Xanthosoma robustum commonly known as “elephant ear” or “pata” is a neotropical species found in disturbed or open, sunny habitats. It is in the Araceae family, subfamily Colocasiodeae, tribe Caladiae (Goldwasser 2000; Madison 1981). The genus Xanthosoma has forty-five species and is therefore the most diverse genus in this family (García-Robledo 2005). Like most species in the Colocadiodeae subfamily, X. robustum is terrestrial and has a large, tuberlike stem (Madison 1981). This species is characterized by this general appearance and by its floral thermogenicity, which is a pollination strategy in which a flower heats up to release odors as a to attract pollinators and is very common in the Araceae family (Meeuse et al 1988). Besides representing an interesting biological pollination system, this species is also economically important because humans and livestock consume both the leaves and starchy stems (Madison 1981).
This aroid is a large, terrestrial monocot herb that can grow taller than an adult human and has large leaves that can be up to a meter square. It does not often have many leaves; usually having no more than four (Goldwasser 2000) X. robustum has a characteristic spike inflorescence with a spadix covered by tiny flowers surrounded by a hood like bract called a spathe. The spathe has a constriction that creates a chamber at the base of the spadix. The plant is monoecious with the female flowers found within the chamber and the male flowers found above the constriction. In addition, there is a ring of sterile flowers below the constriction and above the female flowers (Goldwasser 2000).
X. robustum is a neotropical species with the center of diversity of the genus in the Andes (Milius 2003; Madison 1981). It is found from Mexico to Northern Argentina and has also been introduced to Hawaii (García-Robledo 2005). It is intolerant of cool climates, and therefore is rarely found above 1800 m (Madison 1981).
X. robustum is a sun-loving species that grows well in open areas that are also wet and sunny (Goldwasser 2000, García-Robledo 2005). It can be found in vacant lots, dirt roads and pathways, plantations, and near highways (Díaz-Betancourt et al 1999).
This plant species has an important association with its pollinators that requires a high level of coordination between the thermogenic attraction mechanism of the plant and the scarab beetles pollinators (see pollination section). This coordination is especially important because instead of the beetles moving from one plant to the next closest inflorescence, they often fly longer distances to other plants (Goldwasser 2000).
Scarab beetles are not the only insects associated with the inflorescences. The spathe chamber can provide important habitat for other arthropods. Initially, the chamber is home to terrestrial arthropods that take advantage of the shelter and food sources such as detritus and plant sap. Then later the chamber may fill with rainwater and more aquatic arthropods colonize the chamber (García-Robledo 2005).
These other visitors play three distinct roles in their relationship with the plant. There are “interlopers” like mites, sucking bugs, and myrids that are parasitic to the plant or the beetles or both. There are also “predators” like spiders, larval staphylinids, and the predatory larvae of syrphids, which eat other visitors that come to the plant. Finally there are comensal species including small wasps, drosophilids and other flies, earwigs, thrips, and psocids (lice), which are found on the plant or the beetles but seem to have no negative affect on any of the associated species (Goldwasser 2000).
Even though there may be no direct relationship between the beetle pollinators and these other arthropods, the other arthropods are dependent on the beeltes. If the inflorescence is not pollinated, it is aborted by the plant and the arthropods must migrate to a new inflorescence (García-Robledo 2005). Although, if the inflorescence is pollinated, the arthropod colonies may continue to inhabit the spathe chamber for many weeks (García-Robledo 2005).
Pollination
The plant is not self pollinated and is instead reliant on beetle pollinators. The pollination system is strongly mutualistic; the plant is optimally out-crossed and the beetle recieves the necessities for its entire lifecycle from food and mating sites for adults to brood sites (García-Robledo 2004). Over the course of a year, mainly during the rainy season, the aroid produces between five and fifteen inflorescences. Only one inflorescence is produced at a time and there is usually a nine-day lag time between flowering events (Goldwasser 2000). The pollination event spans over two days. First, at around six pm on the first day, through a process called thermogenic respiration, the plant increases its metabolic rate and the spadix heats up, peaking at about 40-42˚C (García-Robledo 2004, Goldwasser 2000). This may seem like a wasteful system because it requires a large energy input, but this heat is essential to release strong odors to attract the beetle pollinators (Meeuse et al 1988). Beetles in both the Dynastinae andNitidulidae families have been observed visiting and pollinatingthese plants, although Dynastinae seem to be the most important pollinators (García-Robledo 2004). Within these beetle families, the number and species that visit the aroid varies within the plant’s geographic distribution (García-Robledo 2004). This peak in temperature is important in reducing pollination competition because it is reached at specific times for different species within this family (Meeuse et al 1988).
The beetles, attracted by the odor, arrive and crawl down into the chamber of the spathe. In these inflorescences the female flowers mature first and therefore the plant must have a unique way to keep the pollinators around for a short time in order for them to carry away pollen and therefore to make the system function (Milnius 2003). Any pollen that the beetles are carrying when they arrive from another flower sticks on the mature female flowers as they enter the chamber. Then, during the night and the next day the spadix cools and the spathe constricts, trapping the beetles inside. (Goldwasser 2000). While trapped inside the beetles take advantage of the heat produced by the inflorescence in order to lower their own metabolic rate, therefore expending much less energy as they mate and eat (Seymour 2003). On average there are seven beetles found within the chamber but this number can be as high as thirty eight (Goldwasser 2000). During the afternoon of the second day the spadix heats up, the male flowers mature and release pollen, and the spathe constriction opens. Then the beetles, covered in a new layer of pollen, crawl out and fly to the next inflorescence (Goldwasser 2000).
Dispersal
After the pollination event is completed, the fruit begins to mature. The male part of the spadix falls away and the female flowers mature within the spathe chamber. When ripe the fruit resembles a cob with many soft green fruits, each containing up to thirty seeds (Goldwasser 2000). Bats consume the fruits and the seeds are dispersed through their feces (Goldwasser 2000). In addition, land mammals like coatis may also act as dispersers.
