The World of the Weddell Seal

Weddell Seals Diving [Wiki Creative Commons 2013]
Weddell Seals Diving [Wiki Creative Commons 2013]

The Weddell Seal is a common sight around the coastline and surrounding sea ice of Antarctica, and is well adapted for life throughout the Antarctic year.  Weddells live on flat areas of ice attached to the Antarctic mainland, and on freer-floating areas of pack ice, and they dive beneath the ice to feed on fish, crustaceans and molluscs.

During the Antarctic summer the extent of the sea ice shrinks to about 2.6 million km², but in winter it grows to about 14.2 million km², and becomes thicker.  In the summer the temperature around the Antarctic coast and the sea ice ranges between 0°C and -4°C, and in winter between -10°C and -15°C.  These temperature ranges are mild in comparison to the brutal extremes of the inner Antarctic continent, where temperatures plummet to -70°C  or worse in the winter.  But life on the Antarctic coast and sea ice presents great challenges to a mammal, which must be able to cope with the cold air on land, the much greater chilling effect of swimming in the cold ocean, and the challenge of finding food through the barrier of the sea ice.   Weddell Seals, however, have a range of physical and behavioural adaptations that allow them to cope well with the cold air, the cold water and the sea ice.

A Weddell at a breathing hole [Giuseppe Zibordi, Michael Van Woert, NOAA NESDIS]
A Weddell at a breathing hole [Giuseppe Zibordi, Michael Van Woert, NOAA NESDIS]

Their bodies are insulated by a thick layer of blubber, which also aids in buoyancy.  They have a streamlined, slug-like shape which gives them the smallest possible surface area in relation to their body mass, which means that they lose less heat to their surroundings.  They also conserve heat in the way that blood circulates around their body: the arteries that carry warm blood out from the heart run very close to the veins that bring cool blood back from the body extremities, and much of the heat is transferred from arteries to veins, so minimising the heat that is lost at the skin.  When basking in the sun, Weddells have also been observed to lie at right-angles to the sun, to maximise the warmth they receive.

Adults have a thin fur layer, which provides insulation on land and in the water, though the insulating properties of fur are more effective when dry.  Pups have a thicker layer of fur, which insulates them well from the cold air as they spend the first stage of their life on land, being fattened by their mothers’ rich milk, until they have grown and built up a blubber layer sufficient to insulate them in the water.  At this stage they shed their pup fur.

A Weddell Seal pup in its natal fur [Jerzy Strzelecki 2000]
A Weddell Seal pup in its natal fur [Jerzy Strzelecki 2000]

The fat content of Weddell Seal milk is extremely rich, in common with all mammals that need to develop a thick layer of blubber.  The Weddell pup is able to rapidly pack on a thick blubber layer which will insulate it when it takes to the sea, and both add to its body mass and streamline its body shape to maximise its ability to retain body heat.  (Penguin chicks are able to pile on the weight in an analogous way, as the regurgitated fish supplied by their mothers is very rich in fish oils.)

Weddells need access to gaps in the sea ice, so that they can dive down to hunt and re-emerge to breathe.  This becomes a greater challenge as the sea ice grows and thickens in the winter, but Weddells can take advantage of natural gaps in the ice, either cracks or natural openings called polynyas, which are kept open by winds and water currents.  They also create and maintain breathing holes in the ice, keeping them open by gnawing at them with their canine teeth.

Phytoplankton [NOAA MESA Project 2009]
Phytoplankton [NOAA MESA Project 2009]

The cold Antarctic seas are surprisingly rich in life. Cold water contains much more oxygen than warm water, and the polar oceans are also rich in other nutrients essential for life, such as nitrates, silicates and phosphates. Winds and waves have the effect of mixing the upper layer of the ocean, which helps to distribute these nutrients. These provide the conditions for vast quantities of microscopic organisms called plankton to survive and thrive, and these plankton are the basis of a rich marine ecosystem. And it’s not just the sea which is rich in microscopic life – the sea ice itself may look devoid of life, but it can contain a thick layer of algae, as rich in plant matter as an especially lush field of grass!

Plant plankton harness the sun’s energy, especially in the polar summer, to provide the first stage of the food chain. They in turn may be eaten by carnivorous plankton, and these in turn by a kind of shrimp called krill, which is enormously abundant in Antarctic waters. These krill then provide food for a whole marine food web, including many species of fish well adapted to such cold waters, squid, whales, penguins, seabirds, and a range of seal species including the Weddell.

Article for OU Module S175 The Frozen Planet © Andrew Murray

‘The Stem Cell Challenge’ by Lanza and Rosenthal

A PROMPT Analysis of ‘The Stem Cell Challenge’ by Lanza and Rosenthal (Scientific American 2004)¹

Human embryonic stem cells [Nissim Benvenisty - Russo E. (2005)]
Human embryonic stem cells [Nissim Benvenisty – Russo E. (2005)]

Given that Lanza and Rosenthal are both in the stem cell research business, and that they finish the article on an upbeat tone, they’ve produced an article which seems to me to be reasonably balanced in tone – an overview of many potential avenues for research and possible treatment, with the significant caveats (mostly scientific but also ethical) associated with most, if not all of them.

It’s clear that the whole field is at an, ahem, embryonic stage.  It’s like the first days of a new Yukon gold rush – the sharp-eyed can see vistas of potential opening up, but nobody has charted or excavated beyond relatively superficial levels.  The possibilities are thrilling, but bewildering – I suspect not just to me as a lay-reader but sometimes to the scientists themselves!  So many ways that we might obtain stem cells, create stem cells, modify stem cells – so many possible medical applications, but so many hurdles, failures, such a high failure to success rate at this early stage…

Nerve cells [Nissim Benvenisty - Russo E. (2005)]
Nerve cells [Nissim Benvenisty – Russo E. (2005)]

I don’t have a sharply-defined ethical stance on the whole issue.  I don’t have a neat soundbite answer to it all.  I’m excited but confused.  The one certainty is that this research is going to continue.  Once the world knows that there’s gold in the Yukon, nothing can stop them digging.

Presentation

The article is presented to a high professional level. Highly complex topics have been assembled and summarised in a way that is accessible to a scientifically literate general reader, in a way that strikes a good balance between substantial content and general intelligibility.

Relevance

The article succeeds in answering the question presented in its subtitle: ‘What hurdles stand between the promise of human stem cell therapies and real treatments in the clinic?’ It summarises a wide range of potential stem cell techniques and applications, but states the nature and magnitude of the obstacles facing each avenue of research at time of writing.

Objectivity

Debatable. Both authors are stem cell scientists, and it is clearly in their interests to accentuate the positive when positing the future of their field of work. Nevertheless I felt that their discussion of every aspect of research has been accompanied by a clear assessment of the problems facing it at time of writing. At no point does this read like a puff-piece of PR advocacy. It could be argued that the writers focus mainly on the scientific aspects of the field, with less attention being given to the ethical issues – but I feel that they have correctly kept more emphasis on the science – scientists must present the hard facts to society at large, to allow society to make as scientifically informed an assessment of the ethical issues as possible.

Method

The article gives the impression of being a fair and wide-reaching assessment of the state of stem cell research at time of writing, but the lay reader does have to invest a measure of faith in the writers. There is no explanation of the methodical means by which the authors have collated a great deal of primary data, and with this article alone to depend on, the reader does have to take a substantial amount on trust. More specific citations to the primary publications would have been very welcome. This is, however, a secondary article, with less stringent requirements for explanations of methodology than would be the case in a primary scientific paper.

Provenance

The article’s provenance is basically clear but could do with more detailed references. A box titled ‘The Authors’ clearly identifies the authors, their academic positions and their fields of work, and the end of the article gives links to associated publications written by them. A large number of references are made in the body of the article to the work of other scientists at various research institutions, but the article could do with specific referencing of the primary research published by these other scientists.

Timeliness

Not good by 2013. This is clearly a dynamic and fast-moving field, with vast fortunes being invested in getting rapid results, and nine years is much too long for this article to be of use as anything more than a general overview. Any or all of the information in the article could have been refuted, revised, disputed or abandoned altogether by now.

¹Lanza, R. and Rosenthal, N. (2004). ‘The Stem Cell Challenge’, Scientific American June 1 2004: 93-99

Article for OU Module SDK125 © Andrew Murray 2013

Sickle-Cell Disease and Natural Selection

How could natural selection increase the number of children born with sickle-cell disease in certain regions when these individuals are unlikely to survive and produce offspring?

 

Normal red blood cells next to a sickle cell

Natural selection is at work on a species if three necessary and sufficient conditions are met: that there is a struggle for existence, and therefore some individuals die before successfully breeding; that there is variation in characteristics that can confer advantages or disadvantages in the struggle to survive and breed; and that these characteristics can be passed onto an individual’s offspring. ‘Fitness’, in an evolutionary sense, describes the ability of an individual to survive and reproduce, and for those offspring to successfully reproduce, relative to other members of the species, in an evolutionarily competitive environment. So how can we explain the apparent paradox that natural selection has, in West Africa, produced so many children with such a serious disease as sickle-cell anaemia, most or all of whom will fail the ‘fitness’ test of being able themselves to survive and reproduce?

 We know that the sickle-cell allele can be inherited, so the third of the necessary and sufficient conditions for natural selection is met. Let’s look at the other two conditions more closely. The first is that there is a struggle for existence: in poor, tropical regions like West Africa malaria is still a major killer. So here – to a greater degree than in rich industrialised countries – an evolutionary struggle for existence still applies. Many individuals here are dying before being able to have children, because of malaria.

Sickle-cell disease is inherited in the autosomal recessive pattern [Cburnett - Own work in Inkscape]
Sickle-cell disease is inherited in the autosomal recessive pattern [Cburnett – Own work in Inkscape]

Secondly, there must be a variation in characteristics that can confer evolutionary advantage and/or disadvantage. This is where the sickle-cell allele demonstrates a striking ‘trade-off’ between benefit and cost. Individuals who carry just one sickle-cell allele have significantly greater resistance to malaria than individuals without any sickle-cell allele – a major advantage where malaria is so lethal, giving them a greater chance to stay healthy long enough to have children. But individuals who happen to inherit two sickle-cell alleles suffer the crippling disadvantage of contracting the disease, and are very unlikely to survive to have children themselves. So this ‘trade-off’ applies, not to an individual, but rather to a genetic population – conferring a major advantage to individuals lucky enough to inherit just one sickle allele, but punishing the individuals unlucky enough to inherit two sickle alleles.

 So which side of this trade-off wins out overall – the advantage to the sickle-cell carriers, or the disadvantage to the sickle-cell disease sufferers? An important factor concerns the probability of offspring genotypes: if two sickle carriers (already the recipients of a malarial advantage over non-carriers) have a child, that child is twice as likely to have a heterozygous carrier genotype as a homozygous disease genotype – so it is statistically twice as likely to inherit a malarial advantage than a sickle disease disadvantage. This 2:1 ratio of overall advantage to disadvantage helps to explain the spread of the sickle allele throughout the malarial regions of Africa.

Modern day malaria distribution

The sickle-cell allele must have appeared as a spontaneous mutation once, or at most in a few independent cases. The fact that an allele that carries such a heavy evolutionary disadvantage as full-blown sickle-cell anaemia has spread to become so prevalent across a huge region, testifies strongly that, in terms of natural selection, to be a sickle carrier confers a fitness benefit relative to non-carriers, and relative to the counter-balancing disadvantage of full sickle disease. This resolves the paradox of this essay’s title – natural selection can indeed increase the numbers of individuals who carry a serious evolutionary burden (sickle anaemia sufferers), if in the population as a whole their disadvantage is outweighed – in this case in a population ratio of 2:1 – by the advantage held by their more fortunate siblings (sickle carriers).

Article for OU Module S104 Exploring Science © Andrew Murray 2012

The Search for a Unified Theory

Scientists currently believe that there are just four fundamental interactions influencing matter and radiation – the electromagnetic, weak nuclear and strong nuclear forces, and gravity. One of science’s great quests is to unify all four into one fundamental force, such as may have existed at the extraordinarily high energy levels of the embryonic Universe. If we were to travel back in time to the very earliest moments after the Big Bang, with the energy levels of the Universe rising as we go, it’s believed that first we would experience the unification of the electromagnetic and weak interactions, then farther back the grand unification of the electroweak force with the strong nuclear force, then finally the superunification of these three with gravity.

Unified Theory 1
The energy levels of particles in the Universe have slowly declined since the Big Bang

A unified electroweak theory must encompass the three weak quanta, the W and Z bosons, which have mass, and the electromagnetic quantum, the photon, which is massless. Current thinking postulates the existence of four fields, one corresponding to each quantum: three of these fields give mass to the three weak quanta, but the fourth, instead of giving mass to the photon, will manifest as a particle – the much-hypothesised Higgs boson – with a mass energy between 100 and 1000 GeV. In other words, in this energy range the electromagnetic and weak forces will appear to be unified. The chance to prove or disprove electroweak unification is now within our reach, with the Large Hadron Collider at CERN and other similarly high-powered particle-smashers seeking to discover the elusive Higgs boson – or perhaps something more surprising instead!

The next force to be reconciled with the electroweak is the strong nuclear force. According to quantum electrodynamics, the strength of electromagnetic interactions increases slowly with increasing energies; according to quantum chromodynamics, the strength of strong interactions decreases with increasing energies. It is believed that at energies of around 1015 GeV the strengths of the two forces become comparable, and the electromagnetic and strong forces may achieve a grand unification. To achieve this unification another particle, termed the X boson, is needed, and it’s predicted that the X boson would allow quarks to change into leptons, matter to change into antimatter, and vice versa. We have never witnessed an X boson at work, because its activity would be extremely weak at the energies we can currently manufacture on Earth – it would get properly busy only at the tremendous energies around the 1015 GeV mark. These energies are currently beyond our means to recreate – but we may be able to glimpse the exotic processes that are hypothesised to occur in these conditions, such as the decay of protons, because they may occur, very occasionally, at the more mundane energies of the world around us today. Monitoring a huge enough sample of protons, say 1033, for a few years, may allow us to see a few proton decays.

View inside the detector of the Large Hadron Collider, CERN
View inside the detector of the Large Hadron Collider, CERN

Even if we are able to unify the electromagnetic, weak and strong forces, one final, perhaps most recalcitrant, force awaits unification – gravity. Gravity is the most difficult of the four forces to interpret at a quantum level, one great challenge being how we can take the fundamental quantum idea of uncertainty – uncertainty of position and movement of subatomic particles – and somehow apply that idea of uncertainty to space and time themselves. A popular current theory – or rather five versions of the same theory! – speculates that particles such as quarks, leptons and photons may exist as ‘strings’ rather than points – strings which exist in ten dimensions of space-time, whose vibration patterns across all these dimensions can provide all the characteristics needed by such particles, such as mass, colour charge and electric charge. The five string theories may be manifestations of a yet deeper theory, termed M-theory, which postulates objects called ‘branes’ – membranes which can exist in up to eleven dimensions. Strings and branes are still only theories: to experimentally probe superunification would require energy of 1019 GeV – termed the Planck energy – and we are a long way from being able to achieve this.

 So tools such as the Large Hadron Collider currently give us good hope of proving (or disproving) the existence of the Higgs boson, thereby gaining a much clearer insight into electroweak unification. Understanding grand unification is a tougher goal, though as mentioned above we may be able to gain insights from experiments that don’t require such exotic energies as grand unification itself. And as for superunification, it’s a noble summit for today’s theoreticians – and tomorrow’s experimenters – to seek to scale!

Andrew Murray 2012

Photos courtesy the Open University and Wikipedia Commons

Indonesian Primates at Risk

Analysis of the relative endangered statuses of a group of Indonesian primates

 

Our tutor group eliminated Tarsius dentatus and Eulemur coronatus as being at lower risk, so I have considered the three remaining species below, in terms of the vulnerability of the species (including IUCN status, population, geographical range and reasons for decline) and the phylogenetic value of the species (length of evolutionary isolation of genus, number of other species in genus, and vulnerability of these other species):

Golden lion tamarin

Leontopithecus rosalia: the golden lion tamarin’s IUCN Red List status is Endangered (IUCN Red List, 2012). Wild population stands at a mere 1000+, and human deforestation has cut its range to a fragmented 5000km2, but long-term conservation efforts have led the IUCN to downgrade its status from Critically Endangered. Its IUCN Population Trend is Stable.

The genus Leontopithecus has been isolated from others in the Parvorder Platyrrhini for an intermediate period of evolutionary time (Tree of Life Web Project, 2012). Of the other three species in Leontopithecus, two are classed as Endangered and one as Critically Endangered.

Celebes crested macaque

Macaca nigra: the Celebes crested macaque is listed as Critically Endangered on the IUCN Red List, with a Decreasing Population Trend. Discounting an introduced population elsewhere, in recent generations its indigenous numbers have been reduced by 80% due to human habitat encroachment and hunting (as a pest and for bushmeat), and are now estimated at 4000-6000 (Primate Info Net, 2012).

The genus Macaca is of high overall phylogenetic value, for it has evolved in isolation longer than any other genus within the family Cercopithecidae – but Macaca is the most widespread primate genus other than humans, and contains 22 species, of which 7 are considered of Least Concern or Near Threatened.

Sumatran orangutan

Pongo abelii: Our tutor group has identified the Sumatran orangutan as being of the highest priority for conservation. To compare it to the other two species:

Pongo abelii can be considered of greater conservation need than Leontopithecus rosalia because the IUCN give it a worse Red List Status (Critically Endangered) and a worse Population Trend (Decreasing). Its numbers have fallen by over 80% in the last 75 years, to around 7,300, and most individuals live in areas of high risk of logging and forest encroachment. Moreover, being highly arboreal, Sumatran orangutans seem even more vulnerable to habitat fragmentation than their more ground-venturing Bornean relatives.

The genus Pongo is of greater phylogenetic value than Leontopithecus because it has been isolated for longer, there are only two species within it (rather than four), and both are considered Critically Endangered (as opposed to three Endangered and one Critically Endangered).

Although it scores the same IUCN Red List Status and Population Trend as Macaca nigra, Pongo abelii can as a species be considered of higher risk because although the population figures quoted above are comparable, Pongo abelii’s native range is more fragmented than that of Macaca nigra. (Macaca nigra also has an introduced population elsewhere of 100,000 which could be called on as a breeding resource.)

Pongo abelii can also be considered of greater conservation need because of its higher phylogenetic value and risk. The genera Pongo and Macaca have both evolved in long isolation, but Pongo numbers but two species, both Critically Endangered and of severely depleted and decreasing habitat ranges (as opposed to Macaca with 22 species, several of low concern, and with a very large overall habitat range).

References:

IUCN Red List 2012, Leontopithecus rosalia, available from http://www.iucnredlist.org/apps/redlist/details/11506/0 (Accessed 18 February 2012)

IUCN Red List 2012, Macaca nigra, available from http://www.iucnredlist.org/apps/redlist/details/12556/0 (Accessed 18 February 2012)

IUCN Red List 2012, Pongo abelii, available from http://www.iucnredlist.org/apps/redlist/details/39780/0 (Accessed 18 February 2012)

Tree of Life Web Project 2012, Platyrrhini, available from http://tolweb.org/Platyrrhini/16292 (Accessed 18 February 2012)

Tree of Life Web Project 2012, Cercopithecidae, available from http://tolweb.org/Cercopithecidae/16297/1999.01.01 (Accessed 18 February 2012)

Tree of Life Web Project 2012, Hominidae, available from http://tolweb.org/Hominidae/16299 (Accessed 18 February 2012)

Primate Info Net 2012, Macaca nigra, available from http://pin.primate.wisc.edu/factsheets/entry/crested_black_macaque (Accessed 18 February 2012)

Article for OU Module S104 Exploring Science © Andrew Murray 2012