Fairies, Unicorns, and Playing Devil's Advocate

Every now and then, someone will revive the discussion on the iNaturalist forum about the supposed uselessness of the male/female annotation option for plants. They aren't common but they are out there! Botanically speaking, these plant species are dioecious meaning male and female reproductive structures are present on separate individuals. So let me present a compelling case of using the male and female annotation for plants for citizen science.

Meet the fairywand, Chamaelirium luteum, also known as devil's bit, false unicorn, blazing-star, and helonias (all very confusing common names as they are shared with other plants). Its range extends over much of Eastern North America all the way from parts of New England and Canada down to Florida and west to Arkansas. A long-lived perennial, it takes about 5-7 years for a seed-grown plant to reach flowering age. The plants grow as basal rosettes of leaves with about 10-20% of the population bolting with tall flower stalks blooming in spring. As you might suspect already, this species is dioecious with male plants making staminate flowers only and female plants making pistillate flowers only. Interestingly, these male and female flowering plants are not evenly distributed but show a generally male-biased sex ratio. Why is that?

Chamaelirium luteum morphology, based on Britton & Brown's Illustrated Flora (public domain) and iNaturalist observation photo of a male (left) and female (right) plant (Milo Pyne, Creative Commons).

The sex distribution in this species was studied by Thomas Meagher at Duke University, NC, and summarized in several papers published in the early 1980s. The first study reported on the spatial distribution of males and females at four different study sites in North Carolina over the span of six years and found that plants of each sex tend to cluster together in same sex groupings with males usually closer to each other while females tend to grow more isolated (PDF). The second part of the study then took a closer look at the sex ratios at different developmental stages from seedlings to adults. While propagated seedlings started out with near equal representation of both sexes, flowering plants were found to show a significant bias with on average 3.5x as many males as females (PDF).

A third study classified the plants into three groups, juveniles of unknown sex, males, and females, and took a closer look at the possible explanations for the observed gender bias. It found differences in mortality rates with seedlings having the highest mortality rate, followed by juveniles and then females before males. Males started to flower at a younger age compared to females, and females grew consistently taller flower stalks with more cauline leaves then males, followed by a reduction in plant size the following year suggesting a higher reproductive cost of flowering and fruiting for females. While males were able to flower in subsequent years with some individuals blooming for every year of the study, females flowered at most every other year. (Unfortunately, this article is not freely available as a PDF but you can get it from Wiley or JSTOR.)

Having read about the sex ratio bias in this species, I was curious whether that was something that would show up in iNaturalist observations as well. So I set out on a little annotation project, marking male and female plants in the Southern Appalachian Mountains (to limit time investment). Overall, iNaturalist observations show a good overlap with the range map for this species on BONAP (the outlier in New Hampshire is a rosette of leaves that looks to be something else misidentified as this species).

Observations of Chamaelirium luteum recorded on iNaturalist (left) compared against the distribution map from BONAP. The yellow shaded area represents the "Southern Appalachian Mountains" place boundary on iNaturalist.

There are no sex chromosomes so the only way to distinguish between male and female plants is by flower morphology. Fortunately, this is fairly straight-forward on typical iNaturalist pictures due to the sexual dimorphism of the flowers as shown in the images below.

Comparison between male (left) and female (right) flowers of Chamaelirium luteum, using a collage of Creative Commons licensed pictures from iNaturalist observations. Male flowers can be recognized by the presence of anthers atop six stamens, while female flowers have a three-parted stigma leading to a swelling ovary that eventually turns into the seed capsule. Both male and female flowers have six white, linear petals surrounding their reproductive parts.

So what's the verdict? I think the case is pretty clear: iNaturalist observations support that there is a sex ratio bias towards more males than females. As reported in the papers above, peak bloom time was in May, which also coincided with peak observation time on iNaturalist. I only annotated flowering and fruiting plants (155 or ~69% of 226 RG observations in May; most of the remaining plants showed rosettes of leaves with no flowers) that were clearly identifiable as either male (128 observations or ~83%) or female (9 observations or less than 1%) for a bias of a whopping 14x more males than females observed at peak bloom. I was undecided about the sex on 18 (~12%) flowering observations, mostly due to the photos being too far away or too blurry to see the necessary details, but even assuming that these plants are females the gender bias is still pretty clear.

Phenology (A) and sex (B) distribution data for Chamaelirium luteum observations in the Southern Appalachian Mountains.

And now that I've successfully procrastinated until midnight, I've got to get ready for final exams tomorrow... o_o (Fortunately, I'm the instructor, not a student, haha!)

Posted on December 08, 2022 05:21 AM by annkatrinrose annkatrinrose


Fascinating! Thank you for writing this up.

Posted by tsn about 1 year ago

Good use of iNat annotation data!

Posted by trscavo about 1 year ago

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