On Moose Who Reproduce: To Use Or Not To Use

Guest post by Endre Grüner Ofstad

Opposing fitness consequences of habitat use in a harvested moose population (2019) Ofstad, Markussen at al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13221

The Crux

At the heart of our understanding of animal behaviour is Optimal Foraging Theory. It’s related to the core concepts of population ecology, and essentially asks which life history trait a species is more concerned with – survival or reproduction. For instance, often for a herbivore, the areas where they will find the most food is also the area where the most predators will be lurking. This presents them with two options – eat lots, reproduce a lot, and die young, or eat less, reproduce less (at least per year) and live longer.

On a species level we can compare mice and elephants. Yet these differences also occur within species and populations. Some individuals are more prone to high-risk strategies, while others prefer the low-risk strategy. Which strategy is the best will depend on the prevailing environment. For instance, a situation with few predators (or hunters) will favour the more risk-prone strategy, while a strong presence of predators will favour the risk-averse strategy. Populations who experience environmental variations are expected to have a composition of strategies that varies accordingly. 

A classic life history trade-off is that one expects individuals to trade-off between how many offspring they have and how big offspring they get. A mother only has that much energy to invest. But quite often it doesn’t seem like animals in the wild have read that part of the textbook. Mothers who get the largest offspring often also get the most. Theory suggests that this might result from individuals differ in how much resources they acquire. 

In this paper we wanted to know whether moose’ use (say that five times over) of agricultural land was related to important life history traits: survival, offspring number and size, as well as lifetime reproductive success. These terms are also known as fitness, or fitness components and proxies. 

What We Did

We closely monitor the moose population on the island Vega in northern Norway. This includes annual marking events where we weigh individuals, collect tissue samples and (re)collar animals with GPS-collars. We also observe the number of offspring each female produces. This is a harvested population where the local management decides the annual harvest quotas of the population. From the harvest we obtain carcass weights and tissue samples. Based on the collected tissue samples and detective work – including collecting tissue from a stuffed individual at the local mayor’s office – we have managed to establish a pedigree of the population since it was founded in the late 1980s by three moose swimming over from the mainland (with regular immigrations after that). 

We also looked at how consistently individual moose use their habitat between the summer and hunting season, and between subsequent summers. Establishing how reapeatable/consistent individuals are is necessary to see whether changes in moose behaviour can drive their natural selection. 

Subsequently we related individual habitat use of agricultural land, i.e. open grasslands, in summer to survival and reproduction outcome the following season. Agricultural lands in this system represent abundant, high-quality forage, but also leaves the moose very exposed to hunters.

Did You Know: The Darwinian Demon

The concept of trade-offs follows from the assumption of finite resources. You only have given number of hours to live, or only so much energy you can invest in your offspring. Yet imagine a world in which these constraints did not exist. No constraint in number of reproductive events, or resources you can invest in offspring. The hypothetical product of this is called a Darwinian Demon – an organism that reproduces directly after birth, and lives and reproduces indefinitely.

What We Found

We found that individuals are quite consistent in the amount of agricultural land they use – both inside and outside of hunting seasons, and also from year to year. Meaning that moose who use agricultural land intensively one year are likely to also use them intensively in subsequent years. If this behaviour correlates with fitness then we can mark it down as a driver of natural selection. 

Among males we found no relationship between habitat use and reproductive success, whereas among females we found that increasing the use of agricultural lands increased both offspring number and size! However, among both males, females and calves there was also a decrease in survival with increasing use of agricultural land. Meaning that there was a trade-off between reproduction and survival. Across individual life spans however, we found no influence of habitat use on lifetime reproductive success. 

Screenshot from 2020-05-13 08-06-28

The Moose population on Vega was founded in 1985 by three individuals swimming across from the mainland. It may seem unlikely, but moose are also known to graze on algae in shallow lake and costal waters (Image Credit: Endre Gruner Ofstad, CC BY 2.0)

Problems?

When dealing with space use scale is always an issue. Here we assumed that fitness components were related to space use during summer, but winter might also be important. We also assumed that all individuals have equal access to all forage opportunities. Being on a relatively small island we are quite confident that individuals can physically access the whole island, but social structure or other stuff might influence accessibility. Moreover, we assume that mortality and reproduction are influenced by mechanisms at the same scale, which might not be the case. Mortality could for instance be influenced by you deciding which part of the island to use most often, whereas reproduction might mostly be influenced by which part of the forest you use.

So What?

When behaviour is repeated so often both across seasons and across years, hunting can strongly influence the population dynamics of the population with regards to being selective towards individuals who produce more and larger offspring. Moreover, it also suggests that harvested population being more shy might not stem from individuals learning to avoid open areas, but that the ones who used open areas are dead. 

A lack of a trade-off between the size of the calves and the number of them is often attributed to differences in acquisition. Some females have more than others – whether it is more resources or some genetic predisposition – making it impossible to detect a trade-off when comparing females.  Here we have shown how a lack of a trade-off might come about through behavioural differences, with more efficient habitat use meaning that moose with more calves have enough to have larger calves as well. 

Studies are often restricted to relating habitat use or behaviour to one fitness component, such as reproduction. This is usually due to logistic constraints, i.e. it might be difficult to separate mortality from dispersal in an open population, whereas counting number of offspring is more feasible. Our paper shows that this is really only part of the story – and your conclusions might be dead wrong as the individual might compensate for whatever pros or cons you are observing. 

Title Image Credit: Endre Grüner Ofstad, CC BY 2.0, Image Cropped

Endre Grüner Ofstad has spent time getting a Masters degree in Natural Resource Management and a PhD in ecology, spending most of his time researching spatial ecology in ungulates (even dabbling in sea trout ecology for a while). Good friends with GIS, R & Co., he has now ventured into the fantastic and scary realms outside of academia and works for the County Governor in southern Norway. Here he gets to influence how societal practices affect the natural world. WOW! Follow him on Twitter @egopavilleveier.

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