Range Expansion: Towards the end of the last ice age, the receding glaciers exposed new areas of land in the north that were previously inaccessible to many animals and plants. During this time, many species expanded their ranges north, making use of the newly accessible resources. Many islands, such as what is now Prince Edward Island, were peninsulas connected to the mainland by a land bridge. As sea levels rose however, these land bridges disappeared below the waves. Animals currently present on these islands offer a snapshot of some of the first animals to colonize the area before becoming isolated. It was believed that the pickerel frog was a latecomer to the north until 2006 when two individuals were found on Prince Edward Island. These two frogs changed the narrative, confirming that pickerel frogs were some of the first frogs to conquer the north. Some argue however, that these two frogs were brought to the island through anthropogenic means such as fishing bait although there is a lack of big game fish that would be attracted to such bait.
Predator Defense: Unlike many frogs in North America, the pickerel frog will often lay their eggs in wetlands with fish present, to avoid becoming prey, they have evolved a number of strategies. Of these include laying their eggs close to shore, being much shallower, the eggs are inaccessible to larger predatory fish like more vertical sunfish. After hatching, the pickerel frog tadpoles become increasingly aware of their surroundings. Pickerel frogs pick up on chemical cues, detecting the chemicals released by fish, and change their behaviour accordingly. When fish are not present, the tadpoles are regularly moving, finding as much food as possible to facilitate development. When fish are present however, the tadpoles reduce their movements so to avoid detection. When all else fails, the tadpoles also develop toxins that many predatory fish find distasteful. These chemicals persist into adulthood as well with fully developed frogs producing toxic chemicals when threatened. These chemicals are also distasteful, can cause irritation to eyes, and even kill other frogs when in high concentrations.
Sources:
Holomuzki, Joseph R. "Oviposition Sites and Fish-Deterrent Mechanisms of Two Stream Anurans." Jstor, Copeia, 18 Aug. 1995, www.jstor.org/stable/1446757?casa_token=3Dmb3ZvDv8cAAAAA%3AUZyVfEtOAOyix41DwXX71sl5_iRtMAyaJku5PIPfPXTRERIQTFKzM-uB6CaRsoR8d46tle8mh2891Im90_8q7Ejz47S4MMqA806VEd55Cbl4rutoh90&seq=2. Accessed 17 May 2023. McAlpine, Donald F., et al. "Occurrence and Biogeographic Significance of the Pickerel Frog (Rana palustris) on Prince Edward Island, Canada." Research Gate, Herpetological Natural History, 2006, www.researchgate.net/profile/Rosemary-Curley/publication/316169154_McAlpine_Donald_F_Robert_W_Harding_and_Rosemary_Curley_2006_Occurrence_and_biogeographic_significance_of_the_pickerel_frog_Ra. Accessed 26 Jan. 2022. "Pickerel Frog." Canadian Herpetological Society, 2022, canadianherpetology.ca/species/species_page.html?cname=Pickerel%20Frog#:~:text=The%20Pickerel%20Frog%20is%20found,eastern%20Texas%20in%20the%20south.. Accessed 26 Jan. 2022. |