The standard process of invertebrate growth is that when you have matured and reached adulthood your skeletal system becomes static, and you stay roughly the same size. Although this is generally true, there are some animals that pay no attention to this way of growth and can actually shrink and grow in the space of months – incredible!
First, we head to the Galapagos archipelago. The Galapagos consists of 13 major and 6 smaller islands and is home to some of the most pristinely preserved ecosystems in the world. Much of the incredible biodiversity found here is endemic to the islands, one such species are the marine iguanas (Amblyrhynchus cristatus). These iguanas are unique among lizards in their ability to swim, but they can also hold their breath for up to an hour.
Each of the islands play host to slightly different marine iguanas, each as distinct as their island. They were described by Darwin as the “most disgusting, clumsy lizards”, but there is beauty comes in their incredible adaptions. Feeding exclusively on underwater algae and seaweed, they have developed claws that allow them to grip to the rocks to avoid being buffeted away by the currents. While this way of feeding means a lack of competition, it also means ingesting a large amount of salt from the water. To overcome this, they have evolved glands near their nose that clean their blood of the extra salt, and regularly sneeze to expel the salt creating quite the display.
While these adaptations are fantastic, but they have another completely astounding skill – the ability to change the length of their spine over a matter on months.
Due to their location, the archipelago benefits from a high exposure to upwelling of nutrient rich water from the deep ocean. This water fuels photosynthesis, thus phytoplankton and algal growth, and drives the islands ecosystems. When El Niño occurs, the temperatures increase, the upwelling is reduced and the trade winds that drive the whole weather system is reduced. This puts the iguanas under a lot of stress due to the scarcity of food, so by shortening their spines they reduce energy expenditure and increase their foraging efficiency. They are the only vertebrate known to be able to do this, and two studies found that individuals became shorter by up to 20% during these time periods.
When La Niña conditions occur, the upwelling increases, trade winds get stronger, and algal food becomes more abundant and the iguanas regain the spine length they lost. These weather patterns occur every fourth of fifth year, so these iguanas are constantly changing. The mechanisms are still unclear, but reabsorption of bone material could be a potential mechanism.
Although the marine iguanas are the only invertebrates known to be able to lengthen and shorten their spine, there are a few other documented cases of bidirectional size changes, and not just of the skeletal kind.
In 2017 a team of researchers monitored 12 red-toothed shrews and found that their braincase and even their brain changed size! They found a summer peak in brain and braincase size, a winter decrease and a spring regrowth. These size changes of up to 20% seem to be part of their adaptive process to increase their chance of survival in the winter months. It is hypothesised that this is their alternative to hibernation or migration, their metabolic rate reduces which compensates for the energy budget increase in cold temperatures and reduces their need for food. The increase in Spring then makes them competitive during their reproductive period so they can protect their territories, however some studies have shown that they do not fully regain their full brain capacity.
Another mammal which has this power is the weasel. Weasels are phylogenetically distant to shrews but have also been shown to increase and decrease their braincase and brain size over their first two years. They found all the weasels had a “growth overshoot” in their first year before shrinking down in winter, but only male weasels grew in their second summer. This pattern suggests the same pressures of resource scarcity during the winter months and the reproductive pressure in the spring.
Each animal appears to be responding to environmental pressures, so these discoveries could lead to new ways to monitor the impact of environmental changes on different species. It could also provide new avenues to explore in terms of degenerative bone research, joint and brain conditions, and potentially new therapeutic options.