# Ursids evolved dietary diversity without major alterations in metabolic rates

## TL;DR

Ursid bears have evolved diverse diets without significant changes in their metabolic rates, allowing them to adapt to different food sources.

## Contribution

This study reveals that dietary diversity in ursids is achieved without major alterations in energy metabolism.

## Key findings

- Brown bears, polar bears, and giant pandas have similar mass-specific daily energy expenditure despite differing diets.
- Giant pandas save energy through low-cost stationary foraging, enabling their bamboo specialization without altering metabolic rates.

## Abstract

The diets of the eight species of ursids range from carnivory (e.g., polar bears, Ursus maritimus) to insectivory (e.g., sloth bears, Melursus ursinus), omnivory (e.g., brown bears, U. arctos), and herbivory (e.g., giant pandas, Ailuropoda melanoleuca). Dietary energy availability ranges from the high-fat, highly digestible, calorically dense diet of polar bears (~ 6.4 kcal digestible energy/g fresh weight) to the high-fiber, poorly digestible, calorically restricted diet (~ 0.7) of giant pandas. Thus, ursids provide the opportunity to examine the extent to which dietary energy drives evolution of energy metabolism in a closely related group of animals. We measured the daily energy expenditure (DEE) of captive brown bears in a relatively large, zoo-type enclosure and compared those values to previously published results on captive brown bears, captive and free-ranging polar bears, and captive and free-ranging giant pandas. We found that all three species have similar mass-specific DEE when travel distances and energy intake are normalized even though their diets differ dramatically and phylogenetic lineages are separated by millions of years. For giant pandas, the ability to engage in low-cost stationary foraging relative to more wide-ranging bears likely provided the necessary energy savings to become bamboo specialists without greatly altering their metabolic rate.