chiropterophily

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Bat Pollinated Flowers of a Mexican Columnar Cactus

Pollination syndromes – suites of floral traits used to determine potential pollinators and routes of pollination – have been informative in studying plant-pollinator interactions, but are generally too simplistic to tell the full story. Most flowering plants are generalists when it comes to pollinators, whereas pollination syndromes imply specialization. Not all pollinators are created equal though, and some may be more effective at pollinating particular plants than others. In fact, occasionally pollination syndromes ring true and a predicted plant-pollinator combination turns out to be the most effective and reliable interaction.

According to a study published in American Journal of Botany by Ibarra-Cerdeña, et al., Stenocereus queretaroensis, a species of columnar cactus endemic to western Mexico, adheres to this scenario. Stenocereus is a genus in a group of columnar and tree-like cacti called the Pachycereeae tribe. Cactus in this group are generally bat pollinated; however, their flowers are typically visited by various species of birds and insects as well, and in some cases, bats are not the primary pollinator. In their introduction, the authors note that specialization appears to be more common in tropical latitudes, and chiropterophilic (bat pollinated) columnar cacti that occur in temperate regions can be comparatively more generalized. This is because “extratropical chiropterophilic cacti appear to be faced with unpredictable seasonal year-to-year variation in pollinators,” while “cacti in tropical regions” experience “highly reliable seasonal availability of nectar-feeding bats, thereby leading to a temporally stable pollination system.”

Stenocereus queretaroensis is a massive cactus, reaching up to ten meters tall. Several vertical stems rise from a short, stocky, central trunk. Each stem has up to eight distinctive ribs and averages around 15 centimeters in diameter. Groupings of white to grey spines up to four centimeters long appear along the ribs. Flowers are light-colored, around 10 to 14 centimeters in length, and occur along the upper half of the stems, extended well beyond the spines. Flowers open at night – producing abundant nectar – and close by the afternoon the following day. Floral characteristics led the authors of this study to predict bats to be the main pollinator, and they set up a series of experiments to test this.

Stenocereus queretaroensis - photo credit: wikimedia commons

Stenocereus queretaroensis – photo credit: wikimedia commons

Part of their experiment consisted of five treatments involving 130 flowers on 75 plants. One group of flowers was bagged and allowed to self-pollinate naturally, while another group was bagged and self-pollinated manually. A third group was left exposed during the night but bagged in the morning, while a fourth group was bagged during the night and exposed during the daytime. The final group was left alone. For each of these five treatments, aborted flowers and mature fruits were counted and seed set was determined. Nectar samples were taken from a separate group of flowers at two hour intervals from 8:00 PM to 8:00 AM, after which no nectar was produced. A camera was also used to document floral visits. Visits were deemed “legitimate” when the “visitor’s body came in contact with anthers and/or stigma” and “illegitimate” when “no contact with anthers or stigma” was made.

The researchers found S. queretaroensis to be “incapable of self-pollination,” as no fruit set occurred for the first two treatments. The control group and the nocturnally exposed group had nearly identical results, producing significantly more fruits with greater seed set compared to the nocturnally bagged group. During the day, flowers were visited by four species of birds (two hummingbirds, a woodpecker, and an oriole) and several species of bees (mainly honey bees). During the night, apart from illegitimate visits from a nectar robbing hawkmoth, one species of bat was the dominant floral visitor, and the majority (93.8%) of the visits were legitimate. This bat species was Leptonycteris curasoae, the southern long-nosed bat.

Leptonycteris curasoae - photo credit: wikimedia commons

Leptonycteris curasoae – photo credit: wikimedia commons

The abundance of nectar-feeding bats was monitored in the study area over a four year period, and L. curasoae was by far the most abundant species throughout the study period. Nectar produced in the flowers of S. queretaroensis was at its maximum around midnight, which seemed to correlate with observations of bat visits. Even though daytime visitors appeared to contribute to fruit and seed set, the nocturnal treatment produced significantly more fruit with significantly higher seed set, suggesting that bats are the more efficient pollinator. Insects visiting during the daytime, when nectar was decreasingly available, were most likely robbing pollen.

The authors acknowledge that for most plant species, “a wide array of taxonomically diverse fauna such as insects, birds, and mammals usually serve as potential pollinators,” and that “generalized pollination systems are more frequent than specialized ones.” However, in this case, “a close association between L. curasoae and S. queretaroensis [suggests] that the chiropterophilic syndrome is still a useful model.”

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Bats As Pollinators – An Introduction to Chiropterophily

Most plants that rely on animals to assist in pollination look to insects. In general, insects are abundant, easy to please, and efficient at transferring pollen. Because insect pollination is such a common scenario, it is easy to overlook pollination that is carried out by vertebrates. Birds are the most prominent pollinator among vertebrates, but mammals participate, too. The most common mammal pollinator is the bat.

About a fifth of all mammal species on the planet are bats, with species estimates numbering in the 1200-1300 range. Bats are the only mammals that can truly fly. They are not blind, nor are they flying rodents, and they are not going to suck your blood (except in very rare cases!). Most bats eat insects, but a small, significant group of them are nectarivorous. Their main food source is the nectar produced within flowers. In the process of feeding, these bats pollinate plants.

Out of 18 families in the order Chiroptera, only two include species with morphologies that set them apart as nectar-feeders. The family Pteropodidae, known commonly as Old World fruit bats or flying foxes, occurs in tropical and subtropical regions of Africa, Asia, Australia, Papa New Guinea, and the Pacific Islands. The family Phyllostomidae, known commonly as American leaf-nosed bats, occurs in tropical and subtropical regions of the Americas. For simplicity’s sake, the former are referred to as Old World bats, and the latter as New World bats. While both groups are similar in that they consist of species that feed on nectar, they are only distantly related, and thus the nectar feeding species in these families have distinct behavioral and morphological differences.

Grey headed flying fox photo credit: wikimedia commons

Grey headed flying fox (Pteropus poliocephalus), a floral visiting bat from Australia (photo credit: wikimedia commons)

More than 500 species of plants, spanning 67 plant families, are pollinated by bats. This pollination syndrome is known as chiropterophily. In general, flowers that use this approach tend to be white or dull in color, open at night, rich with nectar, and musty or rotten smelling. They are generally tubular, cup shaped, or otherwise radially symmetrical and are often suspended atop tall stalks or prominently located on branches or trunks. In a review published in Annals of Botany, Theodore Fleming, et al. state “flower placement away from foliage and nocturnal anthesis [blooming] are the unifying features of the bat pollination syndrome,” while all other characteristics are highly variable among species. The family Fabaceae contains the highest number of bat-pollinated genera. Cactaceae, Malvaceae, and Bignoniaceae follow closely behind.

The characteristics of bat pollinated flowers vary widely partly because the bats that visit them are so diverse. Between the two bat families there are similarities in their nectar-feeding species, including an elongated rostrum, teeth that are smaller in number and size, and a long tongue with hair-like projections on the tip. Apart from that, New World bats are much smaller than Old World bats, and their rostrums and tongues are much longer relative to the size of their bodies. New World bats have the ability to hover in front of flowers, while Old World bats land on flowers to feed. Old World bats do not have the ability to use echolocation to spot flowers, while New World bats do. Fleming, et al. conclude, “because of these differences, we might expect plants visited by specialized nectar-feeding [New World bats] to produce smaller flowers with smaller nectar volumes per flower than those visited by their [Old World bat] counterparts.”

Pollination by bats is a relatively new phenomenon, evolutionarily speaking. Flowers that are currently pollinated by bats most likely evolved from flowers that were once pollinated by insects. Some may have evolved from flowers that were previously bird pollinated. The question is, why adopt this strategy? Flowers that are bat pollinated are “expensive” to make. They are typically much bigger than insect pollinated flowers, and they contain large amounts of pollen and abundant, nutrient-rich nectar. Due to resource constraints, many plants are restricted from developing such flowers, but those that do may find themselves at an advantage with bats as their pollinator. For one, hairy bat bodies collect profuse numbers of pollen grains, which are widely distributed as they visit numerous flowers throughout the night. In this way, bats can be excellent outcrossers. They also travel long distances, which means they can move pollen from one population of plants to an otherwise isolated neighboring population. This serves to maintain healthy genetic diversity among populations, something that is increasingly important as plant populations become fragmented due to human activity.

Pollinating bats are also economically important to humans, as several plants that are harvested for their fruits, fibers, or timber rely on bats for pollination. For example, bat pollinated Eucalyptus species are felled for timber in Australia, and the fruits of Durio zibethinus in Southeast Asia form after flowers are first pollinated by bats. Also, the wild relatives of bananas (Musa spp.) are bat pollinated, as is the plant used for making tequila (Agave tequilana).

Durio sp. (photo credit: wikimedia commons)

The flowers of durian (Durio sp.), trees native to Southeast Asia, are pollinated by bats (photo credit: wikimedia commons)

There is still much to learn about nectarivorous bats and the flowers they visit. It is clear that hundreds of species are using bats to move their pollen, but the process of adopting this strategy and the advantages of doing so remain ripe for discovery. Each bat-plant relationship has its own story to tell. For now, here is a fun video about the bat that pollinates Agave tequilana: