Aarhus University Seal / Aarhus Universitets segl

Zoophysiology - Theses and projects

Hearing of animals without ears

Christian Bech Christensen, Ph.d. student.

Sound consists of both a pressure component and a particle motion component of the medium in which it travels. An animal must therefore be able to detect either the pressure or the particle motion in order to hear a sound. Animals solve this task in different ways. In water animals often detect  the particle motion, though for example fish with a swim bladder are able to detect both particle motion and pressure. I air it is on the contrary often the pressure component from at sound source that is detected. Thus, it requires an adaptation for animals in water to hear sound in air. The most common adaptation to aerial hearing is a tympanum and middle ear bones. The sound pressure sets the tympanum in vibrations, which is conveyed through the middle ear bones to the inner ear fluid. In the inner ear, these vibrations deflect hair cells which sends signal to the brain.

Is it possible for animals, as snakes, that lack an outer ear and a tympanum to hear sound?

My project concerns whether snakes of the genus royal python (Python regius) are able to detect the sound pressure per se or whether it is the sound induced vibrations that they detect. Snakes cannot be trained to respond to a sound, and therefore we measure the auditory brainstem response. By inserting three electrodes subcutaneously on the head of the snakes, it is possible to measure the neural signals and thereby find out whether the snake can detect the sound or not. By comparing the head vibrations induced by sound and vibration, it is possible to find out if they can detect the pressure component.

The project shows that snakes in spite of the lacking tympanum and outer ears are able to hear loud sounds. But it also shows that it is not the pressure component per se they detect, but rather the sound induced head vibrations.

Python Regius - Ball Python

Royal python on shaker. Vibration detection is measured by measuring the auditory brainstem response. Head vibrations are measured with a laser.

Osmoregulation in the Asian swamp eel: Effects of increased salinity on plasma osmolality and survival

Biological project in the Mekong delta in Vietnam by Pil Pedersen and Kasper Hansen. Superviser: Tobias Wang.

The swamp eel, Monopterus albus, is an air-breathing teleost with highly reduced gills. The Asian swamp eel is normally considered a freshwater species, but can also occasionally be found in brackish water, making their physiology of osmoregulation interesting because the gills normally represent the primary site of osmoregulation in teleosts. Being a popular fish for human consumption, Monopterus is cultured extensively in freshwater ponds in the Mekong Delta of Vietnam, contributing to the strong competition between aquaculture and rice production. Very little is known about the salinity tolerance of Monopterus. We therefore studied how increased salinity affects survival, blood osmolality, hematocrit and plasma ions. All eels survived prolonged exposure to 5 and 10 ppt, although plasma osmolality increased at 10ppt. Further elevation to 15 and 20ppt was associated with significantly elevated mortality, with a corresponding increase in plasma osmolality and ion concentrations. Elevated drinking rates in hyposmotic or haemorrhaged swamp eels, indicate that drinking is activated primarily by baroreceptors. Our results show that swamp eels thrive in 5ppt, but that this species is unable to adapt to further increases in salinity.

Rice Eel

The Asian swamp eel is an air-breathing fish with very reduced gills. It is a popular fish for human consumption.

The cardiovascular system of reptiles

Bjarke Jensen, Ph.d.-student

Reptiles may be viewed as an intermediate evolutionary stage between our ancestors, fish, and the higher vertebrates, mammals and birds. So when we describe the genetics, anatomy and physiology of the reptile heart we obtain insights that may not be limited to reptiles but to general trends in cardiac evolution as well. We have described the anatomy of a typical reptile heart (in the South American rattlesnake) and the anatomical foundation of the atypical blood pressures of pythons (which are more mammalian-like than most other reptiles). In collaboration with cardiologists and physicists from Skejby hospital (Denmark) we visualize intracardiac blood streams with echocardiography and MR-scans and since the reptilian heart is not completely divided as in mammals and birds the blood streams may mix and have complicated routes. At the Heart Failure Research Center at Universiteit van Amsterdam we stain cardiac gene expressions in lizard embryos and from histologic sections we are thus able to make digital 3D models describing development of cardiac anatomy and the underlying genetics.

Lizard-heart

model af firbenhjertet halvvejs igennem fosterudviklingen
Lizard-heart

Regeneration of Tissue

Henrik Lauridsen, masterstudent

Mammals, including humans, possess a very limited ability to regenerate lost limbs and organs. Other vertebrates, however, can restore amputated limbs and body structures to perfection. This applies in particular to a Mexican salamander, the axolotl (Ambystoma mexicanum), being able to regenerate an entire new limb within a few months after amputation. This ability is interesting from a medical perspective, and by understanding the mechanisms behind tissue regeneration in the axolotl, it is possible that future regenerative treatments in humans can be developed.

Previous studies of the regenerative phenomenon in the axolotl have mainly depended on histological and molecular techniques. By these means, a fundamental understanding of the processes involved in amphibian regeneration has been achieved. However, both histological and molecular studies on tissue regeneration are highly invasive, relying on the removal of a newly regenerated limb at different times in the regeneration process. There exists currently no non-invasive imaging technique that can monitor the process of regeneration in the axolotl or other animals. Therefore, this study investigates whether magnetic resonance imaging (MRI) is a suitable modality for this monitoring purpose. The hypothesis is that a functioning MRI methodology for visualisation of the regenerative process in the axolotl and other model animals could be transferred and used for monitoring in future clinical regenerative therapies in humans.

Axolotl

Axolotl kan regenerere mistede lemmer
The Axolotl can regenerate lost limbs.

Metabolic depression during long term starvation and nutrient selection in the following meal in the Giant Brazilian White Knee Tarantula Acanthoscurria geniculata.

Peter Skødt Knudsen, Masterstudent

Many spiders posses the ability to cope with long periods without food. Hence they are good objects for studying the physiological machinery that supports a high capacity to deal with long term starvation and recovery upon the next meal. Such adaptations might have been evolved as a response to living in an environment with great fluctuations in the abundance of food and other resources.

Spiders differ from other sit-and-wait predators like constrictor snakes and crocodiles by their unique feeding behavior labelled as extra-oral digestion where the food is catabolized outside their own body. In what way this interesting feature affects the gastrointestinal system during starvation and recovery is not known. The question is if the spiders are forced to keep the organs up-regulated to always maintain a sufficient level of digestive juice ready to pour over the following meal or if the can shut the system down to save energy.   

These issues are here investigated by multiple approaches using different perspectives and methods. In this experiment I work with the following four ways to gain insight into the mechanisms that allow these Brazilian theraposidaes to exhibit a successful sit-and-wait hunting strategy:

1. Respirometry                  
By respirometry it is possible to follow the spiders’ consumption-rate of oxygen and the excretion-rate of carbon dioxide, and thereby establish a curve that shows how the metabolism is depressed during starvation. It is also a powerful tool for measuring the feeding response (SDA response) where oxygen uptake might be elevated by as much as a factor of 8.

2. MR-imaging                   
MRi-technique gives a non-invasive opportunity to determine the level of atrophy (cell shrinkage) if the midgut mesenteron and diverticulas are down-regulated and how fast these organs are up-regulated again.

3. Digestive enzyme analysis                   
Analysis of the arsenal of digestive enzymes reveals if the spiders prepare for their next meal in a nutrient specific way. This idea has so far never been addressed.

4. Measurements of nutrient compositions in prey and spider               
If the spiders are capable of composing a mixture of digestive enzymes that matches their demand, it should also be seen when the nutrient composition of the spider and the prey is estimated before and measured after a meal.    

Acanthoscurria geniculata

Giant Brazilian White Knee Tarantula

Acanthoscurria geniculata