Many animal species migrate long distances year after year between the areas where they raise their offspring and the regions where they spend the winter. This migration costs a lot of energy and entails high risks – and yet it must mean an evolutionary advantage for the species. What distinguishes migratory species from sedentary relatives? To find out, researchers compared the lifespan, reproductive time, and reproductive strategy of migratory and sedentary birds and mammals around the world. The result: Migratory animals have a lower life expectancy, but compensate for this with an increased focus on reproduction.
Migratory birds sometimes cover tens of thousands of kilometers a year to get from their winter quarters in the south to their breeding grounds in the north and back. There are also numerous migratory species from reindeer to blue whales among mammals. Scientifically, it is still not fully understood why animals wander. Important reasons are seasonal climatic fluctuations with different food supplies, as well as avoiding competition and diseases. But how does that compensate for the high risks and energy costs of further migration? Why do some species have migratory lifestyles while others find it more beneficial to stay in one place and adapt to the circumstances there?
Reproduction more important than individual survival
A team led by Andrea Soriano-Redondo from the University of Exeter has examined these questions. For over 700 bird and 540 mammal species from all parts of the world, the researchers compared the lifespan, the age of sexual maturity and the development time of the young animals. In addition, they recorded how often each species reproduces and how many offspring are produced on average. They put this data in relation to the body size and habitat of the animals.
It was found that compared to related sedentary species, migratory species live shorter on average, become sexually mature earlier and produce more offspring more quickly. “By prioritizing reproduction over survival, species with a fast lifespan have the potential to rapidly increase in numbers, offsetting the risk of many individuals dying during the journey,” the researchers explain. “The shorter lifespan of the migratory species could be an inevitable result of their debilitating lifestyle and increased accidental mortality.”
Big and small hikers
The studies also showed a connection between migration behavior and body size, depending on the type of transport. While flying species such as birds and bats tend to migrate from small species, it is the other way around for walking and swimming mammals. “That is probably related to biomechanics,” the researchers write.
For land animals, the speed of travel increases with body size. A larger body needs more energy, but it can also store more reserves. “Only large animals can store enough energy to cover long hikes in a reasonable amount of time,” the researchers explain. Similar effects can be seen – albeit weaker – in swimming animals. For flying animals, on the other hand, the energy expenditure increases with increasing body weight, so that long migrations are more suitable for smaller species in this case.
Vulnerable to bad weather and climate change
According to the researchers, migratory species are also particularly vulnerable to climate change. “Unusually cold weather in the breeding areas can lead to mass death of migrants, while sedentary species are better able to withstand the weather conditions. There is also evidence that poor weather increases mortality on migrations. ”This further reduces the life expectancy of migrating animals and increases the selection pressure to live faster.
“Given the possible link between the environmental conditions at the hatchery and the life course of migratory species, it makes sense to consider the potential impacts of climate change,” say the researchers. Previous studies have shown that many migratory birds change their behavior in the face of climate change. “The strong link between migratory behavior and the pace of life that we have identified means that migratory species – especially those that cannot adapt within a short period of time – could be living indicators of environmental changes.”
Source: Andrea Soriano-Redondo (University of Exeter), et al., Nature Communications, doi: 10.1038 / s41467-020-19256-0