Larval and metamorph traits of cane toads

As an invasive organism spreads into a novel environment, it may encounter strong selective pressures to adapt to abiotic challenges. We examined the effect of water temperature during larval life on rates of survival and growth of the early life-history stages of cane toads (Rhinella marina) from two geographic regions (tropical vs. temperate) in the species’ invaded range in eastern Australia. If local adaptation at the southern (cool-climate) invasion front has extended the cold-tolerance of early life-stages, we would expect to see higher viability of southern-population toads under cooler conditions. Our comparisons revealed no such divergence: the effects of water temperature on rates of larval survival and growth, time to metamorphosis, size at metamorphosis and locomotor performance of metamorphs were similar in both sets of populations. In two cases where tropical and temperate-zone populations diverged in responses to temperature, the tropical animals performed better at low to medium temperatures than did conspecifics from cooler regions. Adaptation to low temperatures in the south might be constrained by behavioural shifts (e.g., in reproductive seasonality, spawning-site selection) that allow toads to breed in warmer water even in cool climates, by gene flow from warmer-climate populations, or by phylogenetic conservatism in these traits.

Methods
Adult toads were collected at sites in tropical and temperate-zone eastern Australia. The toads were bred in captivity and their progeny raised at a series of controlled temperatures so that we could evaluate effects of larval temperatures on life-history traits.

作为一种入侵生物，当其扩散至新环境时，可能会遭遇强烈的自然选择压力，以适应非生物挑战。本研究考察了水温在幼虫期对来自两个地理区域（热带与温带）的甘蔗蟾蜍（Rhinella marina）早期生命历史阶段存活率和生长速率的影响。若该物种在南部（冷气候）入侵前沿的本地适应性已延长了早期生命阶段的耐寒性，则我们预期在较冷条件下南部种群蟾蜍的生存力将更高。我们的比较研究并未发现此类差异：水温对幼虫存活率、生长速率、变态时间、变态后体型及运动表现的影响在两个种群中均无显著差异。在热带与温带种群对温度响应出现分歧的两种情况下，热带动物在低至中等温度下的表现优于来自较冷地区的同种个体。南方对低温的适应性可能受到行为变化（例如，在繁殖季节性、产卵地点选择）的限制，这些变化使得蟾蜍即使在冷气候中也能在较温暖的水中繁殖，或受到来自温暖气候种群基因流的影响，或受到这些性状在系统发育上的保守性。