Why do female caribou have antlers?

Biologists have long wondered why caribou are the only deer in the world in which females, like males, have antlers.

A study of shed antlers collected from calving grounds in the Arctic National Wildlife Refuge provides a new answer.

Calving grounds are areas where migratory females give birth every year and also where they shed their antlers. Researchers at the University of Cincinnati found evidence that caribou, particularly moms with newborns, gnaw on antlers that were shed years earlier to supplement their diets with crucial minerals.

The study was published in the journal Ecology and Evolution.

Associate Professor Joshua Miller and doctoral graduate Madison Gaetano at the University of Cincinnati studied antlers collected from the Arctic National Wildlife Refuge of Alaska, home to the Porcupine Caribou Herd, which is famous for its epic 1,500-mile round-trip migration.

Antlers are made of bone that grows from the top of the skull The antlers of male caribou can stretch four feet and weigh as much as 20 pounds each, although a female’s are far smaller.

In the cold and dry climate of the Arctic tundra, shed antlers can sit undisturbed for hundreds of years, providing a ready source of minerals such as calcium and phosphorus for foraging caribou at a key time of their epic migration.

Miller collected antlers and bones during scientific expeditions to the Arctic Refuge between 2010 and 2018. He used a rigid inflatable raft, setting up camps with a portable electric fence to ward off curious bears. During the expeditions, it was clear that most of the antlers had been chewed on, but which animals were doing the chewing?

Back in Miller’s lab at UC, researchers examined the tooth marks left on the antlers and bones to identify the culprits. When carnivores such as bears and wolves chew on bones, they leave distinct patterns of damage compared to animals such as lemming or caribou.

UC researchers found that caribou are the prime culprits, chewing antlers they find a little at a time starting at the tips of the tines.

The study found that 86% of the 1,567 antlers they examined showed signs of gnawing and 99% of the gnaw marks were left by caribou.

“We knew that animals gnawed on these antlers, but everyone assumed they were mostly rodents. Now we know it’s really caribou. My jaw dropped when our results started to become clear,” he said.

Researchers observed marks from rodent teeth on less than 4% of gnawed antlers. And they found no evidence of carnivore gnaw marks on antlers in the study.

Researchers also collected 224 skeletal bones from caribou, moose and musk ox in the study area. And unlike the antlers, many of the gnaw marks on these bones were from predators such as wolves and  bears. Caribou gnaw marks were observed on about 12% of the sample bones while just 1% of gnaw marks were from rodents.

Patrick Druckenmiller, a professor from the University of Alaska Fairbanks and Director of the University of Alaska Museum of the North, and National Park Service program manager Eric Wald also contributed to the project. The research was supported with grants from the U.S. Fish and Wildlife Service, the National Geographic Society, the National Science Foundation, the UC Office of Research and the Animal Welfare Institute.

Biologists often point to antlers as a tool for females to defend the choicest grazing spots from other caribou or to ward off predators. But Miller said the role shed antlers play in supplementing a caribou’s diet is an overlooked benefit.

Migrating females collectively drop their antlers within days of giving birth. In this way, females carry their own nutritional supplement that becomes available where and when they need it most.

“These antlers last for centuries or longer in the Arctic and they are a source of nutrients that get revisited again and again. Given the results of our study, this is probably an important clue to a way that antlers benefit female caribou that has gone underappreciated,” he said.

Gaetano said antlers certainly could provide more than one benefit to female caribou. But female caribou are more likely to use their hooves against predators. Reindeer herders she spoke to said their go-to defense is to trample and kick. 

Meanwhile, their antlers can be very small, she said, making them unlikely weapons.

“I think it's reasonable to question how helpful they would be in fighting off a predator,” she said.

“Female caribou shed their antlers right around when they give birth,” Gaetano said. “That means they are antlerless when it would be most crucial to have antlers to defend a young calf if they were a defense mechanism.”

Eventually, over the span of centuries, the minerals from the shed antlers return to the soil where the nutrients help support sedges, grasses and lichens the caribou eat.

“They’re engineering this habitat, seeding the landscape with these super-important minerals that can be quite hard for animals to get enough of,” he said. “Phosphorus in particular is very important for new mothers trying to produce high-quality milk for feeding their young. Caribou bring literally tons of phosphorus to their calving grounds every year.”

Miller said many mammals are known to supplement their diet by gnawing bones, eating clay and salt or drinking from mineral-rich pools.

“It is fairly ubiquitous. I’ll never forget watching a kangaroo eat a dead bird in Australia,” he said. “Herbivores look for nutrients in all kinds of interesting ways.”

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Journal

Ecology and Evolution

DOI

10.1002/ece3.72444 

Wolf reduction boosts caribou survival—but only in rugged terrain

 Reducing wolves to protect endangered caribou doesn't always deliver the expected results—and the shape of the land may be the deciding factor.

That’s according to research led by doctoral student Tazarve Gharajehdaghipour and professor Dr. Cole Burton in UBC's faculty of forestry and environmental stewardship, which examined newborn caribou survival in Itcha Ilgachuz Park in west-central B.C.

Using GPS collars to track animals, the team found that B.C. wolf removals boosted calf survival in steep, mountainous terrain, but made no difference in flatter terrain.

"This study is a note of caution," said Dr. Burton. "Different herds face different conditions. Wolf control may not be reducing calf mortality as effectively as we once thought."

How land and predators affect calf survival

The key factor is which predators can catch calves at different ages. Bears and wolverines hunt newborn calves in the first two weeks, but can’t catch older, more mobile calves. Wolves, by contrast, can catch the older, faster calves.

In rugged areas, this makes a big difference. Bears and wolverines can kill newborns at high elevations, but by the time mother-calf pairs move down to valleys at around three weeks old, the calves are too fast to catch. At this stage, wolves become the main threat.

Before wolves were removed, most calf deaths occurred during this period. Removing wolves eliminated this later cause of death, boosting survival by 41 percentage points—because bears and wolverines cannot hunt the older calves effectively.

In more accessible areas, wolves can reach calving sites throughout the season, including early when calves are young and vulnerable. When wolves were removed, other predators, such as bears and wolverines, simply killed more of these young calves, so overall survival did not improve.

Tracking calves through mothers’ movements

This is the first study to use GPS collars to follow newborn mountain caribou survival. The research builds on tracking methods developed by co-author Dr. Marie Auger-Méthé, a UBC associate professor in the Institute for the Oceans and Fisheries and department of statistics.

They found clear patterns: mothers abruptly start moving less when giving birth to stay close to newborns, then gradually increase activity as calves get stronger, or quickly return to normal movement if a calf dies.

"This method lets us watch the critical first four weeks of life, when calves are most at risk. We also used camera traps to see when predators were on the calving grounds," said Gharajehdaghipour.

The caribou collar data, which this research paper is primarily based on, was collected by B.C. government biologists, and secured through open access. 

Conservation beyond predator control

The research also found that calf deaths before wolf reduction—likely caused by wolves—were linked to horseback and ATV trails and treed valleys. This suggests wolves use these areas to reach calves. Limiting trail development could help reduce wolf-caused deaths in places where wolf control isn’t used.

While B.C.'s wolf reduction program now covers 15 caribou herds, Dr. Burton emphasizes it’s critical to protect habitat.

"If wolf control is sometimes ineffective and diverts attention from habitat restoration, it's a real concern," he said. "Without recovering habitat, you'd have to keep controlling wolves indefinitely."

The researchers recommend that wildlife managers consider the landscape, predator communities, and habitat conditions before starting wolf control programs. They also encourage using GPS tracking and camera traps to better understand calf survival across B.C.

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Journal

The Journal of Wildlife Management

DOI

10.1002/jwmg.70143 

Pumas putting Patagonian Penguins at risk

 Summary:

• Some Argentinian penguins are experiencing high levels of predation from pumas recolonising their historical territory. A new study has quantified the risk on long-term penguin population survival.

• Over four years, pumas at a national park on the Argentinian Patagonia coast are thought to have killed over 7,000 adult penguins (7.6% of the colony’s adult population) – but left many uneaten.
• Long-term, however, puma predation alone is unlikely to threaten colony viability, while low breeding success and reduced juvenile survival appear to be greater threats to the survival of these penguins.
• The findings have been published today (5 February) in the Journal for Nature Conservation.

 

Should we protect an emblematic species if it may come at the cost of another one – particularly in ecosystems that are still recovering from human impacts? This is the conservation dilemma facing Monte Leon National Park, on the Patagonian coast in Argentina.

Since cattle ranching was abandoned in southern Argentina in 1990, pumas (Puma concolor) have been recolonising their historical ranges in this area. For the first time, this brought them in contact with Magellanic penguins (Spheniscus magellanicus) who had moved to the mainland from nearby islands due to the absence of terrestrial predators. Being largely defenceless, the penguins became an easy target for the large carnivore – but it was unknown up to now the exact impact the pumas were having on penguin population numbers.

Since its creation in 2004, researchers from the Centro de Investigaciones de Puerto Deseado of the Universidad Nacional de la Patagonia Austral and rangers from the Monte Leon National Park have monitored the penguin populations in the Park. Over four years (2007-2010), they counted the number of carcasses caused by puma predation. In the new study, they collaborated with researchers from Oxford University’s Wildlife Conservation Research Unit (WildCRU) to analyse the data.

Based on the numbers of penguins found killed, the research team estimated that more than 7,000 adult penguins were killed over the four-year study, most of which were not fully eaten, indicating that not all the penguins were killed for food. This is roughly 7.6% of the adult population (around 93,000 individuals).

Lead author Melisa Lera, a postgraduate student at WildCRU, Oxford University said: “The number of carcasses showing signs of predation we found in the colony is overwhelming, and the fact that they were left uneaten means pumas were killing more penguins than they required for food. This is consistent with what ecologists describe as ‘surplus killing’. It is comparable to what is seen in domestic cats when prey are abundant and/or vulnerable: ease of capture can lead to cats hunting more birds, even when they do not end up actually eating them. We needed to understand if the penguin colony’s persistence could be threatened due to this behaviour.”

However, when the team carried out modelling on the data, this indicated that pumas alone were unlikely to drive the colony at Monte León Park to extinction. Instead, the colony’s future appears to be far more sensitive to factors such as reproductive success and juvenile survival. Population extinction was projected only under hypothetical scenarios combining low juvenile survival (with around 20% failing to reach adulthood) and very low reproductive output (a maximum of one chick per pair). High puma predation was predicted to exacerbate these outcomes.

Study co-author Dr Jorgelina Marino (WildCRU, Oxford University) said: “This study captures an emerging conservation challenge, where recovering carnivores are encountering novel prey. Understanding how these dietary shifts affect both predators and prey is essential to inform conservation.”

Since the models identified breeding success and mortality of the juveniles as some of the key determinants of population viability, the authors highlight the need to understand how environmental factors such as nutrients, food and temperature - known to be influenced by climate change - may affect the penguins’ reproductive success.

As terrestrial predators continue to expand into coastal environments, further mainland colonies of seabirds and other coastal species may become vulnerable. For instance, non-native feral hogs are now key predators of loggerhead sea turtle eggs along the Georgia coast, USA, whilst coyotes in eastern North America are colonising coastal barrier islands, with impacts across these ecosystems.

The authors emphasise the need for sustained monitoring to detect demographic declines early and to guide management actions to prevent severe ecological consequences. The Park authorities continue to monitor Puma and Penguin populations.

The mere smell of predators is enough to change deer behavior

New research shows that the mere smell of predators is enough to change deer behavior and limit browsing damage to tree saplings. The findings offer a potential tool for forest recovery and highlight the important role large predators play. The research is published in the British Ecological Society’s Journal of Applied Ecology.

Research conducted in the forests of south-eastern Germany, shows that the smell of large predators, like lynx and wolves, is enough to make deer more cautious and spend less time eating tree saplings.

In an experiment that involved adding lynx and wolf urine and scat to plots of saplings, researchers from the University of Freiburg found that plots with the scent of predators, particularly lynx, had less browsing damage to young trees than control plots. Deer visited predator-scented plots less and spent less time foraging when they were there.

Over browsing from deer is a major threat to forest regeneration, damaging biodiversity and causing economic losses for the forestry sector. Mitigating browsing damage usually involves substantial human effort and financial resources.

Walter Di Nicola, one of the lead researchers, said: “At a time when debates around large carnivore conservation often focus on conflicts, our study highlights the benefits these species bring to landscapes. The presence of carnivores, even just their scent, could help reduce the ecological and economic problems associated with browsing from overabundant deer populations.”

The research was conducted in forests where both lynx and wolves have been reintroduced. But the researchers say they would expect similar effects in countries like the UK where these predators have long been absent.

Walter said: “In the UK we would expect similar but probably weaker effects. Deer still have some innate fear of predators, even if those predators have been absent for generations. Where predators return, we expect these responses – and their ecological benefits – to become stronger over time.”

The researchers call for conservation strategies that promote large carnivores in forests as a natural, low-intervention solution to over browsing.

To test the effects of large carnivore smells on deer behavior, the researchers set up experimental plots at 11 locations around the forests. Each location comprised of four plots marked with the scent of wolf, lynx, cow and water (as a control). In each plot 30 saplings were planted.

The researchers regularly monitored the saplings to assess browsing damage and used camera traps to record red and roe deer behavior, such as how long and how frequently they visited each plot.

The researchers found that there was a stronger response to the smell of lynx compared to wolf. They believe this could be due to the ambush hunting strategies of lynx, which tend to stalk their prey from close distances, meaning deer might perceive lynx scent cues as more of a threat.

They also point out that wolves were establishing themselves in the area at the time of the experiment, and the deer had much more experience of the threat of lynx.

The researchers caution that their experiment cannot fully replicate the complexity of natural-predator prey interactions. In their set up they used concentrated predator scents, which were easier for deer to detect than in natural conditions. In the wild, real predator cues are more scattered and unpredictable.

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Journal

Journal of Applied Ecology

DOI

10.1111/1365-2664.70267 

Changes to cougar diets and behaviors reduce their competition with wolves in Yellowstone

A new study shows that interactions between wolves and cougars in Yellowstone National Park are driven by wolves stealing prey killed by cougars and that shifts in cougar diets to smaller prey help them avoid wolf encounters.

The study, published at a time of growing overlap between cougar and wolf habitats in the western United States, found wolves occasionally killed cougars, but cougars did not kill wolves.

Researchers also found that cougars tend to avoid areas where wolves have made kills and stay close to escape terrain, such as climbable trees. Cougars responded to a decline in elk in the park by killing more deer, which they consume faster, leading to fewer interactions with wolves.

Published this week in PNAS, the study draws on nine years of GPS data from collared wolves and cougars and field investigations of nearly 4,000 potential wolf or cougar kills in Yellowstone. The findings suggest that coexistence between wolves and cougars depends less on overall prey abundance and more on prey diversity and the availability of escape terrain.

“In North America and worldwide, carnivore communities are undergoing major changes,” said Wesley Binder, a doctoral student at Oregon State University and lead author of the study. “Our research provides insight into how two apex predators compete, which informs recovery efforts.”

For much of the 20th century, government policies in the United States nearly eradicated both wolves and cougars. Cougar populations began to rebound in the 1960s and 1970s in the U.S. under protection. Wolf reintroduction started in 1995, including in Yellowstone. Both species are now recolonizing much of the western U.S.

“You've had these places that in the last 20, 30 years have had cougars come back, and now wolves are coming back as well,” Binder said. “There are a lot of people asking questions like, ‘What are our ecological communities going to look like now that we have both of these large carnivores back on the landscape?’”

Binder began his doctorate at Oregon State in 2022 after nearly a decade working on cougar monitoring in Yellowstone as part of the Yellowstone Cougar Project. His work included setting up a system of 140 remote cameras in the northern part of the park and catching and collaring cats.

The new study builds on decades of research showing that wolves dominate interactions because they live in packs, while cougars are solitary. Previous studies have demonstrated how subordinate carnivores exhibit a tradeoff with dominant carnivores; they suffer mortalities but also benefit from scavenging their kills. Yet cougars seldom scavenge other carnivore kills and are instead efficient hunters themselves, leading to unclear principles that govern their interactions with wolves.

Findings from the new paper provide some answers:

• Researchers investigated 3,929 potential wolf and cougar kill sites: 852 were wolf feeding events, and 520 were cougar feeding events.
  • Wolves made 716 kills and scavenged 136 times, primarily on elk (542), bison (201) and deer (90).
  • Cougars made 513 kills and scavenged seven times, mainly on elk (272) and deer (220).
• Comparing data from 1998-2005 and 2016-2024 revealed major shifts:
  • For wolves, bison increased from 1% to 10%, and elk declined from 95% to 63%.
  • For cougars, elk dropped from 80% to 52%, and deer increased from 15% to 42%.

These kill site investigations were then used to train machine learning models that used GPS data to predict wolf and cougar kill sites. This allowed researchers to pair all wolf and cougar movements with probable kill sites and identify the drivers of their interactions. They found wolf-cougar interactions were highly asymmetric: 42% occurred at predicted sites where cougars killed prey, and only one happened at a site where a wolf killed prey.

The researchers documented 12 adult cougar deaths from 2016-24, two of which were caused by wolves. In both events, no escape terrain was available, and the wolves didn’t consume the cougars but ate the elk the cats had killed. They recorded 90 wolf deaths during the same period, none of which were attributed to cougars. Most were due to natural causes or human actions.

Co-authors of the paper are Joel S. Ruprecht, Rebecca Hutchinson and Taal Levi of Oregon State’s College of Agricultural Sciences; Jack Rabe of the University of Minnesota and Yellowstone Center for Resources; and Matthew Metz and Daniel Stahler of Yellowstone Center for Resources. Hutchinson is also part of Oregon State’s College of Engineering.

 

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Journal

Proceedings of the National Academy of Sciences

DOI

10.1073/pnas.2511397123