Ancient Plants Utilized Heat for Pollination, Study Reveals
New research suggests that some of the earliest plants attracted pollinators by generating heat, which caused these plants to emit infrared light. This study, published in the journal Science, indicates that this heat-based signaling system was present long before the evolution of brightly colored flowers.
Cycads and Their Thermogenic Mechanism
The plants identified in this study are cycads, an ancient group currently found in tropical forests worldwide and classified as one of the most endangered plant groups. Wendy Valencia-Montoya, a cycad expert at Harvard University, noted that cycads are sometimes referred to as 'dinosaur plants' due to their prevalence during the dinosaur era. Fossil evidence spanning over 200 million years suggests that cycads have maintained their morphology with little change over geological timescales.
Cycads, which are related to pines, produce distinct fleshy, cone-like structures on both male and female plants; these structures contain the pollen and the seeds. Valencia-Montoya indicated that cycads represent the oldest known plants with pollen.
For centuries, botanists have observed that the reproductive structures of these plants produce heat, often becoming 15 to 25 degrees Fahrenheit, or more, hotter than the ambient air temperature. This heat production is an uncommon and energy-intensive process within the plant kingdom. Previous hypotheses suggested the heat was either a byproduct of metabolic processes or served to volatilize plant scents. Research from recent decades has also shown that male and female cones heat up at slightly different times, a mechanism that could direct pollinators between them.
Experimental Findings on Infrared Attraction
To understand the role of heat in pollination, Valencia-Montoya and her team conducted experiments. They monitored pollinating beetles, marked with fluorescent markers, and observed their activity in relation to the heating cycles of cycad cones.
To decouple heat from other potential plant signals like scent and humidity, researchers used 3-D printed simulated pollen cones filled with heated sand. These artificial structures emitted infrared light in a manner similar to real plants. These fake plants were deployed outdoors at the Montgomery Botanical Center in Coral Gables, Florida.
Further experiments involved wrapping the simulated cones with plastic wrap, which is transparent to infrared light. This setup allowed insects to perceive the infrared light without direct tactile contact with the heat. The results indicated that the infrared glow alone was sufficient to attract hundreds of pollinating beetles.
Pollinator Adaptations and Evolutionary Implications
Examination of the attracted beetles revealed specialized antennae capable of detecting subtle temperature differences. These antennae are similar to the heat-sensing receptors found in snakes, which detect prey. The study also determined that the beetles' antennae appear to be specifically tuned to the temperature ranges emitted by their respective host cycad species.
Nicholas Bellono of Harvard University, a co-author of the study, proposed that infrared radiation might be the oldest discovered pollination signal. Valencia-Montoya suggested that this heat-based signaling was advantageous for the nocturnal insects with limited vision that were prevalent during the early evolution of plants with pollen. As new groups of pollinators, such as butterflies and bees, emerged with improved diurnal vision, plants likely adapted their signaling strategies to incorporate visible cues like color.
Expert Commentary
Roger Seymour, from the University of Adelaide, who was not part of the research team, characterized the study as an 'important contribution,' highlighting the comprehensive techniques used to demonstrate the ancient link between cycad thermogenesis and pollinator attraction. Seymour also suggested that heat might serve additional functions, such as providing a direct energy reward for insects that remain within warm, thermogenic flowers.
Irene Terry, an ecologist specializing in cycads and insect interactions at the University of Utah, not involved in this research, noted that the understanding of cycads having insect pollinators, rather than relying solely on wind for pollen dispersal, is a relatively recent development. She emphasized the role of specific odorants, which can vary in scent from bubblegum to bell peppers, as important attractants. Terry affirmed that the study provided clear evidence for the key role of infrared signals, drawing parallels to other insects, such as mosquitoes, which utilize infrared for navigation, and underscoring the broader range of sensory perceptions in insects compared to humans.