Monarch misconceptions? An entomological assessment of a popular press news article. BIO 3323 Dr. Jon Houseman February 9 th 2004 University of Ottawa
In the article Butterflies face climate threat, Julianna Kettlewell from BBC News Online reports that monarch butterflies (Danaus plexippus) may lose their current habitat throughout the winter within 50 years due to increased rainfall. This article was analyzed from a scientific perspective by examining the facts presented and comparing them with the results and discussions published in several scholarly periodicals in order to check the validity of the news story. The Monarch s overwintering cycle The article briefly mentions that monarchs have a complex life cycle because of their great feat of travelling enormous distances during migration. In North America there are two populations of monarchs; one breeds in the east and migrates to central Mexico, the other breeds in the west and migrates to California s coasts (Kane 1999). Monarchs migrate to small sites in the Transverse Neovolcanic Belt of the Oyamel forests in central Mexico; however, only the eastern North American monarch butterflies migrate to this location (Hartzler 2000). The author fails to discuss the western monarchs that migrate to California and misdirects the reader into believing that all monarchs migrate from the Rocky Mountains to central Mexico. The Monarch s feeding Kettlewell states that Monarchs from North America feed on milkweed during the summer [and] in the winter they must migrate to somewhere cool enough to keep their metabolism low because there is no milkweed for them to
eat (Kettlewell 2003). This implies that only North American Monarch butterflies eat milkweed and misleads the reader into thinking that adult monarchs feed on it. Larval monarchs feed solely on many Asclepias species such as common milkweed, whereas adults feed on a variety of flower nectar (Hartzler 2000). Due to the lack of nectar sources in the winter, in order to survive, monarchs have a thermoregulatory behaviour that involves resting, basking, shivering, and flying to water which requires at total of 90 days of lipid reserves (Masters 1988). The cool, high mountain, Mexican forests; therefore, provide sufficient conditions for monarchs to live off the conserved lipids (Masters 1988). The Monarch s migration How do Monarch butterflies navigate? The article is correct in stating that Monarch butterflies navigate south not only by using the Sun. Monarch butterflies use the Sun on clear days as a compass by directional orientation (Perez 1997) as well as using polarized light to aid in navigation; however, on overcast days Monarch butterflies still puzzlingly orient south during autumn migration (Perez 1999). A sun compass; therefore, is only answer part of the answer. Monarch butterflies must have some other mechanism of orientation such as a geomagnetic compass when the sun is completely absent (Perez 1997). Experimentation of the butterflies sensitivity to the sun has been conducted by Perez by looking at the flight patterns of subjects exposed to a strong magnetic field and comparing with a control. The treated butterflies
were completely disoriented by the magnetic field and the control group maintained their southwest destination. This was the first study that demonstrated that magnetic field perception was incorporated into the navigation system of the Monarch butterfly. One theory of this is the use of magnetite particles present in the butterfly s thorax which acts as compass needles that are deflected by the earth s magnetic field (Perez 1999). However, there is no evidence to support this during migratory flight. More recent experiments involving a flight simulator have led to another theory that monarch butterflies use their circadian clocks as a time-compensated sun compass for orientation (Mouritsen 2002). The Monarch s mishaps The article states that increased rainfall in the Oyamel forests of Mexico will affect the survival of the butterflies due to the chance of freezing to death. Body temperatures below -2 C start to kill monarchs and 50% die at -7.8 C; however, monarchs have adapted two shivering mechanisms associated with crawling and flying before such temperatures are reached (Masters 1988). Slow crawling and shivering occur between 3.3-5.0 C where the shivering warms the leg muscles which reduce the risks of freezing by allowing them to crawl off the ground onto vertical surfaces (Masters 1988). Monarchs that have moisture frozen from the rain on their exoskeleton have a greater risk of freezing because the ice crystals can penetrate the cuticle and cause internal freezing due to the amount of wetness as well as the low extreme temperature (Calvert
1983). With this in mind, there is confusion when reading the article because the author states that the temperatures of the forests will remain relatively constant over a span of 50 years but monarchs will not be able to endure the rain because it increases the probability of freezing to death. This is extremely ambiguous because the moisture from the rain cannot freeze unless temperatures are below zero. The high altitudes (3000 m) of the forests usually stay above freezing temperatures all winter (Layberry 1998). The article also mentions that the monarch butterflies form enormous clusters on the trunks of the fir trees of the Oyamel forests; however, the article does not mention that the massive colonies actually serve a purpose of acting as a heat blanket and umbrella for shielding against freezing and wetting (Anderson & Brower 1996). Improvement on the news story The news story has provided the reader with several facts about D. plexippus which are indeed true and fascinating but its primary focus of the monarchs habitat loss due only to rainfall increases is rather finicky to a scientist. The only statement the article conveys is that wet increases their chances of freezing to death (Kettlewell 2003). No other explanation is given throughout. The author therefore provides weak support and should have given better explanations as to why increased rainfall would cause monarch butterflies to lose their winter habitat within 50 years. The author however does give the reader a good sense of the monarch s requirements in the winter due to their frost intolerance and restricted winter locations and ends with some notable
issues concerning the deforestation of these habitats for agriculture use. This is an effective means to promote awareness to the public that it is not only Mother Nature imposing on the monarch.
References Anderson, JB, Brower LP. 1996. Freeze-protection of overwintering monarch butterflies in Mexico: critical role of the forest as a blanket and an umbrella. Ecological Entomology 21: 107 116. Calvert, William H, Willow Suchowski, Lincoln P Brower. 1983. The Effect of Rain, Snow and Freezing Temperatures on Overwintering Monarch Butterflies in Mexico. Biotropica 15: 42-47. Hartzler Robert G, Douglas D Buhler. 2000. Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas. Crop Protection 19: 363-366. Kane, E. 1999. "Danaus plexippus" (On-line), Animal Diversity Web. Accessed 4 February 2004 at <http://animaldiversity.ummz.umich.edu/site/accounts/information/dana us_plexippus.html> Kettlewell, Julianna. Butterflies face climate threat. BBC News 10 November 2003. Accessed 18 January 2004 at <http://news.bbc.co.uk/go/pr/fr/-/1/hi/sci/tech/3257917.stm> Layberry, Ross A, Peter W Hall, and J Donald Lafontaine. 1998. The Butterflies of Canada University of Toronto Press. Accessed 5 February 2004 at <http://www.cbif.gc.ca/spp_pages/butterflies/species/monarch_e.php> Masters, Alan R, Stephen B Malcolm, Lincoln P Brower. 1988. Monarch Butterfly (Danaus Plexippus) Thermoregulatory Behavior and Adaptations for Overwintering in Mexico. Ecology 69: 458-467. Mouritsen, Henrik, Barry J Frost. 2002. Virtual migration in tethered flying monarch butterflies reveal their orientation mechanisms. PNAS 99: 10162-10166. Perez, Sandra M, Orley R Taylor, Rudolf Jander. 1999. The Effect of a Strong Magenetic Field on Monarch Butterfly (Danaus plexippus) Migratory Behavior. Naturwissenschaften 86: 140-143. Perez, Sandra M, Orley R Taylor, Rudolf Jander. 1997. A Sun compass in monarch butterflies. Nature 387: 29.