Animal personality is a huge topic in behavioural ecology right now, and it seems like you can’t shake a stick in the literature without hitting another paper on the subject. You may have heard of the term before, but if you’re asking “what is animal personality?”, I’m planning to write on the topic more extensively soon and so I ask that you bear with me and keep an eye out for that. For now, we can go with a sensible version that defines animal personality as “consistent individual differences in behaviour, in time and/or across contexts, for both human and non-human animals” (Dall, Houston, & McNamara 2004). In non-scientist speak, this means (broadly) that animals show personality traits in the same way we think of when we talk about humans; when we say “he’s an aggressive person”, we mean that no matter what context you find him in, the person we are talking about expresses aggression. In a boxing ring, aggression is appropriate, but while it may not be so appropriate in the middle of a grocery store he expresses it anyways. The study of personality traits in animals, where animals who are aggressive or exploratory or shy in one context tend to be so in others, has exploded in recent years and the signal can sometimes get lost in the noise. To that end, I want to highlight a new paper in the Advance Access section of the journal Behavioral Ecology by Adam Reddon, entitled “Parental effects on animal personality”.
(Full disclosure: Adam is a friend from my Master’s lab, and he’s scary smart. He’s currently doing his Ph.D. at McMaster with Sigal Balshine and publishing papers at a rate that most people can only envy. If you’re looking for young behavioural ecologists – or scientists in general – to watch, he should most certainly be on your list).
The point of this paper, an invited forum contribution, is to link the large literature on parental effects and animal personality. Parental effects (though most work has been on maternal effects) cover “the ways parents can shape their offspring’s phenotypes over and above genetic inheritance”, as Adam puts it. These effects can occur in many different ways, which Adam does a nice job of reviewing; examples include nest site selection, the amount of food provided, hormone transfer by birds into their eggs (which can, among other things, manipulate how fast the offspring grows), social interactions, and providing opportunities for social learning. One great example that he provides is of Norway rats. Rat mothers will lick and groom their pups after they are born, and the amount of licking and grooming that the pups experience in the first week will have big effects on how well the pups respond to stress both physiologically and behaviourally. Pups who were interacted with less tend to be “shyer, less exploratory, less social, less aggressive, and less dominant” throughout their lifespan (p.2). This is clearly a parental effect, because pups who were cross-fostered (adopted) to other mothers had stress reactions that came from the licking and grooming of their adoptive mother and did not correlate with their genetic mother. Paternal effects are also quite widespread, having been seen across taxa, including mammals, birds, lizards, and even waterfleas (Daphnia cucullata) and radishes.
Adam’s contribution here is to draw a straight line between the two literatures by connecting developmental processes to animal personality, treating personality as an outcome instead of the starting point. As he states (p. 2-3): “… the parents of a developing organism are in a unique position to guide its development and alter the offspring’s personality to better match the environment it will face”. Parents have acquired information about the environment that may be useful to the child, and if they can translate that into paternal effects that change the offspring’s personality in a way that takes advantage of that information, they may enhance the offspring’s fitness (and by extension, their own chance of seeing grand-offspring). A speculative example might go something like this: parents experience a poor environment because they can’t find food, and this lack of food leads them to manipulate their offspring into having a more exploratory personality so that the offspring will have a greater chance of escaping the poor conditions of the immediate area to find food. This would be a risky strategy, but the idea of being risk-prone in poor environments has a long history in behavioural ecology (especially in foraging, e.g. Stephens 1981).
The upside of this paper is that the connection between them is obvious and powerful, at least in hindsight . As Thomas Huxley was said to have exclaimed upon learning of Darwin’s idea of natural selection, “how extremely stupid not to have thought of that”. The link to paternal effects gives researchers working on personality one potential explanation for the variation they see and a paradigm to test experimentally, and will hopefully energize both literatures. I was also under the impression from my readings that fitness differences in offspring phenotypes arising from paternal effects weren’t well explored (I’m open to correction on this!), so perhaps linking maternal effects to personality variation will provide more data on how these effects affect selection over generations. The only potential downside I can see is that personality research, so far, has been characterised by some confusion over terminology and methodology (which I will touch on in a later post); it might take researchers in this area some time to sort out the best way to combine the two approaches fruitfully. On the other hand, the most exciting moments in science generally emerge out of areas of confusion and doubt, so I hold out hope that exploring the effects of parental decisions on offspring personality will lead to great advances in our understanding of animal behaviour.