Senosry systems II: Perception of magnetic fields

Amazing animal migrations

Can animals sense magnetic fields?

The degree to which animals can sensemagnetic fields unclear

European Robins (Erithacus rubecula)Tend to migrate in a particular direction

Preferred flight direction is maintained in a closed room withno optical references – implies that vision is not necessary

If test cage is moved to all-steel chamber (a shieldagainst magnetic fields), bird no longer finds natural migratory direction

Artificially generated magnetic fields can influence the direction choice ofrobins

Other organisms shown to be affected by magnetic fields:Marine mollusks, Salmon, Salamanders, Turtles, Hornets, Honeybees

Puzzle: The geomagnetic field is weak (ca. 0.5 Gauss). Three possible ways that animals can detect magnetic fields

Mechanical Reception – Same as a compass needle – requires the presence of ferromagnets (small magnetic particles in body)

Electric Induction – Any kind of movement in an induced electric fieldAllows animals to follow magnetic lines; However, electro-sensitive organs not found in all animals who can use magnetoreception

Chemical Reception – Chemical reactions inside body can be influenced by magnetic fields – poorly understood

Honeybees = carry discrete bodies of ferromagnetic material that couldbe affected by earths magnetic field

Certain cells in their abdomen contain iron granules ~ 0.6 micrometersin diameter

These cells (trophocytes) are innervated by axons from the ventralnerve Cord

Mechanism poorly understood

Newts: Can migrate up to five km to and from their natal ponds (and this is after being in a forest for five years)

How? Photoreceptor’s (light sensitive regions in the brain) facilitated migration

Sensory vision with a brightness pattern imposed (due to magnetic field)

Example (Quote from researchers):

"If the newt is pointing north, the newts' photoreceptors would align north-south and would show an excitation response, whereas if the newt points itself to the east it would still show an increased pattern but now the pattern will have rotated 90 degrees because the newt has now rotated,"

Sea turtles migrate for thousands of kilometers to the same beach onwhich they were born

The migratory routes of turtles have unique combinations of magneticfield intensity and field line inclination

Hatchling loggerhead turtles can distinguish between different field intensities and magnetic inclination angles along routes

Turtles thus possess the ability to determine their global positionusing a bi-coordinate map

Vision and sorting of information

Vision and Lateral inhibition:

Lateral inhibition:The horseshoe crab (Limulus) hascompound eye in which individualreceptor units can be stimulated bya fine beam of light

However, the signals in the axon do not completely represent the stimulus

The pattern also depends on the amount of light falling on otherreceptor units

Why? Because each visual receptor is connected to its neighbors andInhibits their activity = lateral inhibition

Potential advantage oflateral inhibition?

Enhanced edge effects

Consider 12 units that are stimulated by uniform lightof two intensities (bright and dim)

Because units 2-5 are borderedby other units getting brightIllumination, they are subject tolateral inhibition

Units 6 and unit 7 are not

Thus, their discharge frequencyis higher

=better picture of edge

Information processing:

Ex. The eye of a frog

Rods and cones are distributed uniformly such that the structure of theretina is similar from place to place

Within the frog eye, there are neurons called ganglion cells. The axonsof these ganglions make up the optic nerve

There are about 500,000 ganglion cells, but there are about 1 millionreceptor cells (rods and cones) in the retina

Thus, there cannot be a 1:1 correspondence between the incoming andoutgoing messages

Fibers in the optic nerve are divided into five different classes on thebasis of the responses of the ganglion cells

On fibers: Respond to the onset of illumination

Off fibers: Respond to only the termination of a light stimulus

ON-off fibers: Respond to either onset or offset of a light stimulus or itstermination

Edge receptors: Respond to the presence of a sharp edge in the visualfield

“Bug detectors”: Respond only to small, dark, moving objectsBut NOT to large dark moving objects or to stationary objects

In fact, switching a light on and off will not affect these final fibers

The retina of the frog contains a matrix of many kinds of specialized fibers,each of which is beneficial for a different purpose

Recent article in Time magazine “The Science of Romance”

Could romance be driven by our senses?

The idea of “love” as a biological phenomenon to promote mating and nurturing of children

May go all the way back to early human culture

Hamsters and pheromones-dictates sexual behavior

Animal senses-A guide to humans?

OK, I thoughtthat we droppedthis!

Within human females, college roommates tend to develop synchronizedMenstrual cycles (pheremones?)

If you apply sweat from a donor to a subject’s lip, the subjectsMenstrual cycle started an average of 9 days away from donor

About 4 months later: the difference was only an average of 3 days

No change in the control group

“DriverHormones”

Pheromones may be linked to MHC genes

Strongly influence the immune system and tissue rejection

Idea: MHC matching

-Conceive a child with a person whose MHC is too similar to yours, and risk increases that the womb will expel the fetus

Find a partner with sufficiently different MHC, and you're likelier to carry a baby to term.

-Lab mice can smell “too-similar” MHC in the urine of other mice

-Will avoid mating with those individuals

Human females smelled T shirts worn by anonymous males and then pick those that were most appealing

They chose the ones worn by men with a safely different MHC

MHC information may be contained within pheremones

Confounding factor: birth control pill

Women who are on the Pill-which “simulates” pregnancy-tend to choose wrong in the T-shirt test

When they discontinue the daily hormone dose, the protective smell mechanism kicks back in

Pheromones may be a “natural” mechanism of partner selection-pregnancy may interfere with this

Preference also relates to faces

Just before, during and after period - when women are least likely to become pregnant - more attracted to more feminine male faces

If woman is on the pill, no preference is shown through cycle

                                             

                                                                    Symmetric Female Face Original Female Face Asymmetric Female Face

Symmetric Male Face Original Male Face Asymmetric Male Face

How do researchers study facial symmetry? Digital manipulation!

                      

Composite of60 Faces

Composite of 15 MostAttractive Faces

Hyper-AttractiveComposite

Facial attractiveness researchers also create “composite” faces

                      

FemininisedFemale Face

MasculinisedFemale Face

FemininisedMale Face

MasculinisedMale Face

Researchers can also make more “male” and “female” faces

Saliva also contains MHC genes

Kissing may be a “taste test” for future compatibility

Testosterone is also present in male saliva-can be passed ontoThe female

Men may use pheromones too…

Evaluation of the subject as sexy

Bo

dy

od

or

What IS attractive?

Deep voice in males (auditory signal)

Research on the Hadza tribesmen found that the richer and lower a man's voice, the more children he had

Sample group of 149 volunteers listened to recordings of men's and women's voices

Ranked them from "very unattractive" to "very attractive."

Voices that scored high on attractiveness also had physical features considered sexually appealing

Ideas of beauty change among culturesand across time

Waist-hip ratio in women

Scientists have claimed that a certain waist-hip ratio is preferable Across cultures

Europe: Women with 0.7 WHR (waist circumference that is70% of hip circumference) are rated more attractive

Marilyn Monroe, Sophia Loren and the Venus de Milo: ratios ~ 0.7

Preferences range from ~ 0.6 in China to 0.8 or 0.9 in South AmericaOr Africa

Things to consider

In many culture, men always have beards-but this has fallen outof favor in modern culture

The male beard – a sexually dimorphic trait

Common in some cultures, but increasingly more rare in ours

Consider all the machinations that men take to get a hairless “attractive”body

Ouch!

There may not be one universal “best qualities” at any one time

If true - then what are pheromones signalling?