Frostbitten toads: Are cane toads adapting to the cold as they move northwards in Florida?
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Reference: Mittan CS, Zamudio KR. 2019. Rapid adaptation to cold in the invasive cane toad Rhinella marina. Conserv Physiol 7: doi: 10.1093/conphys/coy075
Cane toads: widespread conquerors?
Cane toads have been introduced globally through various accidental and purposeful events. The most well-known introduction event was to Australia. Cane toads were introduced as means to help the sugar cane industry because they were predators of cane beetles that had been impacting sugar cane yields. However, no one could have imagined the cane toads’ ability for conquering an entire country. A single cane toad female can lay up to 10,000 eggs. These animals also grow quite large and are known as voracious eaters. Reports have documented that cane toads eat small birds and even snakes! However, a cane toad’s greatest threat comes from its biological defenses. Cane toads have two large glands at the back of their heads that produce powerful toxins that can incapacitate and even kill would-be predators (Figure 1). In Australia this has caused unprecedented effects on the local animals.
Cane toads conquered Florida and have been marching north
Overwhelming evidence shows the impact humans have had over climate patterns. Many researchers have become interested in studying how global climate change can affect plants and animals. For example, as weather patterns change, areas where species have historically thrived may become unsuitable. On the other hand, areas previously too cold or too warm may now become available for a species to move in. In response to these changes, species may move following the areas suitable to them or slowly change to adapt to the new conditions. Specifically, there is growing interest in how species may adapt to be able to live at temperatures that have been historically too cold or hot for them.
Researchers at Cornell University in Ithaca, New York noticed that cane toads in Florida are experiencing colder temperatures than that of their natural home range in South and Central America. Additionally, there are northern toads near Tampa, Florida that live in areas colder than those in southern Florida (Figure 2). Because of this the researchers were able to set up experiments to see if the northern toads are adapting to the colder weather.
Hot n Cold: Are Florida’s northern toads more tolerant of the cold?
The main goal of this new study was to test whether northern toads are more tolerant of the cold. In other words, are northern toads adapting to the cooler weather? To answer this question, the research team took toads from both southern and northern Florida populations and tested the frogs’ tolerance to cold temperatures. Northern and southern toads were split into two experimental groups: the cold acclimation group and the warm acclimation group. The warm acclimation group was kept at 25°C and the cold group was kept at 10°C. These temperatures were chosen because the warm matches the preferred temperature of toads (25°C) and the cold matches the average coldest temperatures in northern Florida (10°C). Tolerance to the cold was measured by gradually exposing toads to colder and colder temperatures and seeing if toads could right themselves when placed upside down. The response to turn your body over has been broadly used a metric of an animal’s ability to tolerate extreme temperatures for that species. When an animal cannot move their body, they are not able to tolerate that temperature. This a safe way to expose animals to extreme temperatures while minimizing harm.
The result of the experiments show that northern toads do tolerate cooler temperatures (Figure 3). They also found that both northern and southern toads tolerated cooler temperatures when acclimated to cold temperatures (kept at 10°C prior to testing cold tolerance). These results show that exposure to colder temperatures can increase the tolerance to the cold in cane toads. This is important because it shows that current populations can increase their cold tolerance by exposure to cold.
Cane conquerors can go even further north
The overall implications of this are that cane toads are still able to continue going north. Because of that, as northern toads experience more colder temperatures, they are better equipped for persisting in these environments. This makes it more likely that the next generation of cane toads will adapt to the cold, in part helped by their parents’ capacity to tolerate colder temperatures through both adaptation and acclimation. Because cane toads have been shown to be highly invasive, early monitoring could help prevent these conquerors from expanding north. Finally, researchers can use the information on how cane toads persist under cold temperatures to make predictions as to how other animals might fare under climate change scenarios.