Indirect and cross-generational effects of predation risk: predator odor and alarm pheromones in the Bank Vole

Prey animals can assess the risks predators present in different ways. For example, direct cues produced by predators can be used, but also signals produced by prey conspecifics that have engaged in non-lethal predator-prey interactions. These non-lethal interactions can thereby affect the physiology, behavior, and survival of prey individuals, and may affect offspring performance through maternal effects. We investigated how timing of exposure to predation-related cues during early development affects offspring behavior after weaning. Females in the laboratory were exposed during pregnancy or lactation to one of three odor treatments: (1) predator odor (PO) originating from their most common predator, the least weasel, (2) odor produced by predator-exposed conspecifics, which we call conspecific alarm cue (CAC), or (3) control odor (C). We monitored postnatal pup growth, and we quantified foraging and exploratory behaviors of 4-week-old pups following exposure of their mothers to each of the three odour treatments. Exposure to odors associated with predation risk during development affected the offspring behavior, but the timing of exposure, i.e., pre-vs. postnatally, had only a weak effect. The two non-control odors led to different behavioral changes: an attraction to CAC and an avoidance of PO. Additionally, pup growth was affected by an interaction between litter size and maternal treatment, again regardless of timing. Pups from the CAC maternal treatment grew faster in larger litters; pups from the PO maternal treatment tended to grow faster in smaller litters. Thus, in rodents, offspring growth and behavior are seemingly influenced differently by the type of predation risk perceived by their mothers.

Chemical communication plays an important role in mammalian life history decisions. Animals send and receive information based on body odour secretions. Odour cues provide important social information on identity, kinship, sex, group membership or genetic quality. Recent findings show, that rodents alarm their conspecifics with danger-dependent body odours after encountering a predator. In this study, we aim to identify the chemistry of alarm pheromones (AP) in the bank vole, a common boreal rodent. Furthermore, the vole foraging efficiency under perceived fear was measured in a set of field experiments in large outdoor enclosures. During the analysis of bank vole odour by gas chromatography–mass spectrometry, we identified that 1-octanol, 2-octanone, and one unknown compound as the most likely candidates to function as alarm signals. These compounds were independent of the vole’s sex. In a field experiment, voles were foraging less, i.e. they were more afraid in the AP odour foraging trays during the first day, as the odour was fresh, than in the second day. This verified the short lasting effect of volatile APs. Our results clarified the chemistry of alarming body odour compounds in mammals, and enhanced our understanding of the ecological role of AP and chemical communication in mammals.

Predator-prey interactions are a major evolutionary driver, affecting not only the direct mortality of prey species, but also their behaviours and reproduction. Prey species behavioural adaptations aim to mitigate the effects of predation and to maximise survival and individual fitness. These adaptations include the ability to signal a threat to conspecifics, e.g. via alarm calls or alarm secretions, or to detect predator presence via odours. In this thesis, I studied the effects of predator odours and conspecific alarm secretions on behaviour and reproduction bank voles (Myodes glareolus), a small mammal species inhabiting boreal forests. My work focused on three major points in comparing the direct predator cue and indirect conspecific cue: first, how the reproductive behaviour is affected by the predator odour or alarm pheromone, second, whether there are transgenerational effects and how they are exhibited in offspring, and third, what the chemical nature of these alarm secretions is. I conducted four experiments, which included both trials in semi-natural enclosures and under controlled laboratory conditions. I found evidence that exposure to conspecific alarm secretions causes a shift in voles’ reproductive behaviour, switching towards terminal investment. This became apparent with an increase in parturitions and an increased growth rate in larger litters, which did not occur when exposed to predator odour. I also found evidence of transgenerational effects, which affect aspects of the offspring’s exploratory and foraging behaviour. Additionally, I discovered that these behavioural effects are context-dependent and do not occur in every environment. Lastly, I identified a group of chemicals from voles’ alarm secretion, which are likely to be responsible for the observed effects. The results of my thesis fill a knowledge gap concerning chemical communication in mammals, and help to further understand the implications of predator presence on prey behaviour and reproduction.

In the predator–prey arms race, survival-enhancing adaptive behaviors are essential. Prey can perceive predator presence directly from visual, auditory, or chemical cues. Non-lethal encounters with a predator may trigger prey to produce special body odors, alarm pheromones, informing conspecifics about predation risks. Recent studies suggest that parental exposure to predation risk during reproduction affects offspring behavior cross-generationally. We compared behaviors of bank vole (Myodes glareolus) pups produced by parents exposed to one of three treatments: predator scent from the least weasel (Mustela nivalis nivalis); scent from weasel-exposed voles, i.e., alarm pheromones; or a control treatment without added scents. Parents were treated in semi-natural field enclosures, but pups were born in the lab and assayed in an open-field arena. Before each behavioral test, one of the three scent treatments was spread throughout the test arena. The tests followed a full factorial design (3 parental treatments × 3 area treatments). Regardless of the parents’ treatment, pups exposed to predator odor in the arena moved more. Additionally, pups spend more time in the center of the arena when presented with predator odor or alarm pheromone compared with the control. Pups from predator odor–exposed parents avoided the center of the arena under control conditions, but they spent more time in the center when either predator odor or alarm pheromone was present. Our experiment shows that cross-generational effects are context-sensitive, depending on the perceived risk. Future studies should examine cross-generational behavioral effects in ecologically meaningful environments instead of only neutral ones. Significance statement We exposed bank voles to odors signaling predation risk to assess the effects parental predation exposure on the behavior of their offspring. Besides predator odor, we also assessed the role of a conspecific alarm cue as a novel way of spreading the predation risk information. Pup behaviors were assessed in the open-field arena, a standard way of assessing animal behavior in a wide range of contexts. We found that also alarm pheromone increased the time pups spend in the center of the arena similarly to predator odor. While previous studies suggested that offspring would be more fearful, our results indicate that the cross-generational effects are very context-dependent; i.e., they differ significantly depending on which scent cue is presented in the open-field arena. This shows the need for better tools or measurements to translate laboratory results into ecologically meaningful frameworks.

In the evolutionary arms race between prey and predator, early risk recognition by the prey species is of paramount importance. Mammalian prey species are able to detect direct predator cues, like odors and to display appropriate defensive behaviors. Not much is known about indirect predation cues in mammals, i.e. the scent of scared individuals detectable by conspecifics, and how they affect recipient behavior. Current theories predict also cross-generational, maternally transferred, effects of increased predation risk or fear to their offspring. To escape predation now or in the next generation, predation risk is suggested to delay or suppress reproduction. However, in theory, enhancement of reproduction, bet-hedging or terminal investment, may be an adaptive strategy as well. Not much is known about cross-generational effects of predation risk on offspring behavior and fitness. We assessed how direct and indirect predation cues, in the form of predator odor or odor of scared conspecifics, alarm pheromones, affect bank vole (Myodes glareolus) reproduction and pup fitness. In our experiment, we exposed males and females either directly to least weasel (Mustela nivalis) odor, to indirect alarm pheromones from weasel-scared male voles, or to control odor. The treatments were started before mating and lasted until the pups were born. Contradictory to our expectations both predator odor and alarm pheromones enhanced reproduction compared to control. Alarm pheromone treated females had a significantly higher pregnancy rate and pups from predator-treated parents were significantly heavier at birth. Stress metabolite levels were similar in the predator odor and alarm pheromone treatment. Our study provides two novel results: compared to a signal of general danger, i.e. predator odor, the odor of a scared conspecific convey an immediate risk of attack and possible death. Both cues can work at the same time and trigger enhancement of reproduction in form of final investment.

Risk recognition by prey is of paramount importance within the evolutionary arms race between predator and prey. Prey species are able to detect direct predator cues like odors and adjust their behavior appropriately. The question arises whether an indirect predation cue, such as the odor of scared individuals, can be detected by conspecifics and subsequently affects recipient behavior. Parents may also transfer their experience with predators to their offspring. In two experiments, we assessed how direct and indirect predation cues affect bank vole (Myodes glareolus) foraging behavior, reproduction, and pup fitness. Weasel (Mustela nivalis) odor served as the direct cue, whereas the odor of weasel‐scared conspecifics, alarm pheromones, was used as an indirect cue and both of those were compared to a control odor, clean wood shavings. Alarm pheromones attracted female voles, measured as time in proximity to the treatment and foraging. Both predator odor and alarm pheromones enhanced reproduction compared to the control odor. Females treated with alarm pheromone had significantly higher pregnancy rates, and pups from predator‐treated mothers were significantly heavier at birth. Our study provides two novel ideas. First, the impact of a predator can be socially transmitted. Second, predation risk likely triggers terminal investment in reproduction.

Abstract Predation involves more than just predators consuming prey. Indirect effects, such as fear responses caused by predator presence, can have consequences for prey life history. Laboratory experiments have shown that some rodents can recognize fear in conspecifics via alarm pheromones. Individuals exposed to alarm pheromones can exhibit behavioural alterations that are similar to those displayed by predator-exposed individuals. Yet the ecological and evolutionary significance of alarm pheromones in wild mammals remains unclear. We investigated how alarm pheromones affect the behaviour and fitness of wild bank voles (Myodes glareolus) in outdoor enclosures. Specifically, we compared the effects of exposure of voles living in a natural environment to a second-hand fear cue, bedding material used by predator-exposed voles. Control animals were exposed to bedding used by voles with no predator experience. We found a ca. 50% increase in litter size in the group exposed to the predator cue. Furthermore, female voles were attracted to and males were repelled by trap-associated bedding that had been used by predator-exposed voles. Movement and foraging were not significantly affected by the treatment. Our results suggest that predation risk can exert population-level effects through alarm pheromones on prey individuals that did not encounter a direct predator cue.