Rising ocean temperatures and acidification are known to be altering the way fish grow and reproduce and now research shows these climate change side-effects may also change how fish think and act. As animals and invertebrates adapt to rising ocean temperatures, their brains might be changing, too.
A recent study by a group at Macquarie University in Sydney studied the effects on Port Jackson sharks, a migratory species that live in the warmer waters off the Australian coast, after they had been raised in water temperatures 1 to 3 degrees Celsius higher than normal.
What the researchers observed was a significant change in the test sharks' lateralization, which is a brain function observed across many species that essentially relegates certain processes to one side of the brain or the other. In humans, lateralization is most colloquially understood through the example of "handedness" the language functions for right-handed people are typically lateralized to the left hemisphere of the brain, whereas for left-handed people it might be bilateral or right-lateralized.
Scientists haven't yet reached a consensus on whether this division of brain labour has an explicit purpose, but previous research has suggested that lateralization of cognitive functions in fish have some evolutionary advantages, such as enhancing schooling behaviour. Species that were more strongly lateralized had greater levels of cohesion and coordination while moving within schools when compared to non-lateralized species.
The Sydney team tested laterality in the Port Jackson sharks by sending them through a maze multiple times and tracking the directions they chose to turn. The group of sharks raised at the higher temperatures displayed significantly stronger laterality than the control group, choosing to turn right much more frequently.
Why would this happen? Because the sharks raised at higher temperatures reached maturity more quickly and had faster metabolisms than the control-group sharks, their brains couldn't develop the same size and complexity as normal. The researchers hypothesized that their lateralization may, therefore, have arisen as an energy-saving mechanism.
Neural processing is metabolically expensive; thus, higher parallel processing abilities could allow animals to save energy during brain development and information processing without compromising function.
Thus, assigning a particular function to one hemisphere could allow sharks to maintain cognitive function, performing multiple actions simultaneously such as navigating while also searching for prey even with relatively smaller brains.
At first, this seems like good news: Species correcting course in unfavourable environs to continue functioning as normally as possible should be a positive sign that a warmer climate isn't the end of the world. As a parenthetical in their report, though, the researchers also note that 42% of the sharks reared in a warmer environment died before testing could commence so better not extrapolate too much about species' ability to adapt to climate change.
The study is an important first step in understanding how end-of-century temperatures may impact shark cognition, says Leon Green Ekelin, an ecophysiologist at the University of Gothenburg in Sweden who was not involved in the new work. Sharks “are underrepresented in climate change research, despite being important for ecosystem health,” he notes. But in order to gain a better understanding of how lateralization affects shark behaviour in different situations, he says they need to be tested in competitive scenarios. “The behavioural shift could be a beneficial response to changes in their environment,” he adds, “or it could simply be brain damage.”
Vila Pouca believes understanding how climate change shapes animal behaviour is important because it can reveal unexpected ways in which they may adapt to their rapidly changing environments. “Looking at lateralization can help us understand brain development and its impact on behaviour,” she says. “To build a complete picture, we need to investigate how climate change may affect other behavioural traits such as escape responses and learning abilities.”
Source: Symmetry — Open Access Journal
