Slow & steady loses the chase: Young bunnies need speed to survive

Growing up as a prey animal is tough. Juveniles are smaller, less muscular, and less experienced than their older counterparts. Not only does this mean that juveniles must compete for resources against older individuals, but they also must avoid becoming dinner for a nearby predator. It’s not surprising that many juveniles in these species often do not survive their first year of life. With all these challenges, how do prey animal juveniles make it through to adulthood?

To better understand how prey animal locomotor performance may be affected by predation pressure, Young et al. (2022) compared acceleration, escape speeds, power production, and survivorship in juvenile and adult eastern cottontail rabbits (Sylvilagus floridanus). They predicted that juveniles would surpass adult acceleration rates so that juveniles could reach adult escape speeds against predators. They also predicted that the juveniles which had the best locomotor performance would have higher survival rates in the wild compared to juveniles that didn’t perform as well.

The researchers measured the ground reaction forces and locomotor kinematics of wild-caught rabbits sprinting on a runway. They also measured the dimensions of several muscles in the limbs to estimate muscle power across juveniles and adults. Finally, rabbits were fitted with radio telemetry collars and released back into the wild. The collars were used to track how long each rabbit survived against predation.

Figure 1. (Adapted from Figure 7 in Young et al. 2022). A: Ontogenetic trajectory of acceleration (aCOM) relative to rabbit body mass. B: Ontogenetic trajectory of escape speed (vesc) relative to rabbit body mass. The authors suggest that acceleration and escape speed peak in a rabbit’s late juvenile stage. This peak corresponds to when juveniles leave the nest and are most likely to experience death by predation.

Juvenile cottontail rabbits accelerated faster than adults, which may in part be due to their ability to produce more mechanical power per unit of body mass. In fact, when the authors modelled the locomotor performance of adults and juveniles, they found that juveniles are consistently predicted to outperform adults in escape behaviors. Despite differences in locomotor performance between age groups, other factors could affect survival in these groups. Adults may engage in other behaviors to better avoid predation, like changing their activity patterns or hiding better in their surroundings. These behaviors could prevent the need to escape from a predator, and thus conserve energy and increase the likelihood for survival. When measuring survival rates of juveniles in the wild, there was a positive relationship between higher survival rates among the juveniles with the best escape performance. Juvenile rabbits between three and four months old reach their peak escape performance, which is incidentally around the same time that death rates spike due to predation.

Figure 2. (Adapted from Figure 8 in Young et al. 2022). While not statistically significant, there was a positive association between juvenile peak escape performance and survivorship, where juveniles with greater performance were likely to survive longer in the wild.

These comparisons of juvenile and adult locomotor performance reveal that when selective pressures from predation are high across ontogeny, juvenile performance should excel. Juvenile rabbits exhibit greater mechanical power and acceleration compared to adults, which may give them an edge to survive against predators in the wild. This allows them a greater chance to reach reproductive age and maintain fitness. We see that adults may use other approaches to survive, such as behavioral changes to avoid predation in the first place. These results give us insight to how natural selection may affect prey species, especially across age groups. This study emphasizes that selection across ontogeny should be taken into account when looking at whole-animal morphology and performance biomechanics.  

Amanda M. Palecek-McClung is a PhD student at Clemson University. She studies functional morphology, biomechanics, and adhesion mechanisms in fish and other vertebrates. You can find more about her at ampalecek.weebly.com or contact her at apalece@g.clemson.edu.

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