Mucuna holtonii: An Echolocation Plant

While many "bat plants" are pollinated by other animal species, some plants have evolved to be pollinated exclusively by bats and thus rely on them for survival. Such is the case of the Central American legume Mucuna holtonii. This plant is so highly specialized to attract bat pollinators, that it has evolved many structural and chemical adaptations truly unique to bat plants (Tschapka and Dressler 2002). Mucuna holtonii is an epiphytic vine found on rainforest edges and lining creeks throughout Central America. The plant has hanging inflorescences consisting of 3-8 flowers that bloom only at night. The flowers have 5 petals; two keel petals, two lateral petals, and a fifth, highly specialized petal that forms a concave, triangular vexillum that is only raised in mature flowers (Fig. 1; von Helversen and von Helversen 1999). 

Fig 1. von Helversen and von Helversen, 1999. Petal structures of mature M. holtonii with erect vexillum (left) and immature bud (right). Accessed 19/5/2015; available: http://www.nature.com/nature/journal/v398/n6730/pdf/398759a0.pdf.

When a flower of M. holtonii is mature and has completed nectar and pollen production, its fifth petal raises upwards at night and forms a concave vexillum. The vexillum serves as a reflective tunnel for bat echolocation waves, collecting them and directing them backwards toward the source. This signals to the bat that a mature flower with a virgin food source is within range. The bat continues to emit sonar and receive signals from the vexillum until the plant is located by the bat, at which point it lands and proceeds to extract nectar from the nectary (Fig 2; Simon et al. 2011). When the bat lands, it will cause the keel petals to burst and the staminal column of the plant will launch most of its pollen load onto the bat's back and rump. The bat will leave the flower once it has extracted approximately 100 microliters, carrying a large load of pollen to the next flower it will seek for nectar (Simon et al. 2011). Once the flower receives a bat visitor and has unloaded pollen, the vexillum petal will lower and thus will not attract bats to its nectary. This ensures that bats will not extract valuable, energy-intensive nectar without receiving a full pollen load, since the flower has already unloaded its pollen stores on the previous bat visitor (von Helversen and von Helversen 1999).

Fig 2. Tuttle, M.D. 2012. Glossophaga commisarisi visits a mature flower of Mucuna holtonii and extracts nectar. The staminal column of the flower will release a full load of pollen onto the bat once it bursts the keel petals. Accessed 19/5/2015; available: ngm.nationalgeographic.com.

These specialized adaptations of M. holtonii demonstrate the incredible specificity some organisms display as a result of coevolution. 

References

SIMON, R., HOLDERIED, M.W., KOCH, C.U. and VON HELVERSEN, O., 2011. Floral acoustics: conspicuous echoes of a dish-shaped leaf attract bat pollinators. Science (New York, N.Y.), 333(6042), pp. 631-633.

TSCHAPKA, M. and DRESSLER, S., 2002. Chiropterophily: On Bat–Flowers and Flower Bats. Curtis's Botanical Magazine, 19(2), pp. 114.

VON HELVERSEN, D. and VON HELVERSEN, O., 1999. Acoustic guide in bat-pollinated flower. Nature, 398(6730), pp. 759-760.