Avoiding Density Stress with Larger Ponds

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By Benedikt Schmidt

(translated from the original post on Swiss Biodiversity Forum)

Conservationists are happy when there are many tadpoles in a pond. Yet at high density, tadpoles are stressed; they need longer to develop, are smaller at metamorphosis, and survive less well as juvenile toads. To reduce the density stress, we need to build larger ponds.

 

A high number of tadpoles in a pond is pleasing but can cause density stress. Using an experimental approach with the natterjack toad, we show that such density stress extends the larval period up to threefold. Extending the larval period increases the risk of failing to reach metamorphosis before the pond dries up. Also, at high densities the tadpoles are more than 50 percent smaller—and on land small toads survive poorly.

For the highly endangered natterjack toad, these findings are particularly problematic. To reduce density stress for this species, it is important to build large ponds (ideally with an area larger than 100 m²). Also, these ponds should dry up, but not until autumn. This can be achieved by building drainable ponds.

Our findings are also important for population monitoring. In a pond with high tadpole density, the tadpoles are present throughout the summer, providing evidence of reproduction. In a pond with low tadpole density, the tadpoles reach metamorphosis after just 20 days. As a consequence, reproduction may go undetected.

Publication: Weber T, Ozgul A, Schmidt BR (2024) Density-dependent performance of larval and juvenile toads: implications for amphibian conservation. Basic and Applied Ecology.

 

Alpine common toads breed on average a month earlier than 40 years ago

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Animals that reproduce once a year during a short breeding season need to get the timing right. Breed too early and the offspring might be exposed to harsh conditions, breed too late and the offspring might not be able to grow enough to survive the first winter. Generally, one of the major factors determining the timing of breeding is the environment. Therefore, due to climate change, we can expect shifts in the timing of breeding and indeed, various studies show that a shift towards earlier breeding in many animal species can be observed globally. This applies also to amphibian species, but in their case, not much is known about populations living at high elevations. We decided to fill this knowledge gap by looking at how the timing of breeding has shifted in the past 40 years in a population of common toads (Bufo bufo) which lives at almost 2000 masl in the Swiss Alps, just below the Grosse Scheidegg (Figure 1). We expect temperatures and amount of snow cover to play an important role. In fact, only once the snow melts and the breeding pond unfreezes can the toads emerge from hibernation belowground and successfully breed.

Figure 1. Breeding site of our focal population. The pond is located at around 1850 masl, and every year in spring it hosts a large number of common toads, grass frogs (Rana temporaria), and alpine newts (Ichthyosaura alpestris) attempting to breed.

Our results matched our expectations. We found that the warm temperatures and low amount of snow in winter and spring were associated with an earlier breeding. Interestingly, both temperature and snow cover have not been changing necessarily as expected over the past forty years at our study site. We observe great year to year variability, and to this day we can observe both late breeding (mid-late June) and early breeding (late April). Despite this great variability, breeding on average occurs earlier than in the 1980s (Figure 2), with an advancement of about 30 days. Further details and information can be found in our article “Four decades of phenology in an alpine amphibian: trends, stasis, and climatic drivers” published in the diamond Open-Access Peer Community Journal (doi.org/10.24072/pcjournal.240)

Figure 2. Trends of breeding phenology over the study period (1982–2021). (A) First day of the breeding season (day of the year, where January 1st = 1) and (B) date of peak breeding (i.e., date where the highest number of toads were observed in a given breeding season).

 

African Savanna Antelopes Need Habitat Space for Resilience to Climate Changes

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New research shows that for antelope populations in East Africa, it is not just about the weather but where they can roam. This highlights why we need big, connected spaces for conservation.

Environmental changes threaten naturally heterogeneous and dynamic ecosystems that are essential in creating and maintaining a rich, resilient, and adaptable biosphere. In East Africa’s savanna, antelope populations are vital for a healthy and functioning ecosystem. They shape the vegetation, disperse seeds, cycle nutrients, and provide food for other animals. A natural dynamic mosaic of vegetation types, water sources, and weather forms a delicate balance with the antelopes that is more and more disrupted by human influences and climatic changes. To protect these hotspots of biodiversity and enable the ecosystem to work properly, it is vital to maintain healthy antelope populations.

Previous studies have shown that densities of savanna antelopes vary based on location, season, and year, but no empirical studies had ever examined all these effects together. Simultaneously studying how environmental variation over space and time affects the local densities of antelope species could resolve whether location, or seasonal or annual variation is the most important factor driving local densities of these wildlife.

Using seven years of antelope monitoring data from the Tarangire Ecosystem in Tanzania, an international collaboration between the University of Zurich and the Wild Nature Institute examined this question. They found spatial factors explained the largest proportion of variation in density for four of the five antelope species they studied. These spatial covariates included proximity to water and human activities as well as vegetation community—suggestive of both bottom-up (resources) and top-down influences (avoiding natural predators) on local densities. The research was published in the journal Population Ecology.

In the Tarangire Ecosystem, antelopes respond to changing climatic conditions and the fluctuating availability of resources by moving across space. Lead author Lukas Bierhoff, a graduate student in the Department of Evolutionary Biology and Environmental Studies at University of Zurich, said “these results demonstrate that antelopes depend upon water and forage availability, but are flexible in their responses to climatic variation when they have the option to move and seek out the necessary resources for the current conditions.”

Helping antelopes move across space to adapt to climate and habitat changes

As natural savanna habitats and climate are rapidly being altered by human activities, effective conservation strategies are needed to ensure the persistence of antelopes and all the services they provide to maintain healthy ecosystems. “This study provides further evidence that the protection of large, connected areas of different habitat types and permanent water sources are the best way to maintain high biodiversity and a functioning biosphere. Providing habitat options for the antelopes enables them to respond to a temporally changing world by moving across space,” Bierhoff said.

The research team also identified guilds of antelopes whose densities co-varied, and that might respond similarly to targeted and coordinated conservation strategies, thus increasing the efficiency of management actions.

“Effective conservation actions include protecting rivers and other water sources from diversion and pollution; reducing bushmeat poaching; keeping and restoring movement corridors; and maintaining the diversity of natural vegetation types” said Derek Lee, Wild Nature Institute principal scientist and senior author of the paper. “Antelopes are critically important to Tanzania’s economy as well as its ecology, so sustaining thriving populations of these animals is a win-win for people and wildlife.”

Citation: Bierhoff L, Bond, ML, Ozgul A, Lee DE. 2024. Anthropogenic and climatic drivers of population densities in an African savanna ungulate community. Population Ecology. https://doi.org/10.1002/1438-390X.12182

Rising Rainfall, not Temperatures, Threaten Giraffe Survival

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Giraffes in the East African savannas adapt well to warmer temperatures. However, they are threatened by increasingly heavy rainfall.

 

Climate change is expected to cause widespread declines in wildlife populations worldwide. Climate anomalies interacting with human pressures can place additional stress on already declining populations, but little is known about the interactions between climate and anthropogenic effects on large African herbivore species despite the growing importance of these pressures. Giraffes are endangered megaherbivores, but the combined climate and human effects on the survival rates not only of giraffes, but of any large African herbivore species, had not been studied. We concluded a decade-long study – the largest to date – of a giraffe population in the Tarangire region of Tanzania. The study area spanned more than a thousand square kilometers, including areas inside and outside protected areas. Contrary to expectations, higher temperatures were found to positively affect adult giraffe survival, while rainier wet seasons negatively impacted adult and calf survival. The results were published in the journal Biodiversity and Conservation.

 

First exploration into the effects of climate variation on giraffe survival

Our research team quantified the effects of local anomalies of temperature, rainfall, and vegetation greenness on the probability of survival of the giraffes. We also explored whether climate had a greater effect on giraffes that were also experiencing human impacts at the edges of the protected reserves.

Studying the effects of climate and human pressures on a long-lived and slow-breeding animal like a giraffe requires monitoring their populations over a lengthy time period and over a large area, enough to capture both climate variation and any immediate or delayed effects on survival. We obtained nearly two decades of data on local rainfall, vegetation greenness, and temperature during Tanzania’s short rains, long rains, and dry season, and then followed the fates of 2,385 individually recognized giraffes of all ages and sexes over the final 8 years of the two-decade period.

Masai giraffes in a heavy downpour, Tanzania. Photo credit: Derek Lee

Surprising effects of temperature on giraffe survival

We had predicted that higher temperatures would hurt adult giraffes because their very large body size might make them overheat, but higher temperatures positively affected adult giraffe survival. This is because the giraffe has several physical features that help it to keep cool, like long necks and legs for evaporative heat loss, specialized nasal cavities, an intricate network of arteries that supply blood to the brain, and they radiate heat through their spot patches. However, temperatures during our study period may not have exceeded the tolerable thermal range for giraffes, and an extreme heat wave in the future might reveal a threshold above which these massive animals might be harmed. So we will continue to monitor this population.

 

Heavy rains may increase parasites while reducing nutritional value of vegetation

Survival of giraffe adults and calves was reduced during rainier wet seasons, which we attributed to a possible increase in parasites and disease. A previous study in the Tarangire region showed giraffe gastrointestinal parasite intensity was higher during the rainy seasons than the dry season, and heavy flooding has caused severe outbreaks of diseases known to cause mortality in giraffes, such as Rift Valley Fever Virus and anthrax. The current study also found higher vegetation greenness reduced adult giraffe survival, potentially because faster leaf growth reduces nutrient quality in giraffe food.

 

Human pressure place additional stress on already declining populations

Climate effects were exacerbated by the giraffe’s proximity to the edge of protected reserves, but not during every season. Our findings indicate that giraffes living near the peripheries of the protected areas are most vulnerable during heavy short rains. These conditions likely heighten disease risks associated with livestock, and muddy terrain hampers anti-poaching patrols, leading to increased threats to giraffe survival.

Climate anomaly effects on seasonal survival of adult Masai giraffes in Tarangire Ecosystem, Tanzania 2012‒2019. MeanDistPA is mean distance from the edge of the protected area (km). M = male; F = female.
Rainfall anomaly effects on Masai giraffe juvenile seasonal survival from the Tarangire Ecosystem, Tanzania 2012‒2019. PA distance is mean distance from the edge of the protected area (km). M = male; F = female, and U = unknown sex.

We concluded that projected climate changes in East Africa, including heavier rainfall during the short rains, will likely threaten persistence of giraffes in one of Earth’s most important landscapes for large mammals, indicating the need for effective land-use planning and anti-poaching to improve giraffes’ resilience to the coming changes.

The paper is available for download at https://doi.org/10.1007/s10531-023-02645-4

Climate change threats to a short-lived primate

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Climate change changes temperature and rainfall patterns, particularly in tropical environments, resulting in consequences for the persistence of wildlife populations. However, the complexities of the effects of changing temperature and rainfall on tropical mammals is not well studied. Together with our colleagues, we studied the demography of the gray mouse lemur in western Madagascar using data collected by the German Primate Center between 1994 and 2020. Climate trends in the lemur’s environment show declining rainfall over the wet season and increasing temperatures in the dry season. These climate trends, we found, led to decreased survival rates as well as increased reproductive rates. The competing demographic trends have prevented population collapse, but have destabilized the population by further speeding up their life cycle which was already one of the fastest among primates. Population projections into the next five decades suggest that when recently observed temperature and rainfall conditions persist, the population size of lemurs may continue to fluctuate, putting the population at increased risk of extinction. Our results show how, even though short-lived mammal species with high reproductive rates are expected to rapidly adapt to changes in their environment, such species can still face threats of extinction from climate change.

Link to the article:  Ozgul A, Fichtel C, Paniw M, Kappeler PM (2023) Destabilising effect of climate change on the persistence of a short-lived primate. Proceedings of the National Academy of Sciences USA

How People, Food, and Water Affect Large Herbivore Distribution In East African Savannas

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To survive, animals must find nutritious food and drinking water—sometimes during long dry seasons or cold periods—and at the same time avoid being eaten. Plant-eating mammals with hooves for feet are an extraordinarily diverse group of animals and are critically important in East African savannas. Yet they must compete more and more with humans for space in a fast-changing world while also evading hungry lions, leopards, and other natural predators. A new study by scientists from the University of Zurich’s PopEcol group and Pennsylvania State University, published in the Journal of Mammalogy, investigated the habitat needs of a community of hooved-mammal species in the Tarangire Ecosystem of northern Tanzania, and how vegetation, water, presence of humans, and risks from predators influenced their use of these habitats.

This was the first study of its kind in the Tarangire Ecosystem, which supports the ecotourism hotspot of Tarangire National Park and is the heart of Maasailand where cattle herders and wildlife have thrived together for centuries. Tarangire differs from other areas where wild ungulates have been intensively studied—like Serengeti National Park or Kruger National Park—in that Tarangire’s wildlife, cattle-keeping people, and farmers all share the landscape, and animals can move unimpeded because the entire region is unfenced.

“Ungulates of different body sizes have different needs and threats,” said the study’s lead author Nicholas James, who conducted the research as a graduate student at University of Zurich. For instance, large ungulates like adult giraffes may have less to fear from natural predators but may face more danger from humans, and smaller animals may have more specialized food requirements. “We wanted to know what features draw each ungulate species to certain areas so we can pinpoint important habitat for each of those species,” James said. This information is important for land managers to maintain thriving populations of wild ungulates and keep the landscape healthy, which is the foundation of Tanzania’s important ecotourism economy.

James and his co-authors counted and mapped six hooved mammal species in dry and rainy seasons over seven years in and around Tarangire National Park and the adjacent Manyara Ranch Conservancy, including unprotected village lands. The ungulates studied included the iconic, massive giraffe down to the little dik-dik—both of which specialize on eating leaves of woody plants—as well as the large, water-loving, grass-eating waterbuck, and three medium-sized antelopes that eat both woody-plant leaves and grass, the impala, Thomson’s gazelle, and Grant’s gazelle. The scientists looked at how the different species used areas depending on the type and greenness of plant food, the thickness of the bushes (where lions often lurk), and how far the areas were from rivers (which provide vital drinking water but also hide predators) and cattle herder settlements (where human disturbance is higher but the humans also keep away predators). The study highlighted the importance of food (vegetation) for all species, as well as nearness to year-round rivers for most but not all. Some species appear to be tolerant of human presence and even congregated close to cattle herder settlements, presumably because of lower predator densities there. The researchers found that antelopes that ate both grass and woody-plant leaves allowed them to avoid areas with high human activity while meeting their dietary needs. Importantly, the presence and number of herbivores were sensitive to short and long-term variation in rainfall suggesting they are vulnerable to drought.

“We show that the focus of research and management should be directed towards the Tarangire Ecosystem’s free-flowing rivers and associated habitat along those rivers,” said Derek Lee, associate research professor at Pennsylvania State University and senior author of the study. “In dry landscapes like East African savannas, water resources are increasingly monopolized by humans, so protection of waterways in human-dominated landscapes, and ensuring sufficient access for wildlife is of primary conservation importance.” Another key finding of the study was that traditional cattle herders and some ungulate species can share the same space and thus appear to be compatible, so long as the human impacts remain relatively low.

Leaving by Staying: Dispersal Decisions of Young Giraffes

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Dispersal, the process where animals reaching sexual maturity move away from family, is important for maintaining genetic diversity and is key to the long-term persistence of natural populations. For most animals, this involves having to make risky journeys into the unknown in the hope of finding new communities in which to settle and reproduce. However, many animal societies—including those of humans—have structured social communities that overlap in space with one-another. These potentially provide opportunities for maturing individuals to disperse socially without having to make large physical displacements. New research published today in the Journal of Animal Ecology shows that this strategy is employed by young dispersing giraffes.

The process of moving away from family is known as natal dispersal. Dispersal is a fundamental biological process that has been shown to reduce the chances of mating with a relative, ensuring that individuals have healthy offspring. However, dispersal is first and foremost a social process. Nevertheless, it has been mostly studied as a spatial process because in most animals, families defend physical areas excluding others, forcing young to have to leave this area to establish their own family. A research team, led by University of Zurich (UZH) postdoctoral research associate Dr. Monica Bond, tested whether animals that live in structured societies comprising social communities that overlap in space with one another could disperse simply by switching communities. Doing so would avoid the risks of moving through the unknown.

The researchers studied a large population of hundreds of giraffes in northern Tanzania. Using data on group composition collected over a huge 2200 km2 area, the team found that most male giraffes leave home once they reach reproductive maturity, and that a significant proportion of these achieve their dispersal by simply switching to new social communities, thereby avoiding the risks of moving far from home. On the other hand, most young female giraffes remained within the same community into which they were born. While sex differences in natal dispersal are well established in animals, this study is amongst the first to demonstrate how living in a structured society provides a unique opportunity for maturing individuals to find a new social community without having to move to new areas.

Complex Giraffe Societies

The team of scientists from UZH and Penn State University previously documented that the adult female giraffes form distinct social communities. The membership to these communities, comprising about 60 to 90 individual females, is very stable over time, despite social groups that are made up of these members merging and splitting throughout each day. They found that these social dynamics have two major consequences. The first is that females maintain enduring social bonds with other females in their community, with bonds likely to last over their entire lifetimes. The second is that these communities are completely structured socially, with different communities using the same physical space. Thus, while individuals from different communities might occasionally encounter one another, they rarely, if ever, form groups together.

“This led us to wonder whether maturing young giraffes might forge relationships with the members of nearby female communities that are different from their birth community, to avoid accidentally mating with their relatives, without having to travel long distances into unknown and possibly dangerous places,” says Dr. Damien Farine, co-author and UZH Eccellenza Professor.

What they found was that, like in most other mammals, dispersal was predominately done by males, with dispersers leaving at about 4 years of age. “The key question was then to ask what strategies young males used to find new communities in which they could search for unrelated mates or avoid conflict with relatives,” says UZH professor Dr. Barbara König, senior author of the study.

Females Stay in the Same Social Networks, Males Switch

The team used social network analyses to quantify the social communities of adult females, and then monitored which community 67 male and 70 female calves associated with as they matured over a 7-year period. The data revealed that while four out of five of young male dispersers switched to social communities different from their birth communities, about one in four of the male dispersers switched communities while staying relatively close their birth site. In other words, they were able to disperse without having to move far at all.

“This type of social dispersal, where males remained close to home but joined different female communities, would not be detected if only spatial movements were measured,” says Bond.

Giraffes may not be unique in being able to disperse socially without having to move away from home. In many other species, including dolphins, elephants, and bats, researchers have reported merging and splitting of groups—called ‘fission-fusion’—within a larger, more stable social community. “It would be interesting to see if dispersing socially within the same physical space is a common strategy that is employed in species that live in societies with many overlapping social communities” Bond says. “Given the importance of maintaining healthy populations, the more we understand the natal dispersal process, the better we can help conserve wildlife.”

According to giraffe expert Dr. Fred Bercovitch, who was not part of the study: “This research has crucial implications for the conservation of giraffes because it demonstrates that the preservation of genetic diversity in giraffes requires saving large ecosystems that allow animals to disperse into different communities, and not the translocation of a handful of giraffes to a new area, where breeding opportunities are limited.”

How to deal with the growing wolf population in Switzerland?

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The newspaper NZZ am Sonntag recently published an interview with our in-house predator specialist and movement ecology group leader Gabriele Cozzi about the current wolf situation in Switzerland and the associated research opportunities. The interview was conducted by Atlant Bieri and originally published in german. Below, we provide an english translation.

 

NZZ am Sonntag: Mr Cozzi, this summer a shepherdess and her dog were growled at by a wolf in Graubünden. There have also been more encounters between humans and wolves elsewhere recently. How dangerous is that?

Gabriele Cozzi: In such an encounter, a wolf possibly growls because it is frightened and thus signals its presence. In its language, this means: “Don’t come near me!” If you follow this signal, nothing can happen. In this situation, a wolf would only attack if it feels attacked and sees no escape options. But that hardly ever happens, you almost have to step on its tail.

 

As a defence, the woman called loudly and made herself appear as big as possible. The wolf then left. Does that always work?

She reacted correctly and used the language of the wolf. A loud voice is equivalent to growling. It means: “I am strong and could be a problem for you. You better leave me alone.” The louder you are, the more dangerous you appear to the wolf. For him, this means he could hurt himself unnecessarily if he attacks.

 

Is it also useful to stand up?

Yes, body size is important. In wolf language, it is synonymous with strength. That’s why you should not crouch or bend down to pick up stones as a weapon. It is better to move your hands above your head or to break branches from trees and waving them. But one should not directly attack the wolf. He might think that it is now too late to flee, and he must fight back.

 

Why is running away not a good idea?

It activates a hunting instinct in the wolf: the image of a fleeing prey. This can be observed very well in cats. A mouse dummy that does not move is ignored. Only when you let it slide across the floor does the cat’s hunting instinct kick in and it strikes. It’s the same with wolves. If I run away, I am the prey. In an encounter, the following message must always reach the wolf: a person is something big that is potentially strong and dangerous. Full stop. Then nothing will happen. You can walk slowly backwards, but always keep your eyes on the wolf without staring.

 

Why not?

Staring signals a challenge: whoever averts their gaze first submits. In an encounter, one should neither enter a competition nor be submissive, but rather remain calm and show self-confidence.

 

On another day, the same woman was surprised by three wolves at once. This time the animals attacked the woman’s dog. Why do wolves behave so aggressively towards dogs?

Wolves see dogs as nothing more than another wolf, but a wolf that is degenerate and weak. Moreover, he is an intruder in their territory. That is why dogs rather than their owners are attacked.

 

Sometimes, however, this also means that the dog is killed.

Yes, that can happen. Large predators are not squeamish about direct competition for food. It also happens that wolves kill other wolves. In Africa, lions kill hyenas without eating them afterwards. It is simply a matter of eliminating the competition. Basically, predators don’t like other predators.

 

In August, a group of hikers in Sufers came across two adult wolves that approached within a few metres. Later, the pups also followed the hikers. Why the approach and the subsequent pursuit?

In Africa, zebras are often seen walking directly towards a pride of lions. That means: “I see you and know what you’re doing.” It’s the same with wolves. Especially with parents, they may scout us to find out if we are a danger to the offspring.

With puppies it’s different. For them, everything is a game. They go after anything that moves. I have seen young African wild dogs chasing giraffes. This can be very dangerous for the young dogs, but they need to make experiences to understand it.

 

In North America, the authorities advise hiking tourists to always carry bear spray (pepper spray) with them. Would that also be conceivable in Switzerland?

Absolutely. It works very well in North America. All it takes there is a short blast and the wolf would takes off. Nobody likes to have pepper spray in their face. Using bear spray would only require a small change in our way of thinking. But maybe there is still some resistance at the moment.

 

Wolves seem to appear in settlements for no reason. Why?

You have to distinguish between migrating individuals and resident packs. Migrating individuals travel long distances every day and don’t really know where they are going. They don’t want to go to the city for sure. But in the densely populated landscapes, they only have to turn left once instead of right and they’re already in a village.

Packs, on the other hand, know their territories very well, and if they find something to eat near settlements, they are very likely to come back. This was a problem in Vättis, for example, where the Calanda pack often appeared. There, meat bait had been put out for fox hunting. The wolves quickly learned that they could easily get food here.

 

What does that mean for settlements?

Don’t leave food lying around. This can quickly lead to problems and unwanted encounters. In Turkey, where I researched bears and wolves, there is a town with a big rubbish dump. Bears and wolves come there every night to feed. It’s like a McDonald’s for them.

 

If wolves approach a settlement too often, they are declared problem wolves and can be shot. Would there be an alternative to this?

You can scare the animals away with rubber shot. But such measures, where the animals have to learn something, always take time. It is clear that this method would be very time-consuming for the gamekeepers and therefore not justifiable everywhere. Ultrasound could be used, similar to a cat or marten deterrent. In recent years, attempts have also been made to restrict the movements of predators by placing urine and droppings of conspecifics in strategic locations. We have to learn to be creative here.

 

Farmers in mountain areas have the same problem. They want to keep the wolf out of their pastures. Would there be more possibilities than fences and guard dogs?

We should try to exploit the technological possibilities. GPS transmitters, for example, could help to record the movements and preferred locations of resident packs. In the alpine region, packs have territories of about two hundred square kilometres. But certain locations are only heavily used during certain times of the year, such as the breeding season. The more information we have, the easier it will be to develop preventive measures.

 

There are currently about a hundred wolves in Switzerland, and the population is growing. Will more farm animals be killed every year?

The number of farm animals killed will probably increase. That is pure mathematics. But the number will not necessarily increase linearly: twice as many wolves does not necessarily mean twice as many lost farm animals. This because in Switzerland there are many wild animals such as deer, roe deer, chamois and wild boar, which play a much bigger role as prey than our sheep and goats.

 

But last year even a donkey was killed by wolves. Isn’t the situation deteriorating?

We should not condemn the wolf because of a donkey. At the end of the day, donkeys are part of its possible prey spectrum. It is important to maintain a rational mindset and take decisions based on facts and not emotions. Only in this way will the relationship between humans and wolves be sustainable.

Giraffe Social Communities are Important to Giraffe Populations

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Female Masai giraffes live in distinct social communities of up to 90 other friends, and although areas used by these ‘girl gangs’ often overlap, they have very different rates of reproduction and calf survival. This means the girl gang social units may be important to giraffe evolution. These findings were published this week in the Journal of Wildlife Management by a team of scientists from the Population Ecology group at University of Zurich and Penn State University, as part of one of the largest giraffe studies in the world. “We used social network analysis of hundreds of females and discovered this girl gang social organization from the giraffe’s own preference and avoidance behaviors,” said Derek Lee, associate research professor at Penn State and senior author of the study. “Gang membership was pretty tight, and even though members of different girl gangs often spent time in the same areas, members of different communities rarely interacted with each other.”

The scientists further found that calf survival and reproductive rates were different among these social communities, even when communities’ home ranges overlapped in space and therefore shared similar environmental conditions. “This shows that population structure can arise from social behavior rather than discrete space use,” noted Monica Bond, lead author and research associate at the University of Zurich. “These social subpopulations have different survival and reproductive rates, so some might have greater competitive abilities than others, like being able to dominate the better-quality food, or there might be cultural differences such as having better strategies for protecting their calves from predators.”

Each giraffe social community exhibited different social characteristics, like how strong the relationships were among the community members. There was also a gradient in environmental characteristics in which the giraffe communities lived: the Tarangire region of northern Tanzania where the study occurred includes two national parks, a livestock and ecotourism ranch, and unprotected lands inhabited by traditional cattle ranchers, as well as several densely populated towns surrounded by agricultural lands. The scientists wondered how the environmental or social conditions experienced by the giraffes might influence their survival and reproduction. “Survival and reproduction together determine whether a wildlife population (or subpopulation like a specific girl gang) increases or decreases and is therefore absolutely critical for conservation,” said Lee.

The team calculated the survival rates of more than 1,400 adult females and calves, and the annual number of calves per female, and examined if there were differences among the social communities. They then investigated if the differences were explained by social factors like the strength of relationships, or by features of the environment, such as how close to people the giraffes roamed, the fertility of the soils, or the kind of vegetation in their ranges.

Giraffe calf survival was higher in social communities that had less area of dense bushlands in their ranges, possibly because lions prefer to hunt in such thickets where they can stalk their prey unseen. “We also found that calf survival and reproductive rates were higher in the social communities that spent more time outside of the national parks,” said Bond, probably also because there are fewer natural predators like lions and hyenas near where people live. Some areas outside the parks also had more fertile volcanic soils and therefore possibly more nutritious food than on other soil types.

“The good news for conservation is that giraffes can survive and raise their offspring in areas close to people,” Lee pointed out. “We can help giraffes to thrive by giving them enough living space in the savanna—both inside and outside of national parks—and by taking care not to disturb them and disrupt their social relationships.”