Big in the tropics

How big can a cold-blooded animal get? It depends how hot it is (perhaps).

Larry Li's team argues that animals have a minimum metabolic rate, below which they become too sluggish to function. This limits their size, because as animals get bigger, their relative metabolic rate declines — which is why a shrew needs to eat its own body weight each day, and you or I get by on about 2% of our weight. But body temperature also has an exponential effect on metabolic rate: a 5 °C rise leads to a 150% increase in metabolic rate. In tropical climates, then, the effects of increased temperature compensate for those of increased size, so animals can get bigger. Li and his colleagues back their idea up with evidence that in a variety of beasts — earthworms, millipedes, cockroaches, frogs and so on — the biggest tropical species is much larger than the biggest temperate species.

I suspect that this idea will be quite controversial. One can think of other reasons why tropical species might be bigger — there could be more food around, for example. It's also often argued that animals get bigger towards the poles, because it helps them keep warm (because they lose heat less quickly). This idea is known as Bergmann's rule.

Also, if I understand their earlier papers correctly (see here and here, these researchers have taken the highly unusual step of arguing that mass-specific metabolic rate does not change with size — i.e. that a big animal's cells do not burn energy more slowly than a small one's, which goes against more than 150 years of thinking about metabolic rate. This also seems to contradict their ideas on body size.

It's another entry into the strange history of metabolic rate, which everyone agrees is a fundamental biological property, but which has also generated more than its share of confusion and disagreement. My book looks at scientists' efforts to understand metabolic rate — which have been going on for about 200 years — as well as some recent powerful theories that seek to explain it (with which Li and co disagree), and the other things in nature that metabolic rate can explain. Check out the metabolic ecology link on the right to know a bit more. Check out my book next year to know more than you'll ever need.

Generality in ecology

The same issue of Oikos contains an interesting paper by Norman Owen-Smith of the University of Witwatersrand, South Africa, promoting what he calls a 'metaphysiological' approach to ecological theory. He argues (I think I have this right) that scientists studying the biology of populations — the rate at which they grow and the reasons they go extinct — have focussed too much on numbers of individuals, and not enough on flows of energy and matter, and that this approach has given their science shaky foundations, liable to fall foul of the basic laws of physics and biology. 'I believe that population ecology … has tended to seek mathematical rigour at the expense of biophysical rigour,' Owen-Smith writes. Getting it right matters, he adds, because our models will play a large part in how we try to conserve endangered populations.

Is everything everywhere?

"...For about a century, microbiologists have believed that the organisms they study are unhindered by geographic boundaries, traveling the world and thriving wherever they find their preferred environment--be it hot springs, freshwater ponds, or rotting fir trees. That view gives researchers who study microbes a rather different perspective on the world. As the Dutch biologist Lourens Bass-Becking put it in 1934: "Everything is everywhere; the environment selects."

Or maybe not. In the past few years, many microbial ecologists have come to believe that microbes are not infinitely mobile. Bass-Becking's dictum is really only "an assumption,"says Jessica Green of the University of California, Merced. "It's based on a confusion of hypotheses for facts."..."

I've got a new feature in Science (registration required) looking at microbial giogeography, and whether the patterns seen for plants and animals - such as the fact that bigger islands contain more species than small, or that the life in two blocks of forest becomes more different as the distance between them increases - hold for microbes. Many have thought that they don't, but the recent boom in studies of DNA fished out of the environment suggests that perhaps they do, in some form or other.

The scientists I spoke to: Jessica Green,Bland Finlay, Claire Horner-Devine, Jennifer Hughes, Tom Fenchel.

Junk food makes gulls dumb

The old wives are right — fish is good brain food. But only if it's the right fish. Kittiwake chicks fed on a diet of smelt, which is low in lipids, are stupider than those that get lots of oily fish.

This, coupled with climatic changes, could help explain why the gull species has been "in precipitous decline" in Alaska over the past few decades, say Alexander Kitaysky of the University of Alaska, Faribanks and his colleagues.

Kitaysky's team fed some captive red-legged kittiwake chicks as much fish as they could eat, but gave others more meagre rations. They also fed some chicks on oily silverside, and others or lipid-poor smelt.

At the end of the test, the birds on a restricted diet showed, not surprisingly, higher levels of nutritional stress, as revealed by their high levels of the hormone cortisone. Oily fish has more calories per-gram than other types, so the smelt-eaters had a harder time getting enough calories.

Hungry birds also showed cognitive impairments. Birds learnt how to open a plastic pot with food in, and then had to learn that black pots had food in, and white ones were empty. Those on the restricted diet did worse in these tests, and the birds on the poorest diets never learned to associate black pots with food.

A diet of smelt exacerbated the effects of nutritional stress alone, showing that both calories and lipid content seem to affect the chicks' learning ability.

Once they fledge, kittiwake chicks must fend for themselves, so slow learners might struggle to survive. Why kittiwakes have declined has been a puzzle — adults have been have having as many chicks as ever, but fewer birds survive to adulthood.

The change in diet is probably linked to a climatic change in the mid 1970s — a switch in a cycle called the Pacific decadal oscillation from cool to warm waters —that caused populations of oily fish— such as herring and mackerel — to decline, and favoured other fish such as pollock.

This finding also lines up alongside a theory to explain the similar decline of Steller's sea lions over the same period. The 'junk food' hypothesis advocated by Andrew Trites at the University of British Columbia and his colleagues argues that sea lions and seals have declined because they do worse on a diet of non-oily fish — experiments show that they have to eat more to get the same calories, and gain weight more quickly, for example. The whole issue of sea lion declines is controversial (registration required); some see this as an attempt to get the fishing industry off the hook, and view Trites' links with the industry with suspicion. (Although for what it's worth, the junk-food idea seems reasonable to me.)

Weirdly, Robert Winston, the avuncular face of British science communication, has recently got into trouble for advertising milk with added omega-3s (i.e. fish oil), and hinting that drinking it might make kids smarter. Perhaps he should market the magic milk to kittiwakes.

Homeric wildlife

The Homeric epics and the founding texts of European civilization, one of the wellsprings of our culture — and, it turns out, a handy field guide to prehistoric Greek wildlife.

Two Greek biologists, Eleni Voultsiadou and Apostolos Tatolas at the Aristotle University of Thessaloniki have gone through the works of Homer and Hesiod, which were written down around 600-800 BC, and recorded what animals are mentioned in the texts. The results tell us both what the ancient Greeks knew about the animals around them, and also how things have changed.

In total, the epics refer to animals 2442 times, and give them 71 different names. The ancient Greeks were, naturally enough, most concerned about useful animals — the majority of references are to domestic or food species such as goat, sheep, pig, dog and honeybee. They also took notice of pests, such as ticks, flies, and the woodworm that ate Odysseus' bow.

But the Greeks also knew their wildlife: lions, wolves and bears appear, as do tortoises, peregrine falcons, swan, geese and the night heron. Sea life is not as well represented as land animals, but dolphins and monk seal appear, and even octopus, sponge and an edible sea squirt. Apart from the eel, all fish were lumped together.

Some animals appear metaphorically. The owl — Athena's bird — appears as a word meaning 'having shining eyes'. People are described as chatting like cicadas.

The epics show which animals have disappeared from Greece in the past three millennia, such as the lion. They also show what hadn't arrived at the time of writing — there are no mentions of the domestic cat or chicken, for example.

Ancient Greeks also had a reasonable knowledge of animals' habits and behaviour. They knew which animals where predators, which their prey, and which parasites, for example. Voultsiadou and Tatolas do not mention if animals were attribute with magical powers, although the myth of the Cyclops, a one-eyed giant, might have their origin in Greeks finding the skulls of the elephants that once roamed their land. These would have seemed freakishly huge, and also have a large hole in the middle, for the trunk, that looks very much like a huge eye socket.

This study, published in the Journal of Biogeography, is reminiscent of a much earlier study by the Scottish scientist D'Arcy Wentworth Thompson. Thompson, who as well as being a professor of biology was expert in ancient Greek, published two works cataloguing the animals in classical literature: A Glossary of Greek Birds (1895), and A Glossary of Greek Fishes (1947). I know this because my forthcoming book, due next year, contains a chapter on his life and work.

We've arrived

Hurrah! Creationists have got round to taking a pop at ecological theory.

What these folks are referring to is a recent paper in Science by Sean Nee at the University of Edinburgh and his colleagues that took issue with a body of theory created by Eric Charnov at the University of New Mexico.

Charnov's ideas are based around a concept called 'life-history invariants'. The concept is that, viewed proportionately, there are features of biology that are roughly consistent across lots of species. Such constants might include the proportion of resources invested in offspring, as a function of body mass; or the age at maturity, as a function of total lifespan.

This seems to show that organisms follow similar rules about how to put their resources into growth, development, reproduction and so on, and that that they have evolved similar responses to trading off say, growth against reproduction. This has an intuitive appeal, particularly to biologists minded to look for generalities.

Now, Nee and his colleagues are saying that these apparent invariants are really just an artifact of the analyses used to compare how one trait, such as age at maturity, varies with another, such as lifespan. They argue that the way these analyses have traditionally been carried out — and then used to support the idea of life-history invariants — makes the link between traits look closer than it really is, and that really species are much more variable in how they allocate their resources.

This does not mean that life-history invariants don't exist — although it certainly looks like a blow to the theory — just that we need to find new ways of looking for them. It also shows the many pitfalls that biologists face in handling their data. For example, there are about half a dozen different ways to do a regression analysis — the technique that the new work accuses the old work of mis-using — and many biologists lack the technical knowledge to pick the most appropriate one.

The new finding is similar to a debate that ran a couple of decades ago around another supposed general ecological principle, the self-thinning law, which claimed to show how the size of plants in an area declined as their population density increased. Here too, errors were made in analysing the data that made the relationship look stronger than it really was. Most plant ecologists now think the old self-thinning law is wrong, although a new version, based on how metabolic rate varies with size, has been suggested in its place — you can't keep generality down.

The (mildly) interesting thing about the attack is that life-history invariants aren't obviously contradictory to a Biblical view of biology in the way of natural selection - any more than gravity, or the double helix, is. But anti-Darwinians are desperate to jump on any hint of dispute among biologists, and hold it up as evidence that the whole edifice is crumbling.

p.s. The Guardian cartoon strip 'Dr Clever's World of Facts' today has the best description of intelligent design I've seen so far - 'Creationism with a website'.