A Rare Case of Ant Pollination in Australia

Photo by Nicola Delnevo [SOURCE]

Ants have struck up a lot of interesting and important relationships with plants. They disperse seeds, protect plants from herbivores and disease, and can even help acquire nutrients. For all of the beneficial ways in which ants and plants interact, pollination rarely enters into the equation. More often than not, ants are actually detrimental to the sex lives of flowering plants. Such is not the case for a rare species of protea endemic to Western Australia called the smokebush (Conospermum undulatum).

The reason ants usually suck at pollination is thanks to a tiny organ called the metapleural gland. For many ant species, this gland secretes special antimicrobial fluids that the ants use to groom themselves. Because ants tend to live in high densities in close quarters, this antimicrobial fluid helps keep their little bodies clean of any pathogens that might threaten their existence. For as good as these fluids are for ants, they destroy pollen grains, rendering them useless for pollination.

Leioproctus conospermi. Photo by Sarah McCaffrey licensed under CC BY-ND 2.0.

As is so often the case in nature, there are always exceptions to the rule and it seems that one such exception is playing out in Western Australia. While investigating the reproductive ecology of the smokebush, researchers noted that ants were regular visitors to their small flowers. They knew that in drier climates, some ant species have evolved to produce considerably less antimicrobial fluids. The thought is that drier climates tend to harbor fewer microbial pathogens and thus ants don’t need to waste as much energy protecting themselves from such threats. If this was the case in Western Australia then it was entirely possible that ants could potentially serve as pollinators for this plant. Armed with this hypothesis, they decided to take a closer look.

It turns out that the floral morphology of the smokebush lends well to visiting ant anatomy. The tiny flowers produce a small amount of nectar at the base. As ants shove their heads down into the flower to get a drink, it triggers an explosive mechanism that causes the style the smack down onto the back of the ant. In doing so, it also mops up any pollen the ant may be carrying. At the same time, the anthers explosively dehisce, coating the visitor with a fresh dusting of pollen. During their observations, researchers noted that ants weren’t the only insects visiting smokebush blooms. They also noted lots of visitation from invasive honeybees (Apis mellifera) and a tiny native bee called Leioproctus conospermi.

(A) White flowers of Conospermum undulatum. (B) Floral details. (C–H) Insects visiting flowers of C. undulatum: (C) Leioproctus conospermi; (D) Camponotus molossus; (E) Camponotus terebrans; (F) Iridomyrmex purpureus; (G) Myrmecia infima; (H) Apis mellifera. [SOURCE]

After recording visits, researchers needed to know whether any of these floral visitors resulted in successful pollination. After all, just because something visits a flower doesn’t mean it has what it takes to get the job done for the plant. By looking at differences in seed set between ant and bee visitors, they were able to paint a fascinating picture of the pollination ecology of the rare smokebush.

It turns out that ants are indeed excellent pollinators of this shrub, contributing just as much to overall seed set as the tiny native Leioproctus conospermi. Alternatively, invasive honeybees barely functioned as pollinators at all. Their heads were too big to effectively trigger the pollination mechanism of the flowers but nonetheless were able to access the nectar within. As such, honeybees are considered nectar thieves for the smokebush, harming its overall reproductive effort rather than helping.

Amazingly, the effectiveness of ants as smokebush pollinators is not because they produce less antimicrobial fluids. In fact, these ants were fully capable of producing ample amounts of these pollen-killing substances. Instead, it appears that the plant itself has evolved to tolerate ant visitors. Smokebush pollen is resistant to the toxic effects of the metaplural gland fluids. With plenty of hungry ants always on the lookout for food, the smokebush has managed to tap in to an abundant and reliable vector for pollination. No doubt other examples exist, we simply have to go looking.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Rodents as Pollinators

Leucospermum arenarium in the field and one of its pollinators, Gerbillurus paeba, feeding on flowers. (A) Pollen presenter contact on G. paeba. (B) G. paeba foraging on L. arenarium [Source]

It may come as a surprise to some that small mammals such as rodents, shrews, and even marsupials have been coopted by plants for pollination services. Far from being occasional evolutionary oddities, many plants have coopted small furry critters for their reproductive needs. Some of the best illustrations of this phenomenon occur in the Protea family (Proteaceae).

Protea nana. Photo by SAplants licensed under CC BY-SA 4.0

The various members of Proteaceae are probably best known for their bizarre floral displays. Indeed, they are most often encountered outside of their native habitats as outlandish additions to the cut flower industry. Superficial interest in beauty aside, the floral structure of the various protea genera and species is complex to say the least. They are well adapted to ensure cross pollination regardless of what the inflorescence attracts. Most notable is the fact that pollen doesn’t stay on the anthers. Instead, it is deposited on the tip of a highly modified style, which is referred to as the pollen presenter. Usually these structures remain closed until some visiting animal triggers their release.

The inconspicuous floral display of Protea cordata. Photo by SAplants licensed under CC BY-SA 4.0

Although birds and insects have taken up a majority of the pollination needs of this family, small mammals have entered into the equation on multiple occasions. Pollination by rodents, shrews, and marsupials is collectively referred to as therophilly and it appears to be quite a successful strategy at that. Therophilous pollination has arisen in more than one genera within Proteaceae.

A therophilous pollination syndrome appears to come complete with a host of unique morphological characters aimed at keeping valuable pollen and nectar away from birds and insects. The inflorescences of therophilous species like Protea nana, P. cordata, and Leucospermum arenarium are usually tucked deep inside the branches of these bushes, often at or near ground level. They are also quite robust and sturdy in nature, which is thought to be an adaptation to avoid damage incurred by the teeth of hungry mammals. The inflorescences of therophilous proteas also tend to have brightly colored or even shiny flowers surrounded by inconspicuous brown involucral bracts.

(C) Flowering L. arenarium with dense, mat-forming inflorescences. (D) Geoflorous inflorescences. (E) Pendulous inflorescences above ground level. [Source]

Contrasted against bird pollinated proteas, these inflorescences can seem rather drab but that is because small mammals like rodents and shrews are drawn in by another sense - smell. Therophilous proteas tend to produce inflorescences with strong musty or yeasty odors. They also produce copious amounts of sugar-rich, syrupy nectar. Small mammals, after all, need to take in a lot of calories throughout their waking hours and it appears that proteas use that to their advantage.

A small mouse pollinating Protea nana

As a rodent or shrew slinks in to take a drink, its head gets completely covered in pollen. In fact, they become so dusted with pollen that, before small, easy to hide trail cameras became affordable, pollen loads in the feces of rodents were the main clue that these plants were attracting something other than birds or insects. What’s more, the flowering period of many of these therophilous proteas occurs in the spring, right around the time when many small mammals go into breeding mode. Its during this time that small mammals need all of the energy they can get.

As odd as it may seem, rodent pollination appears to be a successful strategy for a considerable amount of protea species. The proteas aren’t alone either. Other plants appear to have evolved therophilous pollination as well. Nature, after all, works with what it has available and small mammals like rodents make up a considerable portion of regional faunas. With that in mind, it is no wonder that more plants have not converged on a similar strategy. Likely many have, we just need to take the time to sit down and observe.

Photo Credits: [1] [2] [3] [4] [5] [6] [7]

Further Reading: [1] [2] [3] [4] [5]