Can animals sense magnetic fields? The degree to which animals can sense magnetic fields unclear European Robins (Erithacus rubecula) Tend to migrate in a particular direction Preferred flight direction is maintained in a closed room with no optical references – implies that vision is not necessary If test cage is moved to all-steel chamber (a shield against magnetic fields), bird no longer finds natural migratory direction Artificially generated magnetic fields can influence the direction choice of robins 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 field Allows 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 could be affected by earths magnetic field Certain cells in their abdomen contain iron granules ~ 0.6 micrometers in diameter These cells (trophocytes) are innervated by axons from the ventral nerve 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 on which they were born The migratory routes of turtles have unique combinations of magnetic field 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 position using a bi-coordinate map
Vision and Lateral inhibition: Lateral inhibition: The horseshoe crab (Limulus) has compound eye in which individual receptor units can be stimulated by a 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 other receptor units Why? Because each visual receptor is connected to its neighbors and Inhibits their activity = lateral inhibition
Potential advantage of lateral inhibition? Enhanced edge effects Consider 12 units that are stimulated by uniform light of two intensities (bright and dim) Because units 2-5 are bordered by other units getting bright Illumination, they are subject to lateral inhibition Units 6 and unit 7 are not Thus, their discharge frequency is higher =better picture of edge
Information processing: Ex. The eye of a frog Rods and cones are distributed uniformly such that the structure of the retina is similar from place to place Within the frog eye, there are neurons called ganglion cells. The axons of these ganglions make up the optic nerve There are about 500,000 ganglion cells, but there are about 1 million receptor cells (rods and cones) in the retina Thus, there cannot be a 1:1 correspondence between the incoming and outgoing messages Fibers in the optic nerve are divided into five different classes on the basis 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 its termination Edge receptors: Respond to the presence of a sharp edge in the visual field “Bug detectors”: Respond only to small, dark, moving objects But 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 thought that we dropped this!
Within human females, college roommates tend to develop synchronized Menstrual cycles (pheremones?) If you apply sweat from a donor to a subject’s lip, the subjects Menstrual 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 “Driver Hormones”
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 FaceOriginal Female FaceAsymmetric Female Face Symmetric Male FaceOriginal Male FaceAsymmetric Male Face How do researchers study facial symmetry? Digital manipulation!
Composite of 60 Faces Composite of 15 Most Attractive Faces Hyper-Attractive Composite Facial attractiveness researchers also create “composite” faces
Femininised Female Face Masculinised Female Face Femininised Male Face Masculinised Male 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 onto The female
Men may use pheromones too… Evaluation of the subject as sexy Body odor
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 cultures and 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 is 70% 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 America Or Africa
Things to consider In many culture, men always have beards-but this has fallen out of 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?