Tag Archives: Research stars

Whales, dolphins and porpoises – sleeping with half a brain

Every wondered how and where whales, dolphins and porpoises sleep?

New work by Gateway Antarctica’s Andrew Wright released this week reveals, for the first time, sleeping during diving in harbour porpoises. Part of his PhD work in Denmark before coming to the University of Canterbury, Andrew attached behavioural loggers to porpoises and found a new type of dive in the data obtained. The dives are slow, low energy and low in echolocation clicks – the biosonar that porpoises use to find food.

Cetaceans (whales, dolphins and porpoises), sleep with only half their brain at a time because they spend their lives underwater and must return to the surface to breathe. This unusual behaviour is also seen in many migrating birds that sleep on the wing. Yet life underwater means that we know little about sleeping in wild cetaceans. Applying behavioural criteria for sleep developed in terrestrial mammals to behavioural data from tags, Andrew identified a roughly semi-circular dive form that measured up. Stereotypical in not only dive shape, but also the swimming movements throughout the dive, the dives are typically quiet. This discovery means raises the possibility that animals sleeping at depth might be more susceptible to becoming entangled in fishing nets set at those depths because they are not echolocating.

The work raises some interesting possibilities for resolving the conflict between fishermen and cetaceans around the world, including New Zealand’s own Maui dolphin. For example, it may be possible to reduce entanglement rates if fishermen can avoid setting nets at the depths that the porpoises and dolphins sleep at.

“Although the dives make up less than 10% of all the activities for each animal, even small reductions in fisheries bycatch can make a big difference to the long-term survival of many endangered cetacean species,” notes Andrew.

However, the finding also has implications for scientists themselves. The use of passive acoustic monitoring technologies are becoming commonplace. Detecting marine mammal sounds as the whales and dolphins swim past, such devices were thought to be able to detect all porpoises as they were believed to produce clicks at all times. However, the existence of quiet dives means than not all animals will necessarily be detected. This means the finding also has implications for industries relying upon passive acoustic monitoring to protect marine mammals from harmful effects, such as the oil and gas industry.

Thesis in Three presentation: Jess McHale, Cascading Hazards

Jess McHale (Science) explains her research on Cascading Hazards – the idea that one hazard triggers another, which triggers another. Watch her presentation that won her third place at the UC Thesis in Three finals in August. (video: 3 min 08 seconds)

Each year, the Dean of Postgraduate Research at UC sponsors and organises the Thesis in Three competition for postgraduate students. PhD and Masters students give a three-minute presentation to describe their thesis research with only a single presentation slide. The top three students from each college round go on to compete in the UC final.

Dark energy –  UC supernova analysis  reframes debate

The accelerating expansion of the Universe may not be real, but could just be an apparent effect, according to new research published in the journal Monthly Notices of the Royal Astronomical Society.

The new study—by a group at the University of Canterbury in Christchurch, New Zealand—finds the fit of Type Ia supernovae to a model universe with no dark energy to be very slightly better than the fit to the standard dark energy model.

Dark energy is usually assumed to form roughly 70% of the present material content of the Universe. However, this mysterious quantity is essentially a place-holder for unknown physics.

Current models of the Universe require this dark energy term to explain the observed acceleration in the rate at which the Universe is expanding. Scientists base this conclusion on measurements of the distances to supernova explosions in distant galaxies, which appear to be farther away than they should be if the Universe’s expansion were not accelerating.

However, just how statistically significant this signature of cosmic acceleration is has been hotly debated in the past year. The previous debate pitted the standard Lambda Cold Dark Matter (ΛCDM) cosmology against an empty universe whose expansion neither accelerates nor decelerates. Both of these models though assume a simplified 100 year old cosmic expansion law—Friedmann’s equation.

Friedmann’s equation assumes an expansion identical to that of a featureless soup, with no complicating structure. However, the present Universe actually contains a complex cosmic web of galaxy clusters in sheets and filaments that surround and thread vast empty voids.

Professor David Wiltshire, who led the study from the University of Canterbury in Christchurch, said, ”The past debate missed an essential point; if dark energy does not exist then a likely alternative is that the average expansion law does not follow Friedmann’s equation.”

Rather than comparing the standard ΛCDM cosmological model with an empty universe, the new study compares the fit of supernova data in ΛCDM to a different model, called the ’timescape cosmology’. This has no dark energy. Instead, clocks carried by observers in galaxies differ from the clock that best describes average expansion once the lumpiness of structure in the Universe becomes significant. Whether or not one infers accelerating expansion then depends crucially on the clock used.

The timescape cosmology was found to give a slightly better fit to the largest supernova data catalogue than the ΛCDM cosmology. Unfortunately the statistical evidence is not yet strong enough to rule definitively in favour of one model or the other, but future missions such as the European Space Agency’s Euclid satellite will have the power to distinguish between the standard cosmology and other models, and help scientists to decide whether dark energy is real or not.

Deciding that not only requires more data, but also better understanding  properties of supernovae which currently limit the precision with which they can be used to measure distances. On that score, the new study shows significant unexpected effects which are missed if only one expansion law is applied. Consequently, even as a toy model the timescape cosmology provides a powerful tool to test our current understanding, and casts new light on our most profound cosmic questions.