How Cane Toad Hind Limb Length Influences Its Adaptability - Paper Example

Invasive species undergoes evolutionary selective pressure and can be used to evaluate how the selective process affects its traits and the traits of the taxa with which it interact. Dispersal ability affects the rate at which the invader can spread. In our study, we measured the snout vent length and the hind limb length of cane toads in two populations (population A and population B).  We noted that population A toads had a larger snout  vent length and a  longer hind limb and travelled further distances as compared to population B toads. This indicates that, cane toad have undergone a major shift in sexual dimorphism in relative limb lengths as they spread through population A.

Introduction

As invasive species spread outside its native territory, it may encounter novel selective forces and so it has to undergo a process of rapid evolutionary change which may produce a clear effects over a period of time. The evolutionary selective pressure may modify traits both in cane toads and in the taxa with which they interact (Jeanette Covacevich, 1987). For instance, there might be morphological disparities in origin population and new area population.

Dispersal rate evolve upwards during an invasion, hence, individuals in the invasion vanguard tend to exhibit dispersal-enhancing features. Species that acquire fast-dispersal morphological traits may undergo spatial sorting (successive generations of interbreeding between the fastest dispersers) or natural selection (reflecting fitness benefits to unusually fast-moving individuals) (Cogger).

We studied how cane toad hind limb length influences its adaptability. Our analysis revealed that toads with longer hind limbs travel further distances and therefore limb length is a likely target of natural selection, spatial sorting and sexual selection.

Material and methods

Study species

The scientific name of cane toad is Rhinella marina. The species is a large stocky amphibian with dry, warty skin and it was introduced into Queensland in the 1930s in a failed attempt to control insect pests that were damaging sugar cane crops. An adult giant cane toad ranges in length from 85 180 mm. They eat almost anything they can swallow, including pet food, carrion and household scraps, but most of their food is living insects. Beetles, honey bees, ants, winged termites, crickets and bugs are eaten in abundance. Marine snails, smaller toads and native frogs, small snakes, and small mammals (Lever, 2001). The toads' large body sizes and prodigious appetites encouraged commercial sugarcane growers to import toads to control insect pests in plantations in Puerto Rico. They are invasive species in Australia and pose a threat to native vertebrates by poisoning them. Cane toad have parotoid glad on either side of the head which produces a secretion containing bufadienolides that is toxic to native vertebrates. This chemical defense does not exist in any native Australian anuran. The vertebrates experienced significant population decline, immediately following the invasion of the toad species in Australia but in the long term the population of the vertebrates has improved. This suggest that, the native vertebrates has adapted the presence of the can toad. Toads move in short hops, their hind feet are webbed between the toes but their front feet are not webbed (Flint).

Sampling location

We picked toads from cooloola National Park in southeast Queensland, Australia. The area comprises of 56,000 hectares of rainforest, dry sclerophll forest and open highlands. The soil is sandy covered with thin humus layer. The toads were gathered by hand, placed in an aerated plastic containers labelled as population A and population B respectively. After collection, we transported the toads to the laboratory for experiment. Morphometric measurements (snout-vent length; hind limb length) of the captured toads were taken and recorded.

Statistical analysis

We used R Studio v. 0.99.893, which is an open source software in our analysis. We applied linear mixed models to make generalization of how cane toad hind limb affect the adaptability of the toad.

Each toad was given a unique number putting in factor of variation in snoutvent lengths (SVL) and hind limb length of individuals. We carried out two test during analysis. First, we combined all the collected data from the two population to get the overall pattern of the impact of the hind limb on the adaptability of the toad and secondly we analyzed the data from the two population separately. Results from the study confirmed that indeed the body size and the limb length of between population A and population B do differ. It showed that, population A had a longer hind limb than population B. We concluded that, population A had a longer hind limb for dispersion in the new area since longer hind limbs enables toads to travel longer distances. Explosion of information on evolutionary selective pressure of inversion has attracted several scholars and our findings are similar to the published studies (PA, 2011). The concept that evolution can drive evolutionary change is well supported.

Results

Overall effect of hind limb on the adaptability of cane toad

Data from population A and data from population B was combined to get the overall influence of the hind limb on the adaptability of the cane toad. A wide variation was noted in the adaptability of the toad. The adaptability was a function of hind limb length and the sources of the cane toads. ANOVA indicated that cane toad adaptability was influenced by hind limb length (Warner).

Comparison of population A toad and population B toad

We used a linear mixed-effects ANOVA model to get the influence of the cane toad hind limb on the adaptability of the toad from the two population. Collected data suggested that cane toad from population A have longer hide limbs than those from population B. This showed that toads at the front line of inversion depends on limb for dispersal.

Discussion

Hind limbs influences the adaptability of the cane toad. We indicated that population A toads had relatively long hind limbs and moved faster and further than population B toads which had short hind limbs. From this, we can infer that population A spacemen was collected from a new population and population B was collected from a more established population. Long hind limbs provide high propulsive ability for leaping and hence enabling toads to evade an oncoming predator (Brown GP, 2011b). Long hind limbs are associated with a high incidence of spinal arthritis.

According to studies, selection favors animals with shorter limbs but the process has reversed in populations close to the invasion front. Reversal might be caused by spatial sorting rather than natural selection. Dispersal rate is higher for long hind limb toads and thus, alleles for longer hind limb accumulate in the vanguard of the invasion, regardless of whether or not they enhance fitness of their bearers. Hind limb length decreases over the course of the invasion, but with a reversal close to the invasion front where spatial sorting overrides natural selection (Carroll SP, 1998). Selective advantages that accrue to individuals in the invasion vanguard may favor maximal dispersal rates. Rapid dispersal rate would not confirm the same fitness advantage in longer-colonized areas. This is because, it would not facilitate individuals to reach low-density populations. Extensive sampling revealed a more complex scenario. The invasion driving a reduction in the hind limb length and reversing rapid increase in hind limb length near the invasion front (Hagman M, 2007). Consistent with their greater limb lengths, male toads can travel faster than females toads. The evolution of a faster dispersal during the toad invasion enabled males substantially out-pace female toads and therefore reaping the advantage of food availability at the invasion front but at the cost of a highly skewed operational sex ratio (Hamilton WD, 1977). The high degree of sex-based divergence in hind limb length in cane toads from range-core populations may result from sexual selection. Selection favors larger hind limb length in male toads than females in ancestral populations, a dimorphic condition widespread among anurans and therefore, natural selection in the course of the toad's long hind limbs march across Australia may have favored toads with shorter hind limb length that moved by bounding rather than by leaping (Huey RB, 2005).

References

Brown GP, S. C. (2011b). Methods in Ecology and Evolution.

Carroll SP, K. S. (1998). Evolutionary Ecology.

Cogger, H. (n.d.). Reptiles and Amphibians of Australia. Csiro Publishing.

Flint, D. (n.d.). The Cane Toad Republic. Wakefield Press.

Hagman M, S. R. (2007). Biological Invasions.

Hamilton WD, M. R. (1977). Dispersal in stable habitats.

Huey RB, G. G. (2005). Species Invasions.

Jeanette Covacevich, P. D. (1987). Toxic Plants & Animals.

Lever, C. (2001). The History and Ecology of a Successful Colonist. Westbury Academic and Scientific Publishing.

Mooney HA, C. E. (2001). The evolutionary impact of invasive species.

PA, W. (2011). What invasive species reveal about the rate and form of contemporary phenotypic change in nature.

Robert F. Woolson, W. R. (n.d.). Statistical Methods for the Analysis of Biomedical Data. John Wiley & Sons.

Warner, R. M. (n.d.). Applied Statistics. SAGE.