Science Update: Habitat Preferences of Desert Bighorn Sheep

I have no real personal connection with bighorn sheep (Ovis canadensis). I’ve never seen one, eaten one, and know relatively little about them. Perhaps because of this lack of opportunity to interact with them on some personal level, I’m somewhat fascinated by them. At least a part of this fascination has to do with some pretty remarkable life history, physical characteristics, and habits of the species. I’ve also been reading some pieces by Canadian biologist Valerius Geist in the last little while. Geist spent a great deal of time studying bighorn sheep and I recently bought his book Mountain Sheep and Main in the Northern Wilds, so maybe this post is just the result of the convergence of a few individual interests and information trails. In any case, I came across a recent study on the habitat preferences of female desert bighorn sheep and found that it offered an interesting glimpse into the lives of these species.

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Desert bighorn sheep. Source: U.S. National Park Service

Wild sheep arrived in North America sometime around 750,000 years ago, during the Pleistocene (the ice age period that preceded our current epoch, the Holocene). There are currently two species of wild sheep in North America, Dall sheep (Ovis dalli) and bighorn sheep, with the latter also comprised of a number of subspecies. Historically, the range of bighorn sheep covered much of the western portion of North America from Canada to Mexico. As with many other large mammal species on this continent, wild sheep population abundance and range have fluctuated throughout their history, and much of this has to do with the availability of suitable habitat. Though wild sheep are listed as “least concern” by the International Union for Conservation of Nature and remain unlisted in both Canada and the United States (two subspecies resident to California are listed as “endangered” under the U.S. EPA), the Wild Sheep Foundation continues to work on a variety of initiatives and programs to enhance sheep habitat and distribution throughout wild sheep range. Currently, bighorn sheep still exist across their historic range, but their numbers and the extent of continuous populations has been fairly dramatically reduced.

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Historic range of bighorn sheep from 1850-2012. Source: Wild Sheep Foundation

It’s perhaps not difficult to see why someone would be intrigued by this animal. I’ve never been a fan of the “man vs. wild” or “conquering nature” discourses, but there is something primally attractive about the prospect of being able to navigate and survive in the kind of perilous places wild sheep live that makes hunting them somewhat irresistible to me. Wild sheep live in some of the most precipitous habitat on this continent, generally avoiding predators by spending their time in terrain so steep, rocky, dangerous, and difficult to navigate that it is virtually inaccessible to other species, including many of their predators. As with many other ungulates, male sheep (rams) use their thick, curled, sometimes 30-pound horns to fight one another. I’ve heard that sheep can deliver blows with their horns with a force 40 times what it would take to fracture a human skull.

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Rams fighting. Source: Pinterest

It’s long been understood that the habitat selection preferences of wild sheep is an important factor in their ability to avoid predation, including protecting their young (lambs) from these risks. Predation is the leading cause of mortality in neonate (newborn) ungulates, so appropriate habitat selection by pregnant females is critical to neonate survival. For instance, the amount of visibility within a habitat is an important factor in a female’s ability to avoid predation, particularly during times when young are less mobile and therefore are highly susceptible to predation. Areas of low visibility (e.g. high shrub cover) reduce a predator’s ability to see young, whereas areas of high visibility, while allowing increased visibility for predators, also allow females to detect predators. A new paper published in The Journal of Wildlife Management, Desert bighorn sheep lambing habitat: Parturition, nursery, and predation sites, conducted by researchers in New Mexico has provided some new insight into the parturition and nursery habitats used by female sheep (ewes).

Existing knowledge of desert bighorn sheep parturition habitat had been largely based on observation of the presence of lambs – the belief was that the locations in which lambs were observed was likely reflective of the kind of habitat ewes use for lambing. However, through the use of radiocollars and implants that detect when ewes gave birth, the researchers able to identify more precisely the differences in parturition and nursery habitats used by ewes. It was previously thought that both parturition and nursery habitats were in areas of steep, rugged terrain, high elevation, and high visibility. In contrast, the current study found that parturition sites were more likely to occur in habitats at intermediate slopes and intermediate elevations, whereas nursery sites were more likely to be located in areas with steeper slopes. In terms of visibility, parturition sites were more likely to be located in areas of low visibility and nursery sites were associated with habitats at both ends of the visibility spectrum (but not intermediate visibility).

Bighorn lambs are only immobile for 2-3 days after birth, but are classified as a follower species. On a hider-follower classification, a “hider” species leave their young to hide from predators, whereas “follower” species are mobile earlier after birth and are able to escape from predation. Therefore, this study indicates that ewes and their lambs move from parturition sites to nursery sites at higher elevations and steeper slopes shortly after lambs are born to occupy habitat less accessible to predators. It’s pretty amazing that only days after birth, here is a species that is capable of moving deeper into habitat that predators such as mountain lions find difficult to navigate.

Again, while I don’t have any personal experience with bighorn sheep at this point in my life, I find them to be a fascinating and remarkable species and one I hope to connect with more directly at some point. As someone who is also interested in new knowledge on wildlife, I found this paper particularly interesting as it is the first study to definitively describe desert bighorn sheep parturition habitat characteristics. It’s sometimes easy to think that the answers to all of our questions are simply one Google search away, and it’s exciting to realize that just as we are still discovering entirely new species, we are also uncovering details of species that we have interacted with for thousands of years.

Science Update: Another Chapter in Understanding Coyote Predation on White-Tailed Deer

So here’s the deal with this post. I don’t believe that science is detached from the social, cultural, and political implications of the knowledge it produces; however, these posts are intended to specifically focus on recent updates in scientific knowledge concerning species that hunters might be interested in. In an effort to keep these posts focused and concise, therefore, this post is a two-parter. The research paper I’m talking about here relates to a hotly debated and highly emotive issue, so I felt a bit compelled to also address the social and political aspects of the issue in a companion post. Onwards to the science though.

This post is seasonally timely with the Ontario white-tailed deer bowhunt opener on October 1. In a paper published in August 2016, researchers from the Ecological Research Center in Newton, Georgia reported on a study that began in 2003, to investigate the effects of mesopredators on white-tailed deer (Odocoileus virginianus) recruitment. The paper, titled Predator Exclusion As a Management Option for Increasing White-Tailed Deer Recruitment, reports on a study that used fenced exclosures to provide refuge for deer from predators to assess whether exclosures could be used as a management tool to reduce pressure on white-tailed deer from coyote (Canis latrans) and other mesopredator predation, particularly in cases where hunting coyotes is ineffective.

The researchers compared the neonate/adult female (fawn/doe) ratio in deer that used the exclosures to those that did not use the exclosures. Using neonate/adult female ratios gives an indication of recruitment – how many new individuals are brought into the breeding population. Since fawns are most vulnerable to predation, this ratio is generally a good indicator for population trends (i.e. if neonate survival is low, individuals are not surviving to reproduce, and combined with other mortality, this may lead to a decline in the population).

Exclosures are fenced areas that are just the opposite of enclosures: rather than containing a species, they are designed to prevent certain species from entering the fenced area. In this study, the researchers designed the exclosures with electric wire fencing so that deer were still able to jump the fence into the area, but coyotes, foxes (Vulpes spp.), and other predators were unable to get through the fence.

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Grazing exclosure. Source: The Konza-Kruger Experiment

The effects of coyotes on white-tailed deer is one of the most hotly debated barstool topics among hunters. There are those who swear we should kill every coyote we see and there are those who insist that no matter how many coyotes we kill, we are only removing the transient individuals and having negligible effects on the actual local population. There is also the idea that coyotes will change their reproductive rate and behaviour in response to population changes, increasing litter size as populations decline. Then there is also a polarization of opinions on whether coyotes even present any real threat to white-tailed deer populations.

However, this paper notes that the massive population and range expansion of coyotes in North America throughout the last century has exposed white-tailed deer to increasing predation pressure that has affected neonate recruitment. Further, while there is evidence that removal of coyotes has had positive effects on white-tailed deer recruitment, the evidence also suggests that these effects are quite localized and difficult for most landowners to implement effectively. Coyote populations usually span a much larger area than individual hunters or landowners can affect, which means that individual animals that are killed will be replaced by others from the same population. In other words, coyote removal would need to occur over such large areas that for practical purposes at the property level, killing coyotes is generally not an effective tool to increase white-tailed deer recruitment.

The study found that the average neonate/adult female ratio was greater inside the exclosures than outside. Average neonate/adult female ratios within exclosures was 0.19 and outside exclosures this ratio was 0.09. To put this in more direct terms, this means that within exclosures, roughly speaking, there was 1 fawn for every 5 does; outside exclosures, there was 1 fawn for every 10 does. Hunter success also improved after the construction of exclosure areas. The study doesn’t report harvest statistics, so it’s difficult to know if hunters were killing more mature individuals, which would indicate that adult deer were also surviving longer.

In terms of management lessons, this study indicates that reducing predation pressure on white-tailed deer can increase neonate recruitment into the population. However, it’s notable that the effects of predators doesn’t always take the form of increased mortality. Decreased risk of predation can also result in behavioural changes in deer. Wildlife species are constantly balancing risk and reward – for example, how much energy will it take to locate a food source, and will the energy taken in by that food be greater than what was expended to find it. In terms of predation, prey species will seek refuge from predators, so predation risk can affect both foraging and reproductive behaviour (for a notable example of this, revisit the behavioural effects wolves had on elk in Yellowstone Park).

If increasing local deer population is a management goal, there needs to be an increase in neonate survival. In many cases, lethal control of coyotes is ineffective. In these cases, the authors suggest that predator exclosures may provide a viable option to manage predation risk by reducing “the need for energetically costly antipredator behaviors (e.g., increased vigilance while foraging) by providing year-round reduction in predation risk”. If exclosures provide more effective refuge from predators, it is likely that deer will use these areas to escape predation.

While this research is interesting, I can’t help but find larger questions to ask. For instance, while I have great respect and gratitude for the successes of the North American model of wildlife conservation, I still wonder: are we losing some essence of what it means to have wild life on these landscapes by managing them with the use of fences? At the end of the day, we need to find the best way to manage wildlife for the health of a wide variety of species and habitats; however, there’s a subjective element to this that I can’t escape, which makes me feel a bit uneasy about putting fences in wild places (even if those are on private properties). Then again, I can’t very well suggest that we continue to use potentially ineffective management strategies like lethal control of predators simply because it somehow feel more natural, can I? So uncertainties remain.

Science Update: Sensory Functions of a Narwhal’s Tusk

Many of us who are involved in outdoors activities are interested in new discoveries in wildlife science. A good deal of the popular knowledge about wildlife that we use in our hunting and fishing activities comes from scientific research, and indeed we rely on this knowledge to better understand the animals we pursue and to develop effective hunting strategies. Most scientific research is first published in academic journals. There are thousands of journals and subscriptions to them are expensive. This means that first hand research knowledge is largely unaccessible to the vast majority of the public. I’m lucky that I have access to databases full of journal articles through my work. I want to use this series to discuss new and interesting pieces of wildlife research that are of particular interest to the hunting-conservation community.

As a hunter and outdoorsman, I’m fascinated by wildlife and ecology. Not surprisingly, I have a particular interest in understanding everything I can get my hands on about North American wildlife, especially those species that are also important table-fare in various communities. This first post is not a brand new piece of research anymore, and it may be somewhat removed from the regions and species many of us hunt, but I chose this story because it’s a species that is relevant to the areas I work in the Canadian Arctic. Some people may not even be aware of this species’ existence, but it sure is an interesting animal.

Narwhals (Monodon monoceros) are a toothed whale species (odontoceti) roughly 11 million years old. They are a resident Arctic species, meaning that they spend the entire year in Arctic waters, including during winter ice-covered periods when they rely on openings in the ice to come up for air. Their Canadian range is throughout the central and eastern Arctic waters where they move between summer and winter grounds. Narwhals spend more of their time in deep offshore waters, but also move into more shallow fjords during summers. They eat fish, squid, and shrimp, and do much of their feeding under the ice throughout the winter. Narwhals are hunted by Inuit communities as an important food source. Both of the communities I do research in, Kugaaruk and Pangnirtung, Nunavut, have active summer narwhal hunts. I’ve heard from many people that the narwhal hunt is one of their favourite times of year.

naturepl.com : Bryan and Cherry Alexander : WWF

Copyright: naturepl.com / Bryan and Cherry Alexander / WWF

The most recognizable and magnificent feature of narwhals is the long tusk that protrudes directly from the front of the mouth. The tusk is actually an erupted, protruding left canine tooth that spirals forward, growing as long as 2.6 m / 9 ft, with typically only males growing tusks (as an interesting side note, walrus tusks are also protruded upper canines, whereas elephant tusks are actually elongated incisors). It’s not surprising that this species has been the subject of lore and cultural stories throughout human history.

Since narwhals have been studied by hunters, explorers, and scientists, there has been a great deal of uncertainty over the specific function of the tusk. As may be expected, there is evidence that males use their tusks to attract mates through displays of dominance and to fight other males. In 2014, some of the mystery surrounding narwhal tusks was answered. A study examining the sensory functions of the narwhal tusk discovered that the tusk provides a type of direct line of communication between a narwhal’s brain and temperature and chemical changes in the ocean. Like many mammalian teeth, working inwards from the surface, the outer layer of a narwhal’s tusk is covered with a porous cementum, followed by a dentin layer containing tubes that channel in towards the centre of the tusk. In the core of the tooth, running the full length of the tusk, is a pulp layer full of nerve endings that connect to the brain. In other mammalian teeth, the portion of the tooth that is exposed above the gum line is covered with a hard enamel layer. What sets the narwhal’s tusk apart from other mammalian teeth is that the porous cementum layer is exposed, allowing it to detect changes in the surrounding ocean.

These findings indicate that tusks may fulfill a sensory function to help narwhal locate food sources and mates. Diet analyses show that males and females have different food sources for much of the year, overlapping particularly during the spring-summer mating period. The study also suggests that there may be a number of hypotheses for how tusks could be used to find and attract mates. For example, the tusk may be used to detect females in estrous; to detect food sources that are likely to attract feeding females (which is also supported by the fact that diets overlap during the mating period); or as an indication to females that a male with a well developed tusk could be more successful at finding food for calves.

It’s interesting to learn more about the evolutionary purposes of what is already a stunning physical trait in an elusive species like the narwhal. It’s sometimes easy to forget that we are still discovering the natural world every day. We have a great deal of knowledge about wildlife, but there are also many species we have yet to discover, and it’s a guarantee that we still have much to learn about the species we do know. Each of these little pieces we uncover should remind us to remain both humble and in awe at the diversity of species on this planet. More so, it should remind us of the importance in conserving each of these species and maintaining this diversity.