Posts Tagged evolution

the age of mammals Alison Campbell Mar 06

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The last 65 million years have sometimes been called ‘the Age of Mammals’ (although I’m inclined to think it should be the Age of Insects, or perhaps – as it’s always been – the Age of Bacteria; after all, in terms of sheer number of individuals, bacteria have got to be the dominant life form on the planet…). This gives the impression that mammals are a relatively recent evolutionary novelty.  But just how old is this class of organisms? Just what is the ‘age’ of the mammals?

Mammals began to diversify after the mass extinction event that marks the boundary between two geological ‘periods’ (the Cretaceous & Tertiary) and which carried off a large number of species, most notably the dinosaurs. But they’ve been around for much longer. Much, much longer – the mammal family tree has its roots way back in the Carboniferous, 350-270 million years ago.

Not that those ancestors wandering through the swampy Carboniferous forests would have looked like the mammals of today. Scientists call these ancestral beasts, ’mammal-like reptiles’ ie they were reptiles, but with some mammal-like features, including features of their skulls. Modern mammals, those early mammal-like reptiles, and everything in between, are described as synapsids.

  

Amniote skull types. a) Anapsid eg turtles b) Synapsid eg mammals c) Euryapsid eg ichthyosaurs & plesiosaurs d) Diapsid eg dinosaurs & birds. (NB the ‘amniotes’ are the reptiles, birds, & mammals – all produce an amniotic egg with a number of membranes associated with the yolk & developing embryo.)
 
The synapsid skull (b) has a single opening in the external bone layer, behind the eye – these diagrams can be a bit confusing & certainly my students sometimes find them so; it looks like the hole might go through from one side of the skull to the other, but in fact that’s not so.
 
Early synapsid reptiles roamed the Earth from the Carboniferous period until the Jurassic, and at one time were the dominant land animals. Many groups of these early reptiles were not in our direct lineage; one group that was, was the pelycosaurs - which included the fearsome predator Dimetrodon. (The name Dimetrodon means ‘two kinds of teeth’ – this is significant as most reptiles have a single type of teeth in their mouths, while modern mammals have 3-4.) One group of pelycosaurs evolved into the mammal-like  ’therapsids’, a group that included Thrinaxodon, and the therapsids in turn produced a number of descendant groups. One of them, the cynodonts, was on the line to modern mammals (but not, as it turns out, directly linked to them). 
 
By the way, it’s actually rather difficult to define a clear instant at which an animal becomes a ‘mammal’, rather than a ‘mammal-like reptile’! Therapsids have a number of mammal-like features: compared to the sprawling stance of reptiles, they held their legs more underneath the body (making locomotion more efficient); they had a distinct neck; their mouths contained several different types of teeth – something described as heterodonty; the beginnings of a secondary palate began to separate the airway from the mouth; and one of the bones in the lower jaw (the dentary) was enlarged compared to that in reptiles. But they definitely weren’t mammals.
 
Many therapsids became extinct during the mass extinction event that marked the end of the Permian, around 225 mya. (Incidentally, this was one of the greatest mass extinctions in terms of species lost – David Raup & Jack Sepkoski estimated that up to 94% of all species then alive, died in the end-Permian event. On the mammalian side the survivors included cynodonts like the herbivorous dicynodont Lystrosaurus (the distribution of Lystrosaurus fossils provided early evidence in support of the concept of continental drift) and the carnivorous ‘theriodonts’. The latter were similar to wolves in size, with large serrated canine teeth and skull modifications that provided larger jaw muscle attachment points.
 
During the Triassic cynodonts gradually became more & more like ’true’ mammals. Eventually their lower jaws comprised just a single bone (called the ‘dentary’ because it bears the teeth). Along with this came a change in the way the jaw articulated with the skull (& associated changes in the tiny bones of the inner ear). Another feature was the obvious presence of a diaphragm. OK, you say – this is so unlikely to fossilise, so how can you say this? The evidence lies in the lack of ribs attached to the lumbar vertebrae (the ones that form the ’small’ of your back) – in modern mammals the diaphram is attached to the lower edge of the ribcage, & there are no lumbar ribs. The therapsid Thrinaxodon didn’t have lumbar ribs either, so the diaphragm must have been an early evolutionary innovation in the proto-mammal lineage.

And these beasts were probably furry :-)  Cynodont jawbones are perforated by small holes, similar to those which in modern mammals carry nerves and blood vessels to the whiskers, which implies furriness. (Cuddliness would be highly unlikely!) And a wonderful fossil from the Jurassic, Castorocauda, includes direct evidence of fur. Not only this, but a range of features tell us that Castorocauda spent a lot of time in the water. So, an immediate predecessor of modern mammals swam in the streams that T.rex would have splashed through: the earliest evidence yet of an aquatic mammal.

But the origins of ‘true’ (modern) mammals remains a bit murky. This isn’t helped by the fact that many of the early forms are known mainly by their teeth. (I’m sure it’s been said by someone before, but you could jokingly say that the evolution of mammals is traced through the matings of teeth with teeth…).So to get back to that original question: what is the age of the mammals? The answer has to be – it depends on how you define them :-) 

evolution supressed in new zealand? i think not Alison Campbell Mar 02

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While searching for some background on another post, I happened across this headline on the Herald site: University denies author’s PhD claim. I went on to read the story, as it’s always a bit of a concern to see people claiming credentials and the supposed awarding institution denying that this is the case. And a statement from the person making the claims caught my eye.

“During a period in which ‘evolution’ became a bad word [in New Zealand] punishable by revocation of credentials and confiscation of property [the 1980's], I refused an order from a department chairman to withdraw my books Darwin’s Universe and Time Gate from press,” he told a contemporary writers’ website.

Really? I have to say, that was news to me. The history of attitudes to teaching evolution in NZ has been quite well documented, most notably in works by Numbers & Stenhouse (2000), McGeorge (1992), & Peddie (1995), & it’s an area I’ve followed with interest.. Way bac,k in the 1880s there was an attempt by local creationists to get an Otago academic removed from his post for daring to mention evolution in his lectures (to no avail), and in the 1940s the creationist lobby was successful in getting a radio program for schools that talked about the evolution of life, taken off the air.

But to claim, as this person has done, that in the 1980s you could lose your credentials or even have property confiscated for mentioning the ‘e’ word?! This was the same time that I was teaching evolutionary biology to impressionable young minds in Palmerston North high schools – and I wasn’t run out of town or tarred & feathered ;-) Mind you, the fact that evolution wasn’t taught until the last couple of years of high school, thus exposing the smallest possible number of of impressionable young minds to this dangerous idea of Darwin’s, may have had something to do with that :-) Similar claims have been made in the US, around the infamous movie Expelled. And they’ve been shown to be similarly without substance. (Unless, in some alternate universe…)

Don’t get me wrong – there are problems associated with teaching evolution, particularly in some parts of NZ, as Peddie documented in his 1995 thesis. But they don’t manifest in people being pilloried or losing their belongings. Instead, as Peddie found, teachers may face pressure to skip that part of the curriculum (perhaps made easier to do by the fact that schools don’t have to offer the full number of achievement standards in a subject). Hopefully the fact that evolution is so prominent in the new science curriculum (beginning in primary schools) – being implemented this year, along with all the other curriculum areas – will make it that much harder to avoid teaching about this key biological concept, in state schools anyway.

But as for the claim that began this post? Pure fantasy.

 

 

C McGeorge  (1992) Evolution in the Primary School Curriculum. History of Education 21(2): 205-218

RL Numbers &  J Stenhouse (2000) Antievolutionism in the Antipodes: from protesting evolution to promoting creationism in New Zealand. The British Journal for the History of Science 33: 335-350

WS Peddie  (1995). Alienated by Evolution: the educational implications of creationist and social Darwinist reactions in New Zealand to the Darwinian theory of evolution. Unpublished PhD thesis, University of Auckland.

 

 

the oversized naughty bits of female spotted hyenas Alison Campbell Feb 24

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When I visited Pharyngula today I saw that PZ had posted a video about spotted hyenas. Female spotted hyenas. And that reminded me of one of the late Stephen Jay Gould’s wonderful essays on the same subject. (Gould remains one of my favourite science writers -although, having said that, I do find some of his later work rather overblown – and you can find examples of his work at The Unofficial Stephen Jay Gould Archive.)

If you watch that video you’ll find out that female spotted hyenas look rather like males, especially when they’re aroused. They have a greatly enlarged clitoris that looks just like a penis. (Apparently early European observers didn’t recognise them as female at all.) The females are also large, heavily muscled, highly aggressive & dominant over any males they encounter. A group of spotted hyenas consists largely of females, with a few males hanging around at the periphery. The clan leader is the dominant female, who will pass on her rank to her pups. Females lead the hunt, & once the prey’s been caught, it’s the females who eat first. Social interactions involve much sniffing – and licking – of the dangling appendages.

All of this - the size, aggression, dominance & enormous clitoris - can be put down to the fact that hyenas are exposed to very high levels of testosterone while in the womb. The clitoral enlargement may be a secondary side effect; the heavily muscled bodies & bone-crushing jaws that result from the high doses of testosterone could well have been selected for, carrying the clitoris along with them. (I have to object to the slant – & the possible explanations – put on this by the video’s narrator, who asks why the females ‘put themselves through this’ – of course, they don’t! What we see is the outcome of a long series of selecction events over thousands of years, not a process driven by the needs or desires of female hyenas.) Giving artifically-high doses of testosterone to pre-natal rat & dog female foetuses has the same masculinising effect: larger size & muscle mass, higher aggression – & external genitals that look like those of the males.

The female hyena’s masculine appearance doesn’t stop at the faux penis: at the base of this structure she has a swollen ’scrotal’ sac filled with fat. The apparent penis incorporates not only the clitoris but also the joined labia – and this has what must be an extremely painful consequence for all female spotteds: they urinate, mate, & give birth through a single small opening on the underside of this structure. I remember my eyes watering when I first read Gould’s essay, & they watered again on seeing this video! Apparently up to 20% of females die giving birth to their first pups – this also suggests that there’s some strong adaptive significance to the high levels of testosterone that produced the apparently maladaptive clitoris in the first place.

And the effects of the testosterone exposure extend to the behaviour of the pups themselves. These are no blind, helpless, mewling little softies. Spotted hyena pups come into the world with their eyes open, teeth in their jaws, & a heap of aggression of their own, sorting out their own dominance hierarchy by fighting. Apparently, while they don’t often kill their sibling directly, the subordinate is so scared of the dominant pup that it hides away in the birth tunnel (taken over from other burrowing animals), too scared to approach its mother to feed while the other pup is present – and eventually starves to death.

Disney Planet this isn’t.

meta-analyses – testing relationships Alison Campbell Feb 16

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One of the nice things about working at a university is that there is almost always an interesting talk to go to (supposing you have the time…). Yesterday I managed to go to a fascinating discussion of the use of meta-analyses by a Waikato graduate, Shinichi Nakagawa. (I suspect that Grant knows much more about this technique than I do, but Shinichi’s talk was very post-provoking.)

Shinichi began his studies here a year or so after I joined the University staff. He was invited to do a BSc(Hons), something reserved for the really able students, & it’s a sign of the quality of his research project (on zebra finches) that he’s since published five papers from it. After gaining his PhD at a UK university, Shinichi’s come back to New Zealand to work at the Universit of Otago. Over the last few years he’s begun to use meta-analyses more & more to identify relationships (or the lack of them) between data sets, & this particular research tool & its applications were the subject of his enthusiastic presentation. (He recommended Morton Hunt’s 1997 book How science takes stock: the story of metanalysis as an excellent introduction to the area, written by a journalist for a lay audience.)

I’ve referred to meta-analyses myself, from time to time. The Cochrane Collaboration, for example, makes use of them in examining the (claims for) efficacy of various medical treatments. Essentially the technique involves combining the data from a range of studies (allowing for sample sizes & so on), producing summary results that may allow recognition of a statistically significant pattern in the data. (It may equally show that there’s no real relationship between a set of factors, as Shinichi noted. Alternative treatment modalities such as acupuncture & homeopathy, for example, have been shown to perform no better than placebo as an outcome of  meta-analyses.) The larger sample size afforded by a meta-analysis allows greater confidence about the results.

The technique does have its disadvantages: you could be accused of comparing apples with oranges, for example, although this could be overcome by proper selection of the studies for inclusion in the analysis. There’s perhaps a greater problem (Shinichi described it as the ‘file drawer’ problem) whereby studies with a non-significant result are less likely to be published, thus biasing the pool of studies available for inclusion in a meta-analysis. Shinichi described how he’d had difficulty publishing a paper on sparrow parental care because it had a negative result. Now, there’s nothing wrong with this, & a fairly large proportion of experiments would come up with such an outcome, but unfortunately this doesn’t make good headlines:-) And the editorial attitude described here both skews publications in a particular direction & also skews public perceptions of how science is done.

We then heard about a few research areas where meta-analysis had been applied to a large body of data to test prevailing views. Should we take antioxidant supplements, for example? Not according to a 2007 study, that found an increased mortality rate associated with their use. While this particular study had its critics, a paper published last year in Nature found evidence that antioxidants can actually help cancerous cells to grow. (Mind you, there is a need for caution in interpreting studies like this last one, given that it was done on cancer cells in vitro – there needs to be a fair bit of follow-up work to see if this holds true in the body.)

The final example in Shinichi’s talk looked at the widespread view that a restricted calorie intake can prolong life. (Obviously there would be limits to this one & it’s not a case of living longer & longer on less & less. Eventually there’d be a point in which the lifespan was shortened rather abruptly. And terminally.  Rather like the work of Famine in the wonderful Good Omens by Terry Pratchett & Neil Gaiman.)

Anyway, individual studies of rats & monkeys, fruit flies & nematodes, & even yeast, seem to bear out this idea – it looks like a general biological phenomenon. Our speaker seems to enjoy doing meta-analyses – he commented that a preliminary review f the literature, & an introductory analysis, shows no conclusive evidence that calorie restriction has any positive effect on the length of life, not as a general principle. He found there was no consistency in the data for different species. In the discussion at the end, someone pointed out that almost all the work in the area of dietary restriction’s been done in lab-bred animals, and might not reflect what happens in ‘wild-type’ individuals. And it’s more important to look at the carbohydrate/protein balance in the diet, rather than the overall reduction in calories.

And, of course, the sting in the tail – from an evolutionary point of view, if you don’t produce fertile offspring & thus pass on your genes, the length of life is actually irrelevant. If you lived to 120, but left no fertile children, you’d be an evolutionary non-event…

happy darwin day! Alison Campbell Feb 12

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And for a bit of vaguely scientific fun, you might like to try ‘devolving’ yourself here (found this one via a commenter on the Young Australian Skeptics – whence also came the image above).

grumpiness is best? Alison Campbell Feb 02

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ResearchBlogging.org

Today’s Herald carried a story from the UK’s Telegraph, which looked at some research into the social behaviour of chimpanzees & bonobos (’pigmy’ chimanzees). And – as usual – extrapolated from this to people… Grumpiness, it told is, was a sign of a more ‘advanced’ nature, whereas the happier, more peaceable bonobos were ‘less evolved’.

The article doesn’t begin well: Researchers looked at two different kinds of monkey – the familiar chimpanzee and the less evolved but much more easy going bonobo. Chimps are monkeys??? Arrggghhh! And the ‘less evolved’ really grates. Humans & chimps last shared a common ancestor around 5-7 million years ago and, as the recent swag of publications about ‘Ardi’ have shown, the two lineages have followed different evolutionary paths since then. (Bonobos & chimps subsequently diverged from each other 0.85-2.5 million years ago – & are as ‘evolved’ as each other.) I did wonder if there was a bit of wish-fulfillment there – chimps can be grumpy, & chimps are ‘more evolved’ than bonobos, so grumpiness is a ‘more evolved’ state – good news for all those grumpy human prima donnas :-)

The newspaper story was based on a paper in Current Biology entitled Bonobos exhibit delayed development of social behavior and cognition relative to chimpanzees (Wobber et al. 2010). The researchers compared chimpanzees (Pan troglodytes) & bonobos (Pan paniscus), looking at differences in the animals’ social behaviour & also their cognition in order to test the hypothesis that certain aspects of behaviour or cognition in adult bonobos represent developmentally delayed forms of the traits found in chimpanzees (Wobber et al, 2010:1). In other words – that differences in behaviour could be related to differences in development: adult bonobos show some ‘juvenile’ behaviour patterns (eg play, & non-reproductive sex) compared to adult chimps, & also show a degree of paedomorphism in their cranial structure.

In particular, the team looked at behaviours to do with sharing/competing for food: for each species they gave the animals a food-sharing task, using male:male, male:female & female:female pairs, for a total of 30 chimps and 24 bonobos. The results: older bonobos were as tolerant as juvenile animals when it came to sharing food, but adult chimps were much less so. The adult bonobos were also more playful (including sexual play), which the authors thought might affect their tolerance for sharing food. A subsequent trial of infant & juvenile bonobos & chimps found that pre-weaning bonobos were less adept at social behaviours related to feeding competition than pre-weaning chimps. And finally they looked at the ability of the two species to adapt to ‘role reversal’, where human experimenters switched roles in terms of which possessed a hidden food reward: young bonobos were slower than juvenile chimps, and adults of both species, to adapt each time a researcher switched roles. (It would be nice, though, to see some data from observational field work here – using a human experimenter does introduce new variables that might not apply in the wild.)

Overall, the authors concluded that [u]nderstanding the evolutionary processes by which ontogenetic changes occurred in bonobos may provide insight into our own species’ evolution. [It's been] proposed that the crucial cognitive adaptation of humans relative to other apes is the accelerated development of social skills in infants. Althought the genetic changes that produce such developmental shifts are not well understood, if we can determine the process by which the ontogeny of bonobos evolved, inference can be made regarding analogous evolution in our own species (Wobber et al. 2010: 4) In other words, having an understanding of the control of behaviour development in bonobos may tell us something about the evolution of our own social behaviour.

But it’s a big jump to suggest (as the Telegraph does) that the heightened intolerance for food sharing shown by adult chimps is a ‘higher’ form of behaviour (with the implication that grouchy behaviour in humans is somehow a ‘more evolved’ trait). After all, like bonobos, humans show a reasonable level of paedomorphism in their general appearance, resembling young chimps in their relative facial proportions & in their lighter body hair. But this apparent similarity between humans & bonobos is the result of parallel evolution. Similarly, humans & P.troglodytes have an equally long history of separate evolutionary development, such that any apparent similarities in behaviour (eg grouchiness as a successful food-competition strategy) may well be more perceived than actual.

V.Wobber, R.Wrangham & B. Hare (2010). Bonobos exhibit delayed development of social behaivour and cognition relative to chimpanzees current biology, 20 : 10/1016/j.cub.2009.11.070

of rain and rotifers Alison Campbell Jan 31

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Yesterday we went across to Tauranga to see my in-laws. It was a terrible day for driving; the forecast was for periods of heavy rain & it was pouring down when we arrived. My father-in-law had emptied the rain gauge that morning (23mm, he said), & by 1pm it was back up to 80mm & rising. There was a lot of surface flooding on the road coming home, the roadside drains had turned to foaming brown torrents, and many paddocks were more like pools, with cattle standing close to hedges to avoid at least some of the driving rain.

And seeing those pools my thoughts turned (as they do) – to rotifers.

Rotifers are rather cool little microscopic animals. Each rotifer has less than 1000 cells in their tiny body, and some of them are smaller than single-celled protozoa. They’re found in fresh water just about everywhere (although there are some marine species as well) & will quickly colonise temporary ponds. (Hence the drift of my thoughts.) If you look at some of the rotifer images here you’ll see that while their shapes may differ they all have one feature in common: a crown of beating cilia at one end of that small rounded body. The name ‘rotifer’ reflects the fact that in many rotifers – the bdelloid rotifers - the beating cilia give the impression of a rapidly spinning rotor, or wheel.

I talk a bit about these fascinating wee animals in some of my lectures, on account of their reproductive habits. Why? Because many species (& all of the bdelloid rotifers) reproduce asexually, & have done so for a very long time indeed. "No sex for 30 million years" announced the headline of a rotifer story in our local paper – unfortunately this headline sat next to a photo of the researcher studying them… (My colleague had to put up with a bit of teasing & the journalist concerned was mortified, muttering at length about the iniquitous sub-editor who’d thought up that little gem!)

There’s only one gender in the bdelloid rotifers, & that’s female. Lacking males, they reproduce by parthenogenesis. Each female produces what are known as amictic eggs, which develop without fertilisation to produce daughters that are effectively clones of their mother. Because every individual in the population is capable of producing offspring, such populations can grow in size very rapidly.

Other species have the best of both worlds, combining parthenogenesis & sexual reproduction in quite a complex cycle. A female can produce 2 types of eggs that both develop parthenogenetically – one into females & the other into males, followed by a round of sexual reproduction. Apparently the latter type is produced in response to a change in the environment. This makes a lot of sense, in evolutionary terms. While the environment is relatively constant, producing offspring that are all clones of the parent means that they’ll all be fairly well-adapted to that environment (otherwise, the original ‘mother’ rotifer wouldn’t have survived in the first place). But a changed environment may place different demands on the organism. Producing males in response to such a change allows for genetic recombination, producing a range of individuals some of which may be able to survive in their new situation.

Speaking of changed situations – I was intrigued to discover that bdelloid rotifers can also enter a state called ‘anhydrobiosis’: their tissues dehydrate & in this condition they can survive for some considerable time until rehydrated again (eg after heavy rainfall refills temporary ponds). This is a relatively common strategy, but my favourite anhydrobiotic organism has to be the water bear :-)

I say ... you wouldn't be in receipt of some sort of picnic basket would you?

This image is of a normal, fully hydrated water bear, or tardigrade. As the name suggest, these cute little critters live in fresh water, crawling around on plants. But in the fully dessicated, anhydrotic state they’d win any survival contest, I think: they can cope with being frozen to around -273oC or heated to more than 100oC, exposed to pressures equivalent to 6000 atmospheres or to high doses of radiation, or even the vacuum of space. And then they’ll bounce back to their normal cuddly state on rehydration. The ultimate Survivor of the animal kingdom, methinks.

avatar Alison Campbell Jan 29

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The family finally got its act together & went to see Avatar. In 3D :-) (Actually our act was arranged by friends, who also organised us into an al fresco meal of fish’n'chips beforehand.) I carefully didn’t read anything much about the movie before I went, so I’m aware that what I’ve got to say has probably been said before - but here goes, anyway.  

Wow!

Just, wow!!!

Visually that has to be the most stunning thing I have ever seen in a movie theatre, by a country mile. OK, the story-line was pretty basic & the ending was signalled well beforehand… And the 3D glasses left a dent in my nose… But the way in which the world of Pandora & its ecosystems was rendered was wonderfully and beautifully done. (My friends & I agreed that parts of it were reminiscent of the record cover art of Roger Dean.) And I can see why ecologists like my colleague Bruce Clarkson are so taken with it, & its overtones of the Gaia hypothesis. And as a zoologist-by-training I was enthralled by the Pandoran animals.

But. (There’s always a ‘but’, isn’t there?) That same training also left me with a few questions at the end. And they had to do with the place of the Na’vi in their world. What I mean is, in evolutionary terms, they don’t belong. If they had evolved on Pandora, then it would have been from one of the other animal life-forms. All of which seemed to have 6 legs (2 pairs of forelimbs), 2 pairs of eyes, and nostrils pretty much in their armpits. Which is all great & made them convincingly alien. So – whence the Na’vi, with their very humanoid appearance & the ‘right’ number of eyes & nostrils? In other words, while the Pandoran environment looks great, the evolutionary back-story was a leetle shaky :-)

Didn’t stop me really enjoying the experience, though! (And yes, I am a pedantic science geek!)

_________________________________________________________________________________

& now I see that, in the way of the world, I’m by no means the first to see the apparent derivation from Dean’s work. Or think about the zoology (Tetrapod Zoology has a lovely take on this) Or quibble about the evolutionary ’science’. But we all enjoyed the film :-)

cauliflory (but not with cheese) Alison Campbell Jan 10

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Plants have a fascinating array of adaptations that function to maximise the odds of successful reproduction. Flamboyantly shaped & coloured flowers spring to mind, not to mention nectar rewards & attractive scents (which are not necessarily pleasant to the human nose, but then, Rafflesia isn’t out to attract us!). One of the more unusual adaptations is cauliflory – the flowers are produced on the main trunk or branches of the plant, rather than on a separate flower stem. (Ironically, cauliflower is not an example of a cauliflorous plant. That tightly bunched white head – so yummy with a nice tasty cheese sauce - is derived from a ‘normal’ flower, borne like a lily, orchid or rose on a young leafy stem that develops from an apical or axillary bud.)

I hadn’t come across cauliflory until our trip to the Daintree while on holiday. (Either I didn’t learn about it way back when, or the information fell out the back of my head once exams were over…) Our guide drew everyone’s attention to this tree, a red mahogany:

cauliflory, full trunk.JPG

Those red-gold balls on the trunk are the fruits of this particular species, produced from fertilised flowers that were held on short stalks just above the bark.

cauliflory close-up.JPG

How would this particular adaptation have evolved? After all, ‘typical’ flowers are held out there where they can be easily found by pollinators (or, if they’re wind-pollinated, their anthers hang out there in the breeze). But this ignores the fact that many animals live on the tree trunks themselves – the Boyd’s forest dragon I wrote about earlier is just one example. The ’stem flowers’ of cauliflorous plants are easily accessible to any animal moving about on the trunks below the forest canopy, & this type of flower turns out to be relatively common among rainforest trees, where mammals, birds, reptiles & insects all act as their pollinators. Around 100 different species are cauliflorous, something which seems to have evolved on more than one occasion as it’s found in many different plant families in the tropics (caulifory is less common in temperate-zone plants).

So – something else to add to my lectures this year :-) And hopefully my students will absorb the information better than I did!

trees on stilts Alison Campbell Jan 04

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And no, I’m not talking about triffids here. More a part of the continuing series on plant root adaptations. I’ve mentioned mangroves in passing before (to do with their pneumatophores), but the thing that stood out for me about the mangroves we saw in Queensland was the fact that they looked like they were on stilts:

mangroves at low tide.JPGThese trees looked quite different from the familiar NZ mangrove. Mangroves here belong to a single species, Avecennia marina, which is found around the coastline of the top third of our North Island (reaching its southern limits – linked to average daily temperatures - in Kawhia & Ohiwa harbours). A.marina is just one of a group of species grouped under the name ‘mangroves’ (around 30 different species in Australia), characterised by their habitat as much as anything: they grow in muddy intertidal zones along coastlines & in estuaries. That is, ‘mangrove’ is an ecological rather than a taxonomic classification. ’Our’ mangrove differs from the ones we saw over the ditch in that ‘theirs’ have those rather wonderful stilt roots.

Well, some of ‘theirs’ do. It seems that mangroves can be put into 3 groups: ‘red’, ‘black’, & ‘white’ mangroves. The ‘red’ mangroves grow closest to the water and put down stilt-like ‘prop’ roots, like the ones shown above. ‘Black’ mangroves – belonging to the genus Avicennia – put up pneumatophores to allow them to obtain oxygen from the atmosphere. They tend to grow inland of the ‘red’ trees. "White’ mangroves grow even further inland & don’t usually have either type of specialised roots.

I learn something new every day :-) 

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