Anyone who has seen a middle-aged rock star understands that certain lifestyles take more of a toll than others. Live fast, die young! This rock star mantra is also the premise of life-history theory, one of the central tenants of ecological research. Life-history traits are all of those characteristics related to an individual's lifestyle, such as growth rate, reproduction and lifespan. Life-history theory states that not all life-history traits can be maximized simultaneously, and individuals will need to make trade-offs among competing functions.
Theoretically, one of the key life-history trade-offs may be between growth rate and total lifespan. However, it is difficult to manipulate growth without manipulating other confounding factors, like nutrient availability. Who-Seung Lee, Pat Monaghan and Neil Metcalfe from the University of Glasgow addressed this long-standing question by taking advantage of the biology of juvenile three-spined sticklebacks (Gasterosteus aculeatus). Like all fish, sticklebacks are cold-blooded, and their metabolism and hence growth can be sped up or slowed down by changing water temperature. Furthermore, sticklebacks reproduce in the spring, and as reproductive success is related to body size, they are motivated to attain a large size prior to their first spring.
To examine the effect of growth rate on total lifespan, the scientists used water temperature and photoperiod to manipulate both how much the fish would need to grow and how fast they would have to grow prior to their first spring. The researchers predicted that higher growth rates would come at a cost to overall lifespan. Using juvenile fish captured in November, they experimentally manipulated the period available for growth; half the fish were kept under a normal photoperiod, while the other half were exposed to a delayed photoperiod, so that the fish perceived that they had an extra month before spring. Fish from both groups were then subjected to a ‘cold snap’ (6°C) or a ‘warm spell’ (14°C), or kept at a constant 10°C for 4 weeks. While all fish were fed the same diet, their metabolism, and therefore growth rate, was influenced by temperature. Thus, fish in the cold snap group grew more slowly than those in the other groups. After 4 weeks, all the fish were returned to 10°C for the rest of their lives.
When the fish were returned to 10°C, they faced a resource allocation decision. The fish stunted by the cold snap were much smaller than their counterparts, and to attain reproductive size they would need to grow quickly by directing all of their resources towards growth. Conversely, fish exposed to a warm spell were already larger and could afford to grow at a more leisurely pace. These warm spell fish could use some resources to fuel other processes that are important for overall lifespan, such as the immune system. There was even less pressure on fish that had been subjected to a delayed photoperiod – they had an extra month for growth. But did different growth rates influence overall lifespan?
Excitingly, the results matched the researchers' predictions. All of the fish had similar final sizes, but fish exposed to the cold snap directed more resources towards growth and grew more quickly once returned to 10°C, and consequently had the shortest lives. By replicating the experiment with fish captured in January, with mere weeks before spring, the researchers found that lifespans were shortened still further by the shortened growth time frame. This paper elegantly provides the first experimental evidence that growth rates are directly linked to total lifespan, and this indeed represents a key life-history trade-off. There is now empirical evidence that if you grow fast, you die young.