🧬 biology

Evidence for reward costs in two bee-pollinated Penstemon

This study on two bee-pollinated *Penstemon* species reveals that producing higher floral rewards incurs fitness costs, evidenced by selection to reduce nectar in one species and stabilizing selection on pollen, while also highlighting distinct selection pressures on floral morphology and a lack of strong honest signaling between traits and rewards.

Original authors: Amy L. Parachnowitsch, Kaushalya Rathnayake

Published 2026-09-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Amy L. Parachnowitsch, Kaushalya Rathnayake

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Flowers are not merely static decorations; they are complex negotiation tables where plants and animals meet to exchange services. For a plant to reproduce, it needs to move its pollen to another flower, a task it cannot perform alone. To solve this, plants offer a bribe: a reward in the form of sugary nectar or protein-rich pollen. In return, visiting insects or birds carry the genetic material from one bloom to the next. This exchange has driven the evolution of spectacular floral shapes and colors, leading scientists to long assume that the more generous a flower is, the more successful it will be. The logic seemed simple: a bigger reward attracts more visitors, which leads to more seeds. However, this assumption overlooks a critical biological reality. Producing sugar and pollen requires energy and resources that the plant could otherwise use to grow taller, produce more seeds, or survive harsh conditions. If a flower gives away too much, it might actually hurt its own chances of survival. Understanding whether these rewards are truly beneficial, or if they sometimes become a burden, is essential for grasping how flowers evolve over time.

In a study posted as a preprint, researchers Amy Parachnowitsch and Kaushalya Rathnayake set out to test this idea in the wild. They focused on two closely related species of wildflowers found in the Rocky Mountains of Colorado: the Rocky Mountain penstemon and the Grand Mesa beardtongue. Both plants are visited primarily by bees, which collect nectar and pollen to feed their young. The researchers wanted to see if producing more of these rewards actually helped the plants make more seeds, or if there was a hidden cost to being too generous. To find out, they tracked 90 plants of each species in their natural habitat, measuring everything from the height of the plant and the width of its petals to the exact volume of nectar and the number of pollen grains in each flower. They then waited for the growing season to end to count how many fruits and seeds each plant successfully produced, using this count as a measure of the plant's reproductive success.

The results challenged the simple idea that "more is better." The researchers found that for one of the species, the Rocky Mountain penstemon, producing more nectar actually reduced the number of fruits the plant could make. This suggests that the energy spent on creating extra sugar was taken away from other vital functions, effectively penalizing the plant for being too generous. In the other species, the Grand Mesa beardtongue, the amount of nectar did not show a clear link to reproductive success, indicating that the cost of making nectar is not the same for every plant, even among close relatives. The study also looked at pollen, which serves a double role: it is the male genetic material needed for reproduction, but it is also a food source for bees. Here, the researchers discovered a stabilizing pattern. Plants with either very low or very high amounts of pollen produced fewer seeds than those with a moderate amount. This suggests that while bees might be attracted to flowers with abundant pollen, having too much might encourage bees to visit the same plant repeatedly, leading to the plant fertilizing itself with its own pollen rather than receiving fresh genetic material from neighbors. This self-fertilization can be harmful to the health of the offspring.

Beyond the rewards themselves, the team investigated whether the flowers had any physical features that honestly signaled how much food they contained. It is a common theory that a flower might evolve a specific shape or color to tell a bee, "I have a lot of nectar inside," so the bee knows which flowers to visit. The researchers checked if traits like petal width or the length of the flower tube correlated with the actual amount of nectar or pollen. They found very little evidence to support this. In most cases, a flower's size or shape did not reliably predict how much reward it held. This lack of a clear signal suggests that the relationship between what a flower looks like and what it offers is more complicated than a simple honest advertisement. The study also revealed that the two species, despite looking similar and sharing the same pollinators, were being shaped by nature in different ways. For instance, while both species were selected to produce a moderate amount of pollen, only one faced pressure to reduce its nectar production.

Ultimately, this work highlights that the evolution of flowers is a balancing act. Producing rewards is not an automatic path to success; it is a trade-off that can sometimes work against the plant. The researchers found that the pressure to produce more rewards was often weaker than the pressure to simply grow taller or produce more flowers. This implies that the traits we see in flowers today are not just about maximizing the bribe offered to pollinators, but about managing the cost of that bribe. By measuring these subtle costs in the wild, the study provides a clearer picture of why flowers vary so much in their generosity. It shows that nature does not always favor the most generous donor; sometimes, the most successful strategy is to hold back just enough to ensure the plant survives to bloom again.

Technical Summary: Evidence for Reward Costs in Two Bee-Pollinated Penstemon

Problem Statement
Floral rewards, specifically nectar and pollen, are critical for attracting pollinators and driving angiosperm evolution, yet they are frequently neglected in studies of natural selection on floral traits. While phylogenetic and pollination-system perspectives suggest rewards evolve, estimating the specific phenotypic selection acting on them is essential to understand their future evolutionary trajectories. A significant gap exists in understanding whether floral traits act as "honest signals" of these rewards and how the production of rewards impacts plant fitness, particularly given the potential metabolic costs of reward production. Previous microevolutionary studies have largely focused on signals and pollinator-fit traits rather than the rewards themselves, making it difficult to compare the evolution of rewards with other floral characteristics.

Methodology
The study examined natural selection in two bee-pollinated Penstemon species, Penstemon mensarum and P. strictus, at the Rocky Mountain Biological Laboratory in Crested Butte, Colorado. The researchers selected 90 individuals of each species from distinct populations separated by approximately one kilometer.

Trait Measurement: Over a two-week period during peak flowering, the team measured several floral traits: plant height, flower number (display size), petal width, corolla tube gape width, total corolla tube length, and proximal tube length.
Reward Quantification: Nectar volume was measured using filter paper strips on male-phase flowers to minimize phase-related variation. Pollen number was estimated by collecting unopened buds, drying anthers, and counting grains via hemocytometer.
Fitness Estimation: In early September, female fitness was assessed by counting successful fruits and seeds per fruit (or developing seeds for P. strictus due to maturation issues).
Statistical Analysis: The authors used multivariate regression to estimate linear selection gradients ( $\beta$ ) and quadratic gradients ( $\gamma$ ) for each species separately, following Lande and Arnold (1983). They tested for differences in selection between species using ANCOVA. Additionally, stepwise regression was employed to determine if floral traits served as honest signals of rewards, and correlational selection was tested to see if signals and rewards evolved in tandem.

Key Results
The study found that selection on floral traits and rewards differed significantly between the two species, despite their shared pollinator guild and similar floral syndromes.

Selection on Rewards:
- Nectar: There was significant selection to reduce nectar volume in P. strictus, whereas no such selection was detected in P. mensarum. This suggests that higher nectar production may incur fitness costs in P. strictus.
- Pollen: Both species exhibited stabilizing (non-linear) selection on pollen number via fruit set, where intermediate pollen counts yielded the highest relative female fitness. Both low and high pollen numbers were associated with reduced fitness.
Selection on Morphological Traits:
- Plant Height: Taller plants were favored in P. mensarum but not in P. strictus.
- Floral Dimensions: In P. mensarum, there was a trend toward selection for increased petal width, which was positively correlated with higher rewards. In contrast, P. strictus showed selection for longer total corolla tubes but shorter proximal tube lengths, a trait negatively associated with rewards in that species.
- Flower Number: Selection favored increased flower number via fruit count for both species, but selection via seeds per fruit suggested a trade-off, particularly in P. strictus, where more flowers correlated with fewer seeds per fruit.
Honest Signaling and Correlational Selection:
- The study found little evidence for honest signaling. While petal width in P. mensarum was positively associated with rewards, there was no significant correlational selection between petal width and nectar.
- Similarly, no significant correlational selection was found between rewards and proximal tube length in P. strictus. The authors note that these null results may be due to limited statistical power rather than a true absence of selection.

Significance and Claims
The paper claims that higher floral rewards are not universally beneficial for plant fitness and can be subject to selection pressures that reduce them. Specifically, the findings suggest that producing more nectar or pollen can incur costs, potentially through ecological mechanisms (e.g., altered pollinator behavior leading to increased selfing) or direct metabolic costs.

The authors emphasize that:

Reward Costs Exist: The selection to reduce nectar in P. strictus and the stabilizing selection on pollen indicate that maximizing rewards is not always the evolutionary optimum.
Species-Specific Patterns: Even between closely related species with similar pollinators and floral forms, selection on rewards and associated traits can diverge significantly, making general predictions difficult without empirical measurement.
Integration of Rewards: Including rewards in selection studies is necessary to fully understand floral evolution, as rewards may evolve differently than signals or pollinator-fit traits. The study highlights that while selection on rewards was detected, it was generally weaker than selection on apparency (height) or display size, suggesting that reward evolution might lag behind other floral traits.

The authors conclude that understanding the dual function of pollen as both a reward and a gamete, and recognizing the costs of reward production, deepens the understanding of how hermaphroditic flowers evolve. They caution that their findings regarding honest signaling and correlational selection should be interpreted with caution due to sample size limitations.

Technical Summary: Evidence for Reward Costs in Two Bee-Pollinated Penstemon

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