Fitness costs of reproduction are expected when resources are limited. This can drive the evolution of life‐history strategies and can affect population dynamics, particularly if females change their allocation of resources to reproduction. Dr Marco Festa-Bianchet (Université de Sherbrooke) explains the value of long-term studies for understanding such trade-offs and gives the #StoryBehindThePaper for his recent synthesis article in the Journal of Animal Ecology.
Long-term studies that monitor marked individuals are a valuable source of information in wildlife ecology. But how do they start, and how do they keep going? Here is how we ended up with 104 population-years of data on mountain ungulates in Alberta, Canada, for our ‘Synthesis’ paper: Long-term studies of bighorn sheep and mountain goats reveal fitness costs of reproduction.
In 1971, provincial wildlife biologists drove two small cabins up a fire lookout road on Ram Mountain, and assembled a corral trap with walls built by inmates at a minimum-security prison. They wanted to determine the feasibility of a hunting season on female bighorn sheep, and measure the survival of orphan lambs. In 1981, I was given a dart gun, shown how to use it, and sent to the Sheep River Wildlife Sanctuary in southwestern Alberta. My mission was to investigate whether individual differences in the timing of seasonal migration by bighorn sheep affected lungworm infection and risk of pneumonia. In 1989, provincial wildlife biologists set up traps on Caw Ridge, north of Jasper National Park, to capture mountain goats and determine whether low kid survival may explain a recent decline in many populations in the province.
Fast-forward to 2019. Nearly 2,500 marked sheep and goats, about 200 scientific publications and over 150 field assistants and graduate students later, our studies of mountain ungulates have substantially advanced our understanding of population dynamics, life-history and evolutionary ecology. They also provided key evidence of the evolutionary effects of intense trophy hunting. The long-term data were the raw material for dozens of graduate theses, led to collaborations among researchers from multiple universities, and have had profound influences on wildlife management. They carried on through perseverance of the many people involved, support from funding agencies such as the Natural Sciences and Engineering Research Council of Canada, and sheer luck. At one point, Caw Ridge was supposed to become a coal mine. Forest fires threatened evacuations. Cougar predation came close to reducing each of the three populations to a size that would question the value of continued monitoring. Roads and trails were washed out, camps broken into, equipment stolen. Most ecologists agree on the value of long-term studies, but perhaps not many realize that a major factor in the success of these studies is secure access to study areas. That access can be threatened by natural disasters, changes in land tenure and political decisions.
Our ‘Synthesis’ paper focuses on the fitness costs of reproduction from these long-term studies. Much of what we know stems from our ability to monitor individuals from birth to death, and the changes in ecological variables, from population age structure and density to the occurrence of disease and predation, over the years of monitoring. It is a bit of a running joke among people with long-term data sets that we could at times produce contrasting results simply by cherry-picking a few years of data. Our Synthesis paper emphasizes how individual differences affect allocation to reproduction and our ability to detect fitness costs. Those differences are formidable obstacles for non-manipulative studies.
We partly accounted for those individual differences through repeated measurements of individuals. Repeated measurements transform individual differences from a statistical problem to biological insight. Often, long-term studies face the criticism of limited external validity: you found this here, but would it be the same elsewhere? The comparison of three populations of animals with similar characteristics increases our confidence in how fitness costs become more evident in specific circumstances, such as at high population density or during disease episodes. Our clearest result is that females mostly shift costs of reproduction to their offspring, presumably to avoid compromising their residual reproductive value. That result was expected in these long-lived highly iteroparous mammals, where the residual reproductive value of females is likely much greater than that of offspring. For males, we suggest that the greatest costs of reproduction are in trying to reproduce, which involves competition with other males and a risk of injury.
Many questions remain, such as a better quantification of cumulative reproductive costs over multiple years or the possible role of fathers in maternal allocation decisions. As climate change increasingly affects many aspects of the ecology of our study species, we are well positioned to investigate its ecological and evolutionary consequences. As the studies continue, we will again rely on perseverance, some luck, and people who enjoy (or at least tolerate) fieldwork in difficult conditions.
Festa‐Bianchet, Côté, Hamel, and Pelletier (2019). Long‐term studies of bighorn sheep and mountain goats reveal fitness costs of reproduction. Journal of Animal Ecology, 88(8): 1118-1133. DOI: 10.1111/1365-2656.13002