Species respond in many different ways to a warming world. Some may be producing physically smaller offspring. A number of researchers are finding certain species, including some beetles, sparrows, fish, and mountain goats, may be among those producing smaller offspring as the temperature increases.

This phenomenon correlates with 19th century German biologist Karl Bergmann’s rule which notes populations of species in colder areas generally have larger body sizes than populations of the same species in warmer areas. There are some exceptions to Bergmann’s rule, such as the grasshopper Chorthippus brunneus. But most species do seem to have larger body sizes in populations located where the temperatures are colder and, conversely, smaller body sizes in warmer areas.

Researchers are now assessing whether climate change and rising global temperatures are causing some species to become smaller as their environments change. Professor Michelle Tseng, PhD, at the University of British Columbia, for instance, is analyzing this possibility. Earlier this year, she published study results in the Journal of Animal Ecology examining body sizes of preserved beetle specimens with historical weather and climate records

To conduct the study, Tseng and her team analyzed approximately 7,000 beetle specimens of eight different species preserved at the University of British Columbia’s Beaty Biodiversity Museum. They compared the measurements they collected from the beetles with historical weather and climate records to see if the physical size of the insects changed over time, and if any changes were correlated with weather and climate changes.

Correlation: Some beetles smaller at warmer temperatures

Not every beetle species Tseng studied showed a decline in size, but her team did see a correlation between body size and rising temperatures, with larger beetles showing greater changes. They found one of the larger beetle species lost 20 percent of its body size over the past 45 years. “[We] saw that there was this relationship where the bigger the species was, the more it had been shrinking with temperature,” Tseng says.

These findings confirm what Tseng had suspected based on laboratory studies. “[From] laboratory studies we had a pretty good idea that insects, when you grow them at warmer temperatures, tend to, when they hit maturity, [have an] adult size [that’s] smaller than had you grown them at cooler temperatures, and that is an almost universal response of insects and other cold-blooded things,” Tseng says.

Having the museum’s collection of preserved beetles available for examination provided the researchers access to a huge number of specimens and detailed data about each one, including the collection date and location. The beetles’ physical characteristics also made them a good species to study.

“What’s really useful about insects, Tseng says,  is that as adults “their exoskeleton is very rigid, so their size doesn’t change after they hit the adult stage.” She says she finds the museum specimens to be so useful because they “are a snapshot of what that creature was like when it was alive.”

Study provides useful but limited ‘snapshot’

Tseng notes there are 350,000 beetle species in the world, and her study examined only eight of them, so it’s only a tiny snapshot of what’s out there. However, her study has contributed to the growing body of research showing a correlation between rising temperatures and declining body sizes in many species.

What does this mean for the species and the ecosystems they inhabit? The specific impacts are yet to be determined, but it is possible that changing body size could impact species and ecosystems in a variety of ways.

“Insect body size is highly correlated with a few different other ecological traits,” Tseng says. “The bigger you are, the more eggs you have, for example. If you’re a dung beetle, the bigger you are, the more dung you roll. If you’re a bee, the bigger you are, the more pollen you collect. If you’re a mosquito, the bigger you are, the more blood you feed on and take in. These types of traits are quite relevant in the general food web, and they’re very related to insect body size.” However, Tseng notes that researchers have a lot more work to do before they know exactly how these findings will affect species and ecosystems.

House sparrows: Warmer temperatures correlate to smaller size

The phenomenon of shrinking body size correlations with rising temperatures is not limited to insects. Researchers in Australia recently examined house sparrows (Passer domesticus) to see how the birds’ body sizes change with rising temperatures. Samuel Andrew, who is now a post-doctoral researcher at the University of Helsinki, studied these sparrows as his doctoral project for Macquarie University in Australia, and he recently published his findings in The Auk.

House sparrows are an introduced species in Australia and New Zealand, and they live everywhere from temperate regions to the scorching desert. The birds are frequently studied, and they were the subject of a famous 1898 study focusing on natural selection among birds that survived storms. “House sparrows are one of the most broadly distributed bird species in the world,” Andrew says, noting they live everywhere from Arctic Norway to the deserts of Australia.

To conduct the study, the research team sampled birds at 30 different sites (26 in Australia and data from 4 previously sampled sites in New Zealand). To select study sites, they examined the sparrows’ range and decided to select study sites around 400 km apart, using road availability to help select the final sites.

Andrew and his team found hotter summer temperatures were correlated with smaller body size. They also noted summer high temperatures – which occur during the breeding season – were a better predictor of body size than winter low temperatures. “Normally in colder climates sparrows are bigger,” Andrew says. “We found that general trend to be true as well in Australia … If you looked at maximum temperature during summer, that was the best predictor of the size of the sparrows.” The smaller body size could also be passed between generations. “Body size is very inheritable,” Andrew says.

Scientists do not fully understand why sparrows and other species tend to become smaller with increasing temperatures, nor do they know how these changes will ultimately affect the species and the ecosystems they live in. Although the exact consequences are unknown, it is possible smaller body sizes could have a number of impacts on species fitness, with a cascading effect through various trophic levels. Adaptation is another possibility, but only time will tell how species continue to respond to rising temperatures and what those responses may mean for species and ecosystems.