As a child, we had a limited number of television channels to watch. We ended up watching Jacques Cousteau and National Geographic specials because they were the only thing on. But I learned a lot about the world around us.
In particular, I remember scenes of the massive migration of wildebeests across the plains of Africa and humpback whales migrating along the West Coast. These television shows filled me with a sense of wonder and awe for the natural world.
As a scientist, I chose to study chemistry. In part my decision was because it is much simpler than some other disciplines. Trying to understand behavioural patterns in animals or the structure of a local ecosystem is a lot harder than synthesizing a molecule.
Large scale ecological and evolutionary studies get increasingly difficult when either the animals or the ecosystem are under stress. For example, migratory patterns are affected by everything from human construction of roads, cities, dams, and railways to climate change and modified behavioural patterns of other creatures. The number and diversity of variables involved can be overwhelming.
Last year, Gitanjali Gnanadesikan, William Pearse, and Allison Shaw published an article on the evolution of mammalian migrations for refuge, breeding, and food. In this article they tried to synthesize the data on a wide number of mammals. Of the roughly 5m400 known mammal species, they were able to get definitive migratory data on just 1,062 species.
In part, this demonstrates just how much is actually not known about mammals. Large animals such as wildebeest, reindeer, pronghorn antelopes, and humpback whales have been reasonably well studied but there are a great many smaller creatures we know very little about. The data simply is not available.
That said, they were able to classify 235 species as migratory and another 158 as probables. Within these species they then began to look for common threads in migration behaviour. In essence, they boiled the issue down to three fundamental drives - breeding, refuge, and food.
For breeding, migrants will forage in one location but move to another for the purpose of reproduction. In some cases, this is driven by a change in climatic conditions to locations more suited to the birth of young. For example, the humpback whale feeds in the food rich cool Arctic waters of the northern Pacific but migrate to the warm water off of Mexico and southern California to give birth. The warm waters are more conducive to the development of young whales as they are less demanding on their physiology.
For refuge, some animals breed and forage in the same space but will move away for part of the year due to other influences. For example, Florida manatees breed and forage in the same location but as the water temperature increases, they move north for part of the year.
For food, predatory animals - such as the cheetah - will follow herds of game. In this case, it is animals like the wildebeest which are following a migratory route which gives them access to grazing land. The cheetah simply follow along behind. In other cases, migration is necessary to simply get access to different types of food throughout the year.
These broad classifications can be applied across mammalian species but do not result in a phylogenetically driven classification. That is, different species within the same sub-group of mammals will migrate for different reasons. In some cases, such as the elephant seal, the members of the species will even migrate twice in one year - once for breeding and again for food.
Recently, Science published an article with an impressively long list of authors entitled: Moving in the Anthropocene: Global reductions in terrestrial mammalian movements. This article is a joint effort by a large number of scientists to understand the impact modern humans are having on traditional migratory behaviour.
The authors used a unique GPS-tracking database of 803 individuals across 57 species from an equally large number of individual studies to examine migration. They point out animal movement is critical for the functional health of an entire ecosystem and not just the mammals themselves. After all, foraging in a single location can lead to overgrazing and the movement of animals is a necessary component in the dispersion of seeds for plants among other things.
The conclusion this paper reached is mammalian movement in areas with a significant human footprint is, on average, one-half to one-third that found for the same mammals in low impact regions. When mammals encounter human habitat, their natural ranges are constrained.
This is not always because of restrictions in movement brought about by the built environment. Sometimes it is due to human crops - such as alfalfa - which provide a rich source of food and reduce the need for migration.
But it may mean the next generation of humans will not see the magnificent migrations around us.
