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.”