Adaptation for rodent pollination in Leucospermum arenarium (Proteaceae) despite rapid pollen loss during grooming

Rodent responses to volatile compounds provide insights into the function of floral scent in mammal-pollinated plants

Flowers pollinated by mammals have evolved in many plant families. Several scent compounds that attract bats to flowers have been identified, but the chemical ecology of pollination mutualisms between plants and ground-dwelling mammals is poorly understood. Rodents are key pollinators in South Africa and rely heavily on olfaction to locate food. Our aim was to identify compounds that may function to attract rodents to flowers. Eighteen volatile compounds, including 14 that are prominent in the scent of rodent-pollinated flowers, were used in choice experiments involving wild-caught individuals of four native rodent species. Rodents were generally attracted to oxygenated aliphatic compounds, specifically ketones and esters, but not to some aromatic compounds common in floral scents of insect-pollinated species, nor to a sulfide compound that is attractive to bats. Associative conditioning using sugar solution as a reward had only weak effects on the attractiveness of compounds to rodents. The attractive effect of some compounds disappeared when they were blended with compounds that did not attract rodents. We conclude that aliphatic ketones and esters are likely to play a key role in attracting rodents to flowers. Deployment of these compounds may allow plants to exploit rodent sensory bias that evolved in other contexts such as intra-specific communication and searching for seeds. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

First insights into rodent trapping in Oku village, north-west Cameroon, based on interviews with local hunters

Mount Oku, in the Northwest Region of Cameroon, is known for its rodent diversity. It is located in an area with a high human population density (up to 400/km2), resulting in intense pressure on natural resources. Threats to biodiversity include overgrazing by cattle and goats in grassland areas and montane forests, firewood harvesting, agriculture, bee keeping, debarking of Prunus trees for medicinal uses, and bushmeat hunting. We used data from interviews with 106 local hunters to provide insights into rodent trapping in Oku village. Trapping took place primarily in closed canopy forest. The majority of hunters (88.8%) set at least 100 traps per week, with a mean of 38 rodents trapped per hunter per week. The two most captured species were the Vulnerable Hartwig's praomys Praomys hartwigi and the Endangered Mount Oku rat Lamottemys okuensis, both of which have declining populations. Rodents were harvested mainly for household consumption and/or local trade, but 65.1% of interviewees also used P. hartwigi for traditional medicine. Our findings suggest that rodent trapping in Oku village requires conservation attention, and that further quantitative studies are needed to assess its sustainability.

Scent chemistry is key in the evolutionary transition between insect and mammal pollination in African pineapple lilies

Volatile emissions may play a key role in structuring pollination systems of plants with morphologically unspecialised flowers. Here we test for pollination by small mammals in Eucomis regia and investigate whether its floral scent differs markedly from fly- and wasp-pollinated congeners and attracts mammals. We measured floral traits of E. regia and made comparisons with insect-pollinated congeners. We observed floral visitors and examined fur and faeces of live-trapped mammals for pollen. We determined the contributions of different floral visitors to seed set with selective exclusion and established the breeding system with controlled pollination experiments. Using bioassays, we examined whether mammals are attracted by the floral scent and are effective agents of pollen transfer. Eucomis regia differs from closely related insect-pollinated species mainly in floral scent, with morphology, colour and nectar properties being similar. We found that mice and elephant-shrews pollinate E. regia, which is self-incompatible and reliant on vertebrates for seed production. Mammals are strongly attracted to the overall floral scent, which contains unusual sulphur compounds, including methional (which imparts the distinctive potato-like scent and which was shown to be attractive to small mammals). The results highlight the important role of scent chemistry in shifts between insect and mammal pollination systems.

Pollination and breeding system of the enigmatic South African parasitic plant Mystropetalon thomii (Mystropetalaceae): rodents welcome, but not needed

Unrelated plants adapted to particular pollinator types tend to exhibit convergent evolution in floral traits. However, inferences about likely pollinators from 'pollination syndromes' can be problematic due to trait overlap among some syndromes and unusual floral architecture in some lineages. An example is the rare South African parasitic plant Mystropetalon thomii (Mystropetalaceae), which has highly unusual brush-like inflorescences that exhibit features of both bird and rodent pollination syndromes. We used camera traps to record flower visitors, quantified floral spectral reflectance and nectar and scent production, experimentally determined self-compatibility and breeding system, and studied pollen dispersal using fluorescent dyes. The dark-red inflorescences are usually monoecious, with female flowers maturing before male flowers, but some inflorescences are purely female (gynoecious). Inflorescences were visited intensively by several rodent species that carried large pollen loads, while visits by birds were extremely rare. Rodents prefer male- over female-phase inflorescences, likely because of the male flowers' higher nectar and scent production. The floral scent contains several compounds known to attract rodents. Despite the obvious pollen transfer by rodents, we found that flowers on both monoecious and gynoecious inflorescences readily set seed in the absence of rodents and even when all flower visitors are excluded. Our findings suggest that seed production occurs at least partially through apomixis and that M. thomii is not ecologically dependent on its rodent pollinators. Our study adds another species and family to the growing list of rodent-pollinated plants, thus contributing to our understanding of the floral traits associated with pollination by non-flying mammals.

Novel Consequences of Bird Pollination for Plant Mating

Pollinator behaviour has profound effects on plant mating. Pollinators are predicted to minimise energetic costs during foraging bouts by moving between nearby flowers. However, a review of plant mating system studies reveals a mismatch between behavioural predictions and pollen-mediated gene dispersal in bird-pollinated plants. Paternal diversity of these plants is twice that of plants pollinated solely by insects. Comparison with the behaviour of other pollinator groups suggests that birds promote pollen dispersal through a combination of high mobility, limited grooming, and intra- and interspecies aggression. Future opportunities to test these predictions include seed paternity assignment following pollinator exclusion experiments, single pollen grain genotyping, new tracking technologies for small pollinators, and motion-triggered cameras and ethological experimentation for quantifying pollinator behaviour.

First record of flower visitation by a rodent in Neotropical Proteaceae, Oreocallis grandiflora

There is still much to learn about pollination ecology in areas of high species diversity, such as the Neotropical region. For example, the role of non-flying mammals in pollination is largely unknown in the Neotropical areas, despite the importance of this group in the pollination of plants in other regions. Here we report evidence for flower visitation by a rodent to Oreocallis grandiflora (Proteaceae) in our study site in the southern Andes of Ecuador. For a period of 241 h, camcorders equipped with infrared lights revealed 22 occurrences of floral visitation by Microryzomys altissimus (Cricetidae) to inflorescences of O. grandiflora. Pollen samples taken from the fur of captured individuals showed that M. altissimus carries pollen of O. grandiflora, indicating a potential pollinator role for the rodent. This is the first record of flower visitation by a rodent in the Neotropical Andes. Pollination interactions between non-flying mammals and Proteaceae have been reported in other southern hemisphere continents. This study underscores the need for more research on pollination interactions in the Neotropics, especially in critical Andean biodiversity hotspots.