For animals, ‘Value meals’ are not just about quantity

Animals that hunt or forage for prey can be just as picky as humans about their food, and recent studies show that their choices are based on the quality of food they consume rather than its quantity.

For example, some bugs and spiders have a Spidey sense for regulating their diet to get the nutrients they need without gorging on less nutritious prey. In a recent paper published in PLOS ONE, scientists fed wolf spiders with flies that had been raised on either nutrient-rich or nutrient-poor diets. They found that the spiders hunted both kinds of flies, but chose which ones to eat based on the nutritional quality of the flies, and the spiders’ own dietary history.

Though such carefully controlled experimental diets aren’t always possible in the wild, two other recent studies rely on observations of how changes in food availability affect mammals in oceans and tropical forests.

Unlike wolf spiders, marine mammals are frequently believed to thrive eating any kind of fish, as long as there is enough of it. However, another study published this month in PLOS ONE questions this idea by analyzing the food habits of 11 species of dolphins, whales and porpoises.

These researchers discovered that although some marine mammals ate most kinds of prey, others were more finicky, and only ate certain fast-moving varieties of fish.  Fast-moving fish are more energy-rich as food than slower moving species, but it also takes more effort to catch them. This energy expenditure is, quite literally, the price that marine animals pay for their dinner. According to this study, these preferences appear linked to the muscle performance of the predators- faster moving predators require high-energy prey, while others can afford to be more liberal in their food choices.

Marine life is widely affected by climatic shifts, global warming and fishing, which are causing the populations of high-energy fish to drop and increasing the numbers of low-energy species, a phenomenon the authors describe as “the emergence of junk-food in the ecosystem.” This ‘junk-food’ is likely to have a greater impact on more selective eaters, and the authors suggest that understanding the dietary preferences of these marine mammals could help manage and prioritize conservation efforts.

Changing weather also causes land animals to adopt different diets. Several monkey species switch to fallback foods when the foods they prefer are out of season, and a study published today found that these behavioral changes can have effects beyond just simple changes in nutritional value. Scientists studying blue monkeys in tropical forests found that levels of a stress marker increased in female monkeys when they had to eat lower-quality fallback foods like leaves instead of their first preference of insects or fresh fruit. The dietary change also affected the female monkeys’ reproductive health, although that of the males remained unchanged.

These and other studies reveal how animals’ dinner decisions aren’t just about quantity but quality, and how changes in their food habits impact their health. Read other PLOS ONE papers to learn about salt-loving flies, how brightly colored birds choose foods with more pigments, why pandas eat bamboo, and more.

Photo credits:

Burger by Ewan-M on flickr, wolf spider by jack_246, dolphin by Brian Branstetter, Cercopithecus by Steffen Foerster 


Schmidt JM, Sebastian P, Wilder SM, Rypstra AL (2012) The Nutritional Content of Prey Affects the Foraging of a Generalist Arthropod Predator. PLoS ONE 7(11): e49223. doi:10.1371/journal.pone.0049223

Spitz J, Trites AW, Becquet V, Brind’Amour A, Cherel Y, et al. (2012) Cost of Living Dictates what Whales, Dolphins and Porpoises Eat: The Importance of Prey Quality on Predator Foraging Strategies. PLoS ONE 7(11): e50096. doi:10.1371/journal.pone.0050096

Foerster S, Cords M, Monfort SL (2012) Seasonal Energetic Stress in a Tropical Forest Primate: Proximate Causes and Evolutionary Implications. PLoS ONE 7(11): e50108. doi:10.1371/journal.pone.0050108

PLOS ONE Zoo: Giant robber crabs, pandas, bonobo apes and more!

Whether it is unusual food habits, fighting over females, or snacking between meals, research published in PLOS ONE this week spans these and more diverse animal behaviors. These studies don’t aim to use animal research to interpret human behavior, but their results might nonetheless evoke navel-gazing for us humans.

For example, one study published today uses GPS trackers to reveal an internal ‘GPS’ in giant robber crabs, the world’s largest land-living arthropod. Researchers tracking the enormous coconut-eating crabs, whose bodies can grow bigger than a man’s head, with legs up to a meter in length, have identified previously unknown navigation and homing skills in this species.

Like GPS units, peculiar food choices are also not unique to humans. An ancient bear’s preference for munching on hard plant materials like bamboo has revealed that it might be the oldest known ancestor of the giant panda. The bear’s fossil tooth and jaw were discovered in Spain and are described in another study.

People and other animals go to great lengths to develop elaborate courtship rituals or even physical features, but few will fight to the death over a female. Researchers have found that one species of Australian grasshopper is an exception. Males of the species will resort to grappling, kicking and biting in aggressive fights, and the behaviors exhibited depend on whether the male is challenging another or defending himself (Watch a video here).

Research on bonobo apes tackles a behavior that is likely more familiar to most of us: contagious yawning. The authors find that just like humans, bonobos are more likely to yawn in response to one another’s yawns when they are more closely related than when they are unrelated to the first yawner, and when the first individual to yawn is a senior member of the group. Among other findings, their results support the idea that senior group members play a key role in affecting the emotional states of others.

Defining emotions in non-human species is always challenging, but researchers are beginning to understand what signs of boredom might look like in at least one species, mink. A study published today shows that mink without enough interesting things to do tend to snack on food treats more often between meals, and lie awake for longer periods of time without falling asleep. Providing animals with sufficient stimulation is considered critical to their well-being, but defining what might be considered adequate is still a challenge. As the authors of this study say, “Many people believe that farm and zoo animals in empty enclosures get bored, but since the animals can’t tell us how they feel, we can only judge this from seeing how motivated they are for stimulation.”

If all these behaviors have you thinking, “They’re just like us!” think again. Understanding the animal behaviors described in these studies is only a first step towards sharing human spaces with them, whether in zoos, on farms or on an island off the Indonesian coast.


Krieger J, Grandy R, Drew MM, Erland S, Stensmyr MC, et al. (2012) Giant Robber Crabs Monitored from Space: GPS-Based Telemetric Studies on
Christmas Island (Indian Ocean). PLoS ONE 7(11): e49809. doi:10.1371/journal.pone.0049809

Abella J, Alba DM, Robles JM, Valenciano A, Rotgers C, et al. (2012) Kretzoiarctos gen. nov., the Oldest Member of the Giant Panda Clade. PLoS ONE 7(11): e48985. doi:10.1371/journal.pone.0048985

Umbers KDL, Tatarnic NJ, Holwell GI, Herberstein ME (2012) Ferocious Fighting between Male Grasshoppers. PLoS ONE 7(11): e49600. doi:10.1371/

Demuru E, Palagi E (2012) In Bonobos Yawn Contagion Is Higher among Kin and Friends. PLoS ONE 7(11): e49613. doi:10.1371/journal.pone.0049613

Meagher RK, Mason GJ (2012) Environmental Enrichment Reduces Signs of Boredom in Caged Mink. PLoS ONE 7(11): e49180. doi:10.1371/journal.pone.0049180

Photo credits: 

top: Giant robber crabs on Christmas Island, Bill S. Hannson, Jakob Krieger, Steffen Harzsch

below: Empathetic yawning in bonobos, Elisa Demuru