Guest Post by Stephen Heap, PhD Candidate University of Melbourne
Animals must make decisions about where to breed, since some places are better than others in terms of how much reproduction can be accomplished. Individuals can sometimes make better decisions by observing others and either copying them, if it looks like a good choice, or doing the opposite, if it looks like a bad choice. A recent study by Olli Loukola (Animal Behaviour 83: 629-633) (click on link here) and colleagues aimed to determine if the great tit (Parus major) copied or rejected the nest site decisions made by other breeders of its species.
The researchers set up nest boxes in a habitat used by great tits for breeding. These boxes either had a circle or triangle shape placed over them (see Figure). The researchers faked decisions made by other breeders by putting egg clutches in some of the nests. Other pairs looking to choose their own nest will thus think that another pair has chosen a circle or triangle nest. The researchers used two different clutch sizes in the fake nests. Some had relatively many eggs, making it look like a good breeding attempt, and some had few eggs, making it look like a poor attempt. Breeding pairs were expected to use the information on this phoney nest site to make their own decisions, by either copying or rejecting the symbol above a nearby fake nest. Furthermore, breeding pairs were expected to base their decision on how well the nest appeared to be doing, copying nest sites that had more eggs (apparently a good decision) and rejecting nests that only had a few eggs (apparently a bad decision).
What actually happened was that breeding pairs ignored how many eggs were in the fake nests when making their own decisions. However, pairs that had an older male partner tended to copy the fake decision, but pairs with younger males tended to reject the fake decision (see Figure). That is, older males tended to occupy nests with the same symbols as the fake nest and younger males occupied nests with different symbols.
So why do young males try to be different and old males try to be the same as other breeders? The researchers interpreted the use of the nest site’s symbol as a signal to other breeders. Older males, who are more competitive, may choose a nest with the same symbol as their neighbour as a challenge for dominance. Younger males, on the other hand, may choose a nest with a different symbol to signal their willingness to submit to their neighbour. However, this interpretation makes a lot of assumptions about the use of arbitrary symbols on nest boxes. For instance, why would great tits, who have never being exposed to nest boxes with geometric symbols on them as part of their evolutionary history, incorporate such symbols into a communication system? The behaviour becomes increasingly more puzzling when we consider that decisions about nesting site are ultimately made by the female, not the male, of a pair. Are males coercing females into choosing a site based on some competitive game between neighbouring males? Or is the other way around? Are females taking the age of their male partners into account, pitting competitive males against one another and keeping younger, less competitive, partners from the fray? It could even be that females with younger partners are signalling to their male neighbour that there won’t be much competition if he wants some extra-pair copulations (i.e. cuckoldry). Clearly, more studies are needed to make sense of an individual’s age and sex affecting the manner in which it responds to the behaviour of others, yet this study still highlights the complexity of social information use in animals.