Author Archives: Eric Balke

It’s A Bird…It’s A Reptile…It’s A Mammal…It’s A Monotreme???

In my head, I am thinking of an animal which lays eggs, has venom and dives in lakes & ponds to catch prey. Oh, it also has fur and mammary glands. If you are a little confused right now, do not worry, you are not alone. When European settlers in Australia first brought back pelts and drawings of this curious creature to England, scientists thought it was a hoax!

Rest assured that this animal is real. I am talking about the platypus, Ornithorhynchus anatinus. Native to the eastern and southern coasts of Australia, this semi-aquatic mammal builds its burrows deep into the banks of the bodies of freshwater in which it spends about 15% of its time. About half a metre long, the platypus has one of the highest fur densities of any animal with up to 90,000 hairs per cm2 (sea otters take the top spot with over 150,000/cm2!).

Platypus on display. Picture by TwoWings.

The three extant (i.e., still living) mammalian groups are the eutherians (e.g., humans), marsupials (e.g., kangaroos), and monotremes. Platypuses, along with echidnas are the only living members of the monotreme order. This group diverged from the last common mammalian ancestor about 166 million years ago, thus explaining the bizarre origins of this seemingly paradoxical egg-laying mammal.

As mentioned earlier, platypuses (or platypodes) forage for aquatic invertebrates on the bottom of lakes and streams. When it dives under water it closes its eyes, plugs its ears and does not use olfaction to detect its prey. You might think that it has run out of ways to find food, but that is definitely not the case here!

Platypus underwater. Image from LearnAnimals.com

The skin on the bill of the platypus is highly specialized, containing at least two cutaneous receptors which aid in the location of prey. There are over 45,000 push-rod mechanoreceptors spread almost uniformly over the four surfaces of the bill. These receptors give the bill skin a sense of touch which is almost as sensitive as that on the tips of human fingers. More interestingly, platypus bill skin has over 38,000 sensory mucous gland electroreceptors. That’s right, electroreceptors!

These electroreceptors are so well-tuned that they can detect electrical signals from the muscle contractions of their invertebrate prey in the water. These electroreceptors are arranged in rostro-caudal stripes along the top and bottom bill surfaces (i.e., from tip to base of bill). It is thought that this striping pattern helps the platypus to pinpoint the exact location of its prey. By feeling the environment with the mechanoreceptors and detecting prey with the electroreceptors the platypus is very good at finding food without the use of sight, sound or taste.

When the platypus comes out of the water it is able to close these electroreceptor glands to prevent desiccation of mucous glands which are crucial to the function of these receptors. Platypuses are one of the few electroreceptive animals which have no specialized sensory cell to receive the incoming electrical stimulus. Instead, the electrical stimuli from prey are conducted through mucous and low resistance cells in an epidermal invagination to a bare nerve terminal.

As you can see, the electroreceptive ability of the platypus enables it to find food, therefore is crucial to the survival of this very bizarre animal.

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Sources:

Grant, T.; Dawson, T. J. Physiol. Zool. 1978, 315-332.

Grant, T.; Temple-Smith, P. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 1998, 353, 1081-1091.

Klamt, M.; Thompson, R.; Davis, J. Global Change Biol. 2011, 17, 3011-3018.

Manger, P. R.; Pettigrew, J. D. Philosophical Transactions: Biological Sciences 1995, 359-381.

Manger, P.; Pettigrew, J. Brain Behavior and Evolution 1996, 48, 27-54.

Warren, W. C., et al Nature 2008, 453, 175-U1.

Platypus Parts. YouTube video. Accessed 12 February 2012.

Great Bowerbirds and Great Illusions

Are you having a difficult time finding the right girl for you? Have you ever been excessively concerned that she will not approve of how you organize all of your belongings? Well, maybe it is time you took a page out of the courting-book of a pro!

Greater Bowerbird. Image by Laura Kelley.

Bowerbirds belong to the genus Ptilonorhynchidae, which consists of 8 genera and 20 species found in different parts of Indonesia, Papua New Guinea and Australia. What sets these birds apart are the elaborate courtship displays of the male. Just check out this video of the Vogelkop Bowerbird from the BBC nature documentary series, Life.

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If you thought that was remarkable, just wait until you hear about the Greater Bowerbird of northern Australia! The male creates two parallel walls of sticks which form a bower with an avenue between the walls. The avenue opens up to a collection of grey stones, shells and bones called the court or gesso. This extravagant structure has no use but to attract potential mates. Using elaborate vocal mimicry, a male entices females to inspect his bower. Upon the arrival of a female in the centre of the bower avenue, the male stands in the court just out of the female’s field of view. The male then displays to her a variety of colourful objects and the colourful patch of feathers on the crest of his head. If the female is sufficiently impressed, mating occurs shortly thereafter.

Bower with avenue opening into the court. Image by Laura Kelley.

The Greater Bowerbird stands apart from its bowerbird relatives because of how it arranges objects in its court. In 2010, researchers at Deakin University looked at the geometry of the Greater Bowerbird court. They discovered that males are very particular with the selection and location of objects in the court. Males create a size gradient of objects in the court by placing smaller objects close to the female’s location in the avenue and increasing the size of the objects placed further away. From the female’s position in the avenue an illusion called forced perspective is created where the pattern in the court appears to be more even than it otherwise would appear.

Measuring the characteristics of the court. Note the progression in size of the objects, small to large from right to left. Image by Laura Kelley.

On January 20th another study was published which found that this illusion impacts mating success! Males who create a court with a higher quality gradient gain more mates than males who make a poorer quality gradient. When observed from the female’s perspective, males with the most evenly patterned courts have the greatest number of mates.

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The authors suggest several reasons to explain this phenomenon, but what I find most interesting are the implications. Many animals, especially birds, are very perceptive. There are countless species which perform elaborate courtship rituals. It is very possible that similar forced perspective illusions are used by other species. We may have barely scratched the surface on this one!

Sources:

Anderson, B. L. Bird-Brained Illusionists. Science 335, 292-293 (2012).

Davies, E. Male bowerbirds ‘benefit from optical illusions’. BBC Nature online. Accessed January 20, 2012.

Endler, J. A., Endler, L. C. & Doerr, N. R. Great bowerbirds create theaters with forced perspective when seen by their audience. Current biology 20, 1679-1684 (2010).

Kelley, L. A. & Endler, J. A. Illusions Promote Mating Success in Great Bowerbirds. Science 335, 335-338 (2012).

Visual illusion the key to Bowerbirds mating success. Youtube video. Accessed January 20, 2012.