organisms). So if a stem starts in a particular good time, its growth can easily overtake older trees who struggled to grow fast early on in life, so the biggest trees aren't necessarily the oldest. None the less, from this and other studies of baobabs it's still clear that very large baobabs are often over 1000 years old - the oldest known was over 1275 years old when it died.

You do see baby baobas - this nr Tarangire Nov 2011

Now, why is this longevity perhaps related to the lack of baobab recruitment (i.e. why don't we see baobab seedlings)? Well, firstly let's clarify - lots of people all over Africa have commented on the apparent lack of baobab recruitment (several cited in here), but it isn't actually true that we never see baobab seedlings, just that we don't see many - if you really get to know a park with baobabs well, you probably do know where there are one or two seedlings at least. And now think a little about population biology. All it takes for a population of a species to remain stable is that births equal deaths. If births exceed deaths it's obvious that we'll soon have a growing population, and if deaths exceed births we've got a population in decline. So how often does a baobab have to be successful in having a baby for the population of baobabs as a whole to remain stable? Only once in 1000 years, of course. But it may produce seeds (many!) every single year of that life - only for them all to die - let's say a mature tree produces 1000 pods per year, each with 100 seeds (guesses!) and it does that for 1000 years - that's 100 Million seeds, of which only 1 is expected to survive to maturity! So are we really likely to see lots of baby baobabs if the population is stable? No, I don't think so; especially as we've already got the suggestion that early conditions - for a baobab maybe periods as long as 10 or 100 years - are pretty important, and maybe those conditions only come around ever 4-500 years - nothing that we'd ever expect to experience in our lifetimes. So do I worry about baobabs? No, not really. But I do like to see them, and they're seriously important for the ecology of the areas where they occur, something I think we'll tackle in a later post...

Good for sunset photos too! Tarangire, Oct 2009

Main reference: Patrut, A., Reden, K., Pelt, R., Mayne, D., Lowy, D., & Margineanu, D. (2011). Age determination of large live trees with inner cavities: radiocarbon dating of Platland tree, a giant African baobab Annals of Forest Science, 68 (5), 993-1003 DOI: 10.1007/s13595-011-0107-x