Animals, just like humans, can have personalities. Too bad, that unlike us, they can’t take online personality quizzes! Instead, scientists measure personality traits using behavioral assays.  For example, we can test the “shyness” of an individual cricket by putting him in a box in a new space and see how long it takes him to creep on out and explore his surroundings. Scientists can test for traits like boldness, aggression, or stress coping styles, like reactivity or proactivity.

Like aspects of our human personalities, animal personality traits can vary across contexts. You might imagine you might be shy and relatively quiet with your classmates working on a group project, but love to play the center of attention at parties with your friends. Now imagine that annoying guy that always blurts out in class or group meetings, but also is loud and crazy when you see him at the bar. You can change how much you speak up and how loud, but the class clown can’t change quite as much. We call this behavioral plasticity¹, which is how much a personality trait can, and does, change across different contexts. You’re relatively plastic compared to that other guy.

Now, what might underlie your – or an animals’ – ability to be plastic? Of course, you have to be able to change, but you also need to be sensitive to context. Continuing our example, you’re able to tell that your group members are less comfortable with you than your close friends, but the other dude might feel close to everyone, so can’t tell the difference between a work or party situation.  Animals also can be sensitive to their surroundings. This sensitivity changes how we see their personalities, and might also change how they can adapt to new environments, such as to climate change.

Without further ado, some of the key things sensitive animals do:

1. Distinguish when predators are coming and move away faster.

Increased survival can be a key advantage to being a sensitive animal (and might explain why sensitivity may be selected for).  Imagine a bird that is more attuned to predator risks in the environment, and devotes more attention listening for threats. When a cat starts rummaging around in the bushes, the sensitive bird will be the first to fly away.  This sensory sensitivity (say that five times fast) can come in other forms too, such as a  more keen sense of smell for predator odors, or more acute vision for scanning the landscape.

Given the obvious survival benefits, why would individuals vary in their sensitivity?  Turns out being sensitive isn’t cheap! Having increased eyesight or olfactory bulbs costs energy and time to evolve. Even spending time and attention looking for predators takes time away from foraging for the best food or noticing important social cues in your group.  This also might make you a bit of a scaredy-cat and overreact when threats aren’t there, or aren’t all that dangerous (see #3).

2. Discern the subtly-sexy from the average mate.

The same attention-to-detail that allows sensitive animals to sense risks might also lead them to the rewards – the extra-special mates!  With higher attuned sensory skills, a female bird might better be able to parse out honest signals, such as the rapid trills of a male’s song or the shimmery hue of his feathers.  This increases selection on these sensory traits, since they now contribute to future survival and reproductive success.   If sensitive males also have different signals to attract these females – such as a subtly different song or different dance moves – this can lead to assortative mating, the opposite of “opposites attract”.  Eventually, with all the sensitive animals pairing up and having more sensitive offspring, population – and eventually species level – sensitivity could be much higher to both mating and survival cues.

3. Get really scared and react a lot in the face of stress!

Whenever you experience something stressful – be it a jump scare in a horror film that makes your heart race, or the butterflies in your stomach before a class presentation – your brain tells your adrenal glands (right above your kidneys) to release stress hormones, called glucocorticoids. It turns out animals that are sensitive to subtle changes in their environment and social groups tend to release a bunch of these internal signals, and have higher everyday levels than their less-sensitive peers.  In an actually stressful world, this is great; stress hormones increase your reaction time for “fight or flight” responses, make energy available in the body, and  increase your ability to think fast on your feet. However, if life is tame, having elevated levels of these stress hormones for long periods of time can cause disease.   This is yet another cost to being sensitive, explaining why animals might still differ in this capacity.

4. Have built-in brain and hormone responses in order to do these things better.

   

So how do animals actually become sensitive, within their bodies and cells?  They might have more complex and refined sensory anatomy (as mentioned in #1), like sharper eyes or more delicate noses.  Within their bodies, especially in response to hormonal signals (like the stress hormones in #3), animals might vary in how they respond to these internal signals.  Hormone responses require the signal (coursing through your blood) to bind to receptors on the target organs, be it the brain, adrenal glands, or your stomach.  The way these hormones work is much like a game of freeze tag – where someone moving around (“it”, the hormone) can change your (the receptor) behavior or activity by touching you.  In order for an organ to change action, it has to receive the hormone signal on its receptors.

These receptors can vary between individuals (some receptor types respond stronger than others), and the amount of them can vary too (the more receptors you have, the more that tissue is likely to respond).

Receptor differences have not only been implicated in animal sensitivities to stress, but also in humans. For example, people can have the long or short gene for te serotonin receptor. Serotonin is a chemical that travels between neurons, where it binds to receptors, and leads to feelings of happiness and pleasure in the brain’s reward circuit.  People with the “long” gene form have higher amounts of the receptor, and thus are more sensitive to serotonin. Studies have shown individuals with the long allele are more optimistic, more positive, and less neurotic – important human personality traits³.

As seen in this example, a lot of these physiological traits are hardwired by genetics or determined by your parents, so sensitive animals must…

5. Pass all of this stuff onto their offspring!

In order for selection to act on sensitive traits, there needs to be a heritable, genetic basis to all of these mechanisms.  For receptor types and anatomical complexes, different genes can do most of the work. But if sensitivity should lead to a better fit with the environment, animals should be able to alter their sensitivity early in development depending on the environment its in, a capacity termed developmental plasticity in behavioral ecology language.  What’s especially cool is that various mechanisms exist where moms (and even dads) can “program” the traits of their offspring depending on the environments they experience. This includes different levels of hormones in the egg yolk, or stress cues before birth⁴, or even epigenetic modifications⁵ – little marks on the genes that tell you how to use your genes, without changing the actual DNA sequence. These maternal effects allow a mom to “toggle” her offspring’s sensitivity to be better prepared for the environment she’s bringing them into.

Author Rob Blenk is a 3rd year PhD student in the Graduate Group in Ecology at UC Davis, with an Area of Emphasis on Conservation Ecology. 

Author Victoria Farrar is a 2nd year PhD student in Animal Behavior. She studies the physiological mechanisms of behavior, specifically brain hormones in birds.

Citations:

1. J. West-Eberhard (1989) Phenotypic plasticity and the origins of diversity. Annu. Rev. Ecol. Syst., 20: 249-278
2. Rodriguez-Prieto, I., Martin, J., Fernandez-Juricic, E. 2010. Individual variation in behavioural plasticity: Direct and indirect effects of boldness, exploration, and sociability on habituation to predators in lizards. Proceedings of the Royal Society (B) 278(1703)
3. Nautiyal, K. M., & Hen, R. (2017). Serotonin receptors in depression: from A to B. F1000Research, 6, 123. http://doi.org/10.12688/f1000research.9736.1
4. Rubenstein, D.R., Skolnik, H., Berrio, A., Champagne, F.A., Phelps, S.M., Solomon, J. 2016. Sex-specific fitness effects of unpredictable early life conditions are associated with DNA methylation in the avian glucocorticoid receptor. Molecular Ecology
5. Allis and Junwein (2016) The molecular hallmarks of epigenetic control. Nature Reviews Genetics. 17(8):487-500.