Sunday, November 30, 2014

A Blue Colored Grey Tree Frog

The blue frog pictured above is a Grey Tree Frog, Hyla versicolor or H. chrysocelis.  They are not usually blue (but they are not always grey either), but develop a bluish cast when they freeze for their winter hibernation.  This particular one is really vibrant, the man who first found it on a sidewalk thought it was a toy at first glance. 

They are a lot of insects that hibernate in a frozen or partially frozen state, but not many vertebrates, all of them terrestrial frogs (that I know of).  Not only is freezing a seemingly death defying feat, but the adaptations employed to avoid permanent damage would kill most other animals.

There's a really excellent article written by Janet M. Storey at NatureNorth that explains the process in detail and in easy to understand language.  There's a section on freeze tolerance in frogs in the book "Life in the Cold" by Peter J. Marchand (read it if you are at all interested in how animals and plants survive winter, you'll be more smarter if you do).  So I'm not going to explain the process in detail, just outline some key points, and . . . an animated version of freezing in Grey Tree Frogs!
  • Grey Tree Frogs hibernate under leaves,  which offer very little protection from cold and ice.
  • The changes in a Grey Tree Frog's body only begin when ice actually starts to form in the frogs body.  Most animals anticipate the onset of freezing temperatures through environmental cues by experiencing changes in their physiology and/ or behavior.
  • Freezing is limited to spaces outside of cells, leaving organs and tissues intact when the frog thaws in the spring.
  • When ice starts to form, the liver starts producing glycerol; frogs use glucose, I'm not clear if Grey Tree Frogs use both or just glycerol.  The glycerol aids in possibly three ways: by lowering the freezing point of cells, by strengthening cell membranes, and by reducing cellular dehydration.  The first two help prevent the structural damage that can be caused by ice (think frost bite), the third helps with the fact that with all he water frozen outside of the cell, the water in the cell will flow "downhill", (osmotically speaking).
  • The formation of ice also starts he heart beating faster, which seems counter intuitive; Grey Tree Frogs are cold-blooded so their metabolism should slow down with lowering temperatures.  This would be true if the frog was just experiencing a temperature decrease, but it is also freezing.  When a liquid turns into a solid, there is a release of heat, it's not a lot, but with a Grey Tree Frogs small body it is enough to get the heart pumping faster than normal.  This allows the glycerin to be distributed quickly in the frog's body.
  • When about 60% of the frog's body is frozen (up to 20 hours later), the heart stops pumping, and the little metabolism that occurs does so without oxygen.  The Grey TreeFrog will remain this way until temperatures rise above freezing, at which point it will thaw out and shortly resume normal activities.
(note: the video my not appear on mobile devices.  You can try this link if that's the case)







Friday, November 28, 2014

An Illustrated Life List: Tundra Swans


My favorite spot to watch for Tundra Swans, Cygnus columbianus may seem a little unusual; it's the NE Minneapolis neighborhood I live.  Every year I usually hear and/ or see one flock go by, usually at night, and more often in the spring.  It's one of my favorite signs marking the passage of the seasons.  Seeing them at night gives them a somewhat ghostly or other worldly cast. The passing flock's calls sound wild and out of place in the city.

I don't think I've ever seen Tundra Swans on the ground or swimming in Minneapolis, just migrating past.  But I can be a pretty lazy birder, maybe a swan or two that I have identified as a Trumpeter Swan, C. buccinator has actually been a Tundra Swan - they can be difficult to tell apart.  If I see a flock of swans in the spring or fall I call them Tundra Swans (they usually give themselves away by their call).  If I see a pair, anytime of the year, I call them Trumpeter Swans.  I did a little research on the eBird site to see if this generalization is supported by the observations of others.  I compared year-round sightings of the two swan species in Hennepin County.  The data seems to support my generalization: Trumpeter Swans are seen more often, in smaller numbers, and throughout the year.  Tundra Swan sightings are most likely in the spring and fall and in larger groups.  Of course this is a simplification of the data presented at eBird, but it I think it gives some credibility to my lazy swan identification.

Visit the Distracted Naturalist Red Bubble site for more Illustrated Life List artwork

Thursday, November 20, 2014

Dark Fishing Spider Exoskeleton

 

I found this Dark Fishing Spider (Dolomedes tenebrosus) exoskeleton under a sink a few weeks ago.  Spiders have to periodically shed their exoskeletons, the hard outer layer of their body as they grow. 

I think the exoskeleton gives an interesting perspective on how a spider's body is arranged.  Below, a short discussion of a spiders various parts, color coded for your convenience.

Abdomen
One of the two main sections of a spider’s body.  It’s where the most of the spider’s internal organs are located.  The exoskeleton of the abdomen is pretty soft and flexible and isn’t well preserved.


Cephalothorax
The other main section of a spider’s body.  I’m amazed at how well many details have been preserved in the shed exoskeleton.  It’s seems like shedding an exoskeleton should be a traumatic experience with only broken remnants left behind, but from the exoskeleton in the photograph, it appears the spider just unhooked a little latch, lifted its carapace up, and skipped away.  The carapace is the top portion of the cephalothorax, the underside is called the sternum.


(Maybe a future post on how the molting actually proceeds).

Eyes
Spiders have up to eight eyes.  The number, size, shape, and arrangement of the eyes can be a good clue to the spider’s family.  Spider eyes are labeled as follows.

PLE       Posterior Lateral Eyes
PME      Posterior Median Eyes
ALE      Anterior Lateral Eyes
AME     Anerior Median Eyes

Fishing Spiders are in the family Pisauridae.  This family of spiders generally has eyes that are arranged in two rows of four, with PME eyes that are slightly larger.  The P (posterior) row curve back.  Fishing Spiders are often mistaken for Wolf Spiders (family Lycosidae), but Wolf Spiders have three rows of eyes and noticeably larger PME eyes. 


Chelicerae
Often known as a spider's jaws.  In the photograph, they are easy to spot because they are the darkest part of the exoskeleton.  Like most spiders, a Fishing Spider's chelicerae move side to side, like a pair of scissors.  They hold prey and guide it to the small mouth parts located behind the chelicerae (not visible)  The tips of the chelicerae have fangs that are used to inject venom into prey.  They do not suck the fluid of their prey from their fangs.


Pedipalps
These are the leg like appendages on either side of the jaws.  The base of the pedipalps is actually part of the spider’s mouth structure and may aid in holding or crushing prey.  Male pedipalps are enlarged at the ends and used during mating to transfer sperm to the females.  Sometimes mention is made of the pedipalps being used for sensing too.