Endothermy, the biological production of heat, is familiar in Mammals and Birds, but is also found in a variety of other Animals, such as Bumblebees, some Fish and even some Snakes. It is also known in Plants, where it is usually used to warm flowers rather than the whole Plant, enhancing the emission of scent molecules used to attract pollinators. One recently discovered example of this is the Ivory Palm, Phytelephas aequatorialis, an economically important Palm species found on the western side of the Andes in Ecuador, in which the male flowers have been shown to generate heat; this is unusual as most Flowering Plants in which this behaviour has been observed have flowers with both sets of sex organs, rather than flowers with different sexes, and it is unclear if endothermy is restricted to the male flowers in the Ivory Palm, or whether it occurs in the female flowers as well.
In a paper published in the journal Frontiers in Ecology and the Environment on 1 December 2016, Sylvain Pincebourde of the Institut de Recherche sur la Biologie de l’Insecte at the Université François–Rabelais de Tours, Rommel Montúfar and Erika Páez of the Facultad de Ciencias Exactas y Naturales at the Pontifica Universidad Católica del Ecuador, and Olivier Dangles, also of the Facultad de Ciencias Exactas y Naturales at the Pontifica Universidad Católica del Ecuador, and of the Institut de Recherche pour le Développement, describe the results of an investigation into the female flowers of the Ivory Palm.
Pincebourde et al. visited the Otonga Reserve in Ecuador in February and June 2015 to look for female flowers of the Ivory Palm. The male flowers of the Ivory Palm are large and obvious, forming inflorescences over a meter in length, but the female flowers are much smaller, reaching at most 30 cm. In addition the female flowers are found in the crown of the Palms, typically more than 10 m above the ground. The Palms flower several times per year, but the male flowers are typically open for about 24 hours and the female flowers for only a few days, so that at any one time flowering trees will be widely scattered in the landscape.
The male inflorescence of endemic ivory palm (Phytelephas aequatorialis). The orange material at the top of the image is the remnants of the bud integument. Pincebourde et al. (2016).
However, the Palms are important to local communities, who use its leaves as a roofing material and its seeds to make buttons and other handicrafts, and Pincebourde et al. were able gain the help of local people in finding female flowers of the Ivory Palm.
Initial examinations of these female flowers with infrared cameras proved disappointing; the did not appear to be emitting any heat. However further examination revealed that while the mature flowers produced no heat, the female buds were as much as 10–20°C above ambient air temperature. This led Pincebourde et al. to investigate the buds of the Ivory Palm more closely, revealing that buds of both sexes maintained a constant temperature of about 37°C (comparable to the body temperature of a Mammal or Bird), in an environment where the ambient air temperature ranged between 17°C and 28°C.
Photographs (left) and thermographic images (right) of a male flower bud (top) and female flowers with the bud integument remaining at the base of the flower (bottom) taken on 9 Feb 2015 (at 18:30) and 11 Feb 2015 (at 12:10), respectively. Air temperature was 23.4°C for the male bud and 22.5°C for the female inflorescence. Three female inflorescences at different phenological stages are shown: (a) late stage, (b) senescent, and (c) recently opened. Pincebourde et al. (2016).
The production of heat by the buds of a flower is unexpected; heat in flowers is typically used to help release scent molecules to attract pollinators, but a bud, which is an unopened flower, has no need to do this, requiring a different explanation. Rapid growth can lead to the generation of heat, but this seems unlikely in the case of the Ivory Palm, where a constant temperature is maintained (unlikely if the heat is simply the product of a metabolic process) and this temperature is roughly the same in buds of both sexes, despite the fact that the male buds grow faster and larger than the females.
Local people reported that the open flowers were visited by a wide range of Insects, including a variety of Beetles and Flies. However when Pincebourde et al. monitored Buds of the Ivory Palm of both sexes they found that they were regularly visited by a type of Stingless Bee, Trigona sp. These Bees regularly flew around the bulbs, and often settled on the buds and walked along the groves at their tops. Pincebourde et al. suggest that this species may be involved in the pollination of the flowers, possibly using the heat identify the short lived male flowers before they open, in order to enter and gain pollen, then visit the slightly longer lived female flowers to get nectar.
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