Climate closing the gender gap for this mountain flower | Science News for Students

Climate closing the gender gap for this mountain flower

Males move up the mountain as weather patterns warm
Jul 28, 2016 — 7:00 am EST

Male valerian plants growing at 3,790 meters (12,434 feet). These males have moved to higher elevations in response to hotter, drier weather. That has caused the ratio of males to females to climb over the last 40 years. 


These males can take the heat — an advantage when it comes to global warming. Called valerian plants, they sport small white flowers. One type of bloom grows on male plants, another type on female plants. And in Colorado’s Rocky Mountains, the males and females have responded differently to warmer and drier weather. That’s the finding of a new study.

Valerian (Valeriana edulis) plants grow over a wide range. Some live in hot, scrubby lowlands. Others sprout on cold mountain slopes. In each valerian patch, some plants are male and others female. The exact share of each sex varies with elevation, the new study finds. Males like it warm and dry. Females prefer it wet and cold. So the valerian patches higher up a mountain have tended to have far more females than males. Females may account for as many as eight out of every 10 plants at some upland sites.

But with climate change, warmer weather has been creeping slowly up the mountain slopes. More warm days has resulted in more males and fewer females. Such a rapid change in the share of males and females could be a signal of climate change, the researchers say.

They report their findings in the July 1 Science.

Forty years ago, scientists surveyed a number of the Rocky Mountains’ valerian populations. Around 3,000 meters (9,842 feet), one-third of them were males. This was right in the middle of the plant’s elevation range. Males are now 5.5 percent more common throughout the plants’ range. So someone hoping to find the same share of male plants today would have to hike much higher up the mountain.

“We think climate is acting almost like a filter on males and females,” says Will Petry of ETH Zurich. He’s the plant ecologist who led the study while at the University of California, Irvine. The warmer the climate, the more females it is filtering out. That filter is sweeping up mountainsides. It’s climbing upwards at a rate of 175 meters (574 feet) every decade, Petry and his team find.

Ecologists already knew that the share of male to female plants can change depending on altitude or how much water is around, says Spencer Barrett. He’s an ecologist at the University of Toronto who was not involved in the research. This new study is different. “The idea that a sex ratio is moving upslope,” he says, “nobody’s ever [seen] that before.”

Tracking the sexes

The share of male to female plants has changed at about the same pace as the local climate has since the late 1970s. Today’s winter snows are melting earlier. Summers are hotter and have less rain. That means each upslope area is drier now than it once was. Precipitation levels have moved up the slope by 133 meters (436 feet) every 10 years. Soil moisture has also moved up the mountain even faster pace — at 195 meters per decade.

J. Soule
Judy Soule (here) collected data on valerian plants in the Colorado Rockies from 1978 to 1980. This historical data provided the researchers with long-term perspective for their new study.
J. Soule

So climate and the valerian plants’ sex ratios are changing in lock step. That suggests fast-changing sex ratios could help scientists understand how climate change will affect those species where sex differences depend on weather, says Tom Miller. He’s a population biologist at Rice University in Houston. He was also one of the study’s authors.

Whole species can migrate either in latitude (north versus south) or altitude or both. Researchers view these migrations as an indicator of climate change. But the plants’ sex ratios are changing much faster than a whole species is moving, Miller says. And that, he says, “might be a much more rapid fingerprint of climate change.” Scientists could use that fingerprint to predict the future health of a species’ population in the long term.

Petry and his group counted blooming male and female valerian plants. They studied them at 31 different sites around the Rocky Mountain Biological Laboratory in Crested Butte, Colo. That was in 2011. Then they compared those numbers with historical counts. Nine of those same populations had been surveyed between 1978 and 1980. As soon as Petry saw that the share of males to females had changed, he and his team started thinking about the impacts.

If one sex greatly outnumbers the other, a population might die out, says Kailen Mooney. Mooney works at UC Irvine and also took part in the new study. For instance, if females in lower altitudes died out, then there would be no flowers capable of growing seeds. Those lowland communities would then disappear as the last of the mature male plants died.

Such changing sex ratios can have a subtle effect on how scientists think about species’ migrations due to climate change, Mooney says. The geographic limits for a whole species, then, might be set by the limits of just one sex.

Power Words

biology    The study of living things. The scientists who study them are known as biologists.

climate     The weather conditions prevailing in an area in general or over a long period.

climate change    Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

ecology    A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

elevation    The height or altitude at which something exists.

fingerprint      The unique pattern of raised lines and whorls that on each of an individual’s fingers. Or something that might be used, much as a fingerprint it, to identify the presence or effect of some factor.

latitude    The distance from the equator (north or south) measured in degrees (up to 90).

lowlands      Regions that are below sea level or at lower elevation than most of the surrounding area.

population    (in biology) A group of individuals from the same species that lives in the same area.

precipitation    (In meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

ratio    The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).

sex     The biological status of an animal or plant, typically whether it is male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal or external reproductive organs.

sex ratio     The share of males to females (or females to males)  in a population or given group of animals (and some plants). A 50:50 ratio, for instance, means half are male and half are female. A 90:10 ratio, in contrast, would mean there are nine males for every one female.

species      A group of organisms that share similar traits and ancestry, and can usually breed to produce fertile offspring. It is also the basic rank in a classification system called taxonomy. A species name is usually given with the next highest rank, the genus.

subtle    Some feature that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be visible but subtle — small and hard to distinguish from nearby healthy tissues.

upslope      A region on a mountainside or hillside that is higher up in elevation.


  • MS-LS1-5
  • MS-LS2-3
  • MS-ESS3-3
  • HS-LS2-8
  • HS-ESS3-6


W. Petry et al. Sex-specific responses to climate change in plants alter population sex ratio and performanceScience. Vol. 353, July 1, 2016, p. 69. doi: 10.1126/science.aaf2588.