A possible new
solution to a 163-year-old biology puzzle - why animals grow bigger in cold climates - may have been found, according to researchers who say ecological factors can now be added to
physiological ones.
The results were published
in The
American Naturalist and they say it offers new
insight into 'Bergmann’s rule', an ecogeographic notion that correlates latitude with body mass in animals - animals grow larger at high, cold latitudes than their counterparts closer to
the equator. While traditional explanations have
been based on body temperature being the driving force of this phenomenon, a
group of community ecologists hypothesize that better food makes high-latitude
animals bigger.
Chuan-Kai Ho, a postdoctoral
student at Texas A&M at Galveston’s Armitage&Quigg Laboratory, Steven Pennings, professor at the University of Houston, and Thomas Carefoot
from the University of British Columbia, opened up a new line of study into
Bergmann’s rule. This latest finding came from one of Ho’s doctoral
dissertation chapters.
Studying three different
plant-eating species – grasshoppers, planthoppers and sea snails – collected
from along the Atlantic coast to Japan, respectively, the researchers fed these
herbivores plants from both high and low latitudes and found that they all grew
better when fed plants from the higher latitudes. This indicates that
Bergmann’s rule could reflect that plants from high latitudes provide better
food than those from low latitudes. These latest findings, according to Ho,
indicate that studies of Bergmann’s rule should consider ecological interactions
in addition to the more traditional theories of physiology based on responses to
temperature.
Over the years, they say they have shown that, although low-latitude plants are less nutritious
and better protected by chemical defenses, they experience heavy damage from
herbivores, which are more abundant at low latitudes. Future study, Pennings
adds, should focus on why there are more herbivores at lower latitudes despite
the lower-quality food sources. A likely explanation is that herbivore
populations are limited at high latitudes by a short growing season and high
death rates during cold winters.
“While the explanations
discovered in our current study only apply to herbivores, it may be that
carnivores and omnivores also might grow larger as a consequence of eating
larger herbivores,” Ho said. “Examining such patterns and underlying mechanisms
in nature will help us understand what currently is going on and what might
happen down the line to our ecosystems.”
