The decision of taking risks in the presence of a predator involves complex trade-offs between future reproduction and immediate survival. When faced with a predator, many individuals will seek safety in a refuge and must decide when to re-emerge based on trade-offs. Remaining in the refuge for too long will result in lost opportunities for foraging, courting and mating but on the other hand if an individual emerges too soon they pay lose their life. Many factors influence risk-taking behavior such as size, sex, population density and the presence of potential mates. An individual may adjust their risk-taking behavior depending on the different fitness benefits that may be available in an environment. For example, during mating periods, it may be beneficial for individuals to conduct risky behavior in the presence of a predator because there may be a higher chance of gaining reproductive benefits. Furthermore, it may also be beneficial to conduct risky behavior high population densities since the risk of predation may be reduced by a high number of conspecifics.
This study used male fiddler crabs (Austruca mjoeburgi) and a simulated avian predator that performed a low swoop over the crab habitat. Fiddler crabs occupy and defend territories that contain a burrow which serves as a mating site and a refuge from predators. Males conduct an elaborate and energetically costly courtship display that involves repeated waving to a significantly oversized claw while darting to and from their burrows.
Watch their mating dance to the beat of We Will Rock You here: https://www.youtube.com/watch?v=Z8U89EoUhuo !!
The aim of this study was to examine the risk taking in response to a avian predator fly by across three different treatments.
- Mating v.s. Nonmating Periods
- Female Presence
- Population Density
For each of these three contexts, the researchers also tested whether or not the male fiddler crab took refuge in its burrow in response to a simulated avian predator. If refuge did take place, the latency before re-emergence from its burrow was also measured.
Gruber and colleagues at The Australian National University predicted that after a simulated avian predator fly by:
- Crabs would be more likely to stay above the ground during the mating period or re-emerge sooner from their burrow compared to the nonmating period due to available reproductive benefits
- Crabs presented with females during the mating period would exhibit the lowest propensity to take refuge in their burrows or shortest latency before re-emergence time than any other treatment
- Fiddler crabs in higher population densities would be more likely to stay above ground during a predator fly-by or re-emerge sooner than conspecifics living at lower densities
Results demonstrated that males living at different population densities did not significantly vary in their responses to the predator fly-by, although crabs at high population densities tended to spend less time in their burrows following a predator fly-by than did conspecifics at low densities. Furthermore, only 17% of all tested male crabs remained above the ground despite predator simulation. Males that entered their burrows due to the predator presence during the female presentation treatment emerged significantly faster than did males in any other treatments. Figure 3 illustrates further results for latency until males re-emerged from their burrows in all three treatment conditions.
So what do all these results mean? Contrary to the first prediction, risk-taking behavior did not differ between mating and nonmating periods. Prediction 2 was supported by the results because male crabs that were presented with a female during mating period were more likely to stay above ground during a predator fly-by compared to males not presented with a female. The high propensity of male fiddler crabs to take risks under the threat of predation during female presentation may be explained by the benefit of an increased likelihood of reproductive opportunities when in the direct presence of a female compared with mating and nonmating scenarios where a female is not in close proximity to a male’s burrow.
Furthermore, courting displays by male fiddler crabs are energetically costly therefore they may exhibit the same risk-taking strategy across nonmating and mating periods due to the rarity of receptive females in the population resulting in a consistently low likelihood of mating opportunities. The likelihood of a male fiddler crab obtaining a potential mate during mating season is already low due to how the ratio of males to mate searching females is 45 to 1! Talk about the odds being stacked against you. The lack of variation present between crabs observed in high population densities and those in low population densities may be attributed to the scale at which individual crabs assess the density of conspecifics. One theory presented by the researchers is how the individual crabs may use the vibrations of nearby crabs as nonvisual cues to determine if the predator has passed by.
Adjustment of risk taking under predation threat appears to be based on the costs and benefits of risky behavior within each treatment. This suggests that male fiddler crabs are able to assess complex trade-offs associated with survival and reproduction. When looking at the bigger picture, we must ask how this study fits in the field of sensory ecology and what it contributes. The knowledgeable research gained in this experiment is not only applicable to male fiddler crabs but instead may apply to other aquatic species as well. The light shed on the relationship between trade-offs and behavior in this study is just another piece of the puzzle. Understanding the behavior exhibited by animals when under predator threat allows researchers to study a whole new dimension of decision-making occurring in the animal’s mind as well as how costs and benefits are weighed.
Not only does this research impact sensory ecologists but it also teaches us a thing or two about being brave and courageous! These crabs aren’t going to let a bird stop them from serenading females with their dancing display. Now time to go play Just Dance and learn their signature waving claw move!
Reference: Gruber, Jodie., Kahn, Andrew., Backwell, Patricia R. Y. “Risks and Rewards: Balancing Costs and Benefits of Predator Avoidance in a Fiddler Crab.” Animal Behaviour, Academic Press, 1 Nov. 2019, www.sciencedirect.com/science/article/pii/S0003347219302969.