Food-deprivation-induced changes in sexual behaviour of meadow voles, Microtus pennsylvanicus

Small rodent population cycles and plants - after 70 years, where do we go?

Small rodent population cycles characterise northern ecosystems, and the cause of these cycles has been a long-lasting central topic in ecology, with trophic interactions currently considered the most plausible cause. While some researchers have rejected plant-herbivore interactions as a cause of rodent cycles, others have continued to research their potential roles. Here, we present an overview of whether plants can cause rodent population cycles, dividing this idea into four different hypotheses with different pathways of plant impacts and related assumptions. Our systematic review of the existing literature identified 238 studies from 150 publications. This evidence base covered studies from the temperate biome to the tundra, but the studies were scattered across study systems and only a few specific topics were addressed in a replicated manner. Quantitative effects of rodents on vegetation was the best studied topic, and our evidence base suggests such that such effects may be most pronounced in winter. However, the regrowth of vegetation appears to take place too rapidly to maintain low rodent population densities over several years. The lack of studies prevented assessment of time lags in the qualitative responses of vegetation to rodent herbivory. We conclude that the literature is currently insufficient to discard with confidence any of the four potential hypotheses for plant-rodent cycles discussed herein. While new methods allow analyses of plant quality across more herbivore-relevant spatial scales than previously possible, we argue that the best way forward to rejecting any of the rodent-plant hypotheses is testing specific predictions of dietary variation. Indeed, all identified hypotheses make explicit assumptions on how rodent diet taxonomic composition and quality will change across the cycle. Passing this bottleneck could help pinpoint where, when, and how plant-herbivore interactions have - or do not have - plausible effects on rodent population dynamics.

A neural pathway underlying hunger modulation of sexual receptivity in Drosophila females

Accepting or rejecting a mate is one of the most crucial decisions a female will make, especially when faced with food shortage. Previous studies have identified the core neural circuity from sensing male courtship or mating status to decision-making for sexual receptivity in Drosophila females, but how hunger and satiety states modulate female receptivity is poorly understood. Here, we identify the neural circuit and its neuromodulation underlying the hunger modulation of female receptivity. We find that adipokinetic hormone receptor (AkhR)-expressing neurons inhibit sexual receptivity in a starvation-dependent manner. AkhR neurons are octopaminergic and act on a subset of Octβ1R-expressing LH421 neurons. Knocking down Octβ1R expression in LH421 neurons eliminates starvation-induced suppression of female receptivity. We further find that LH421 neurons inhibit the sex-promoting pC1 neurons via GABA-resistant to dieldrin (Rdl) signaling. pC1 neurons also integrate courtship stimulation and mating status and thus serve as a common integrator of multiple internal and external cues for decision-making.

Impact of food availability and predator cues on meadow vole response to social vs. non-social odorants

The risk of predation and food deprivation may alter the degree to which animals associate with conspecifics. We examined if food deprivation, the risk of predation, or simultaneous exposure to both altered meadow voles’ preference for odour cues in a way that adheres to the terminal investment, safety in numbers, or avoidance hypotheses. Satiated or food-deprived meadow voles were given the choice to investigate either opposite-sex conspecific bedding, same-sex conspecific bedding, clean bedding, or self-bedding when exposed to mink urine or olive oil. Mink urine and food deprivation did not impact the amount of time meadow voles spent with each type of bedding, but meadow voles did begin investigating more quickly when experiencing either or both stressors. However, food deprivation and mink urine did not have an additive impact on any measured variable. Further research is needed to determine if the terminal investment hypothesis is the hypothesis that best describes the mating behaviour of meadow voles facing one or multiple stressors.

Meadow voles differentiate between scents of different sources

Scent marks are an important means of transmitting information between rodents, and they can be produced from several body sources. Previous studies have shown that scents from multiple sources can convey the same information; female meadow voles, for example, have three scent sources that communicate sex. However, possessing three separate sources that convey the same information is likely costly due to the metabolic energy required to produce these signals and the increased chance that eavesdropping individuals may intercept information present in these signals. In this study, we investigated if these scent sources could communicate other information, in addition to scent donor sex, by determining if male meadow voles could distinguish scent marks taken from different sources of a single female scent donor. This was accomplished with a habituation-test method, that allowed us to compare how male meadow voles differently investigate scent from a familiar and novel source of a female scent donor. Male meadow voles could distinguish between faeces and urine scent marks of a female, but could only distinguish mouth from urine and faeces scent marks when first familiarized with mouth scent marks. Our findings suggest that mouth, urine, and faeces scent marks of female meadow voles produce both redundant and distinct information. The overlap in information between scent marks produced from separate sources may be needed to provide social context, which allows receiving individuals to accurately weigh the tradeoffs associated with responding to an olfactory cue. While this overlap in information remains costly, this cost may be minimized by the different fade-out times of scent marks from distinct body sources, which may limit the amount of time information in a scent mark is available to a time period where this information is socially relevant.

Odor Communication and Mate Choice in Rodents

This paper details how chemical communication is affected by ecological challenges such as finding mates. I list several conditions that affect the decision to attract mates, the decision to respond to the signals of potential mates and how the response depends on context. These mate-choice decisions and their outcomes will depend on the life history constraints placed on individuals such as their fecundity, sex, lifespan, opportunities to mate in the future and age at senescence. Consequently, the sender's decision to scent mark or self-groom as well as the receiver's choice of response represents a tradeoff between the current costs of the participant's own survival and future reproduction against that of reproducing now. The decision to scent nark and the response to the scent mark of opposite-sex conspecifics should maximize the fitness of the participants in that context.

The Effects of Food Availability on the Sexual Behavior of Meadow Voles

Two competing hypotheses have been used to explain species and sex differences in the effects of food availability on the sexual behavior of mammals. One hypothesis, the metabolic fuels hypothesis, posits that individuals, primarily females, forgo reproduction when faced with food deprivation and invest their limited resources in behaviors not associated with reproduction. The other hypothesis, the reproduction at all costs hypothesis, states that individuals, males and females, continue to reproduce or increase their reproductive behavior when faced with food deprivation. Results show female voles followed the predictions of the metabolic fuels hypothesis. That is, food-deprived female meadow voles were less attractive, proceptive, and receptive toward males than were female voles that had continuous access to food. Dams that were food deprived late in lactation failed to enter postpartum estrus, a period of heightened sexual receptivity that occurs immediately after parturition. Females that were food deprived during the first or second week of lactation spent less time involved in maternal care compared to dams that were not food deprived. In contrast, male voles mainly followed the reproduction at all costs hypothesis. Food-deprived male voles were as attractive, as proceptive, and as sexually receptive toward females as were males that had continuous access to food.

The time interval between sequential mating affects the response of male meadow voles to previous mates

Male meadow voles differed in their copulatory behaviour, mating and reproductive success, and odour preferences for successively encountered mates, when the interval between pairing with two females was 1 h, 1 day, 4 days, 7 days or 10 days. Male voles had shorter latencies to first ejaculation with female 2 than with female 1 when the interval was 1 h, which may be attributed to heightened sexual arousal in males for female 2. Males also had shorter copulatory bouts with female 2 than with female 1 when the intervals between pairings were 1 h and 1 day. Shorter copulation bouts may allow more mating opportunities for both sexes and encourage multiple mating. Male voles also preferred the scent marks of female 2 to those of female 1 when the interval between pairing was 7 or 10 days, suggesting that these females were treated as distinct entities.

Intermittent Food Absence Motivates Reallocation of Locomotion and Feeding in Spotted Munia (Lonchura punctulata)

Background: Daily feeding and locomotion are interrelated behaviours. The time spent in feeding and rate of food intake depends on food availability. In low food condition, the birds would show intense movement (locomotion) for a longer time throughout the day however during abundant food supply they may chose higher activity and food intake in the morning and evening only. In the present study we hypothesized that in Spotted Munia (Lonchura punctulata), intermittent food availability during day would reallocate their interrelated behaviors, the feeding (food intake) and locomotor activity patterns.

Methods: Two groups of birds (N = 6 each) were kept individually in activity cages under 12L:12D. Group 1 (Control; C) had ad libitum food but group 2 (Treatment; T) had food for 6 hours only (2 h presence followed by 2 h absence; 2P:2A) during 12 hour light period. In the first week, group 2 received food with ‘lights on’ (TI; ZT 0–2, 4–6 and 8–10; where ZT 0= zeitgeber time 0, time of lights ON). In the following week, the food was given 2 hours after ‘lights on’ (TII; ZT 2–4, 6–8, 10–12). The food intake and locomotor activity under each condition were observed.

Results: The results showed that locomotor activity was induced during food deprivation and suppressed during food availability. Also the food deprivation led to increased food intake.

Conclusion: Our results suggest that intermittent food availability/deprivation reallocates the locomotor activity and food intake in Spotted Munia.

The response of rodents to scent marks: four broad hypotheses

This article is part of a Special Issue "Chemosignals and Reproduction". Many terrestrial mammals must be able to distinguish between the myriad of scent marks they encounter in order for them to facilitate or deter direct interactions with their scent donors. I review studies that examine how rodents, mainly meadow voles (Microtus pennsylvanicus), respond when they encounter the scent marks of conspecifics and heterospecifics, and how context, as well as the age and condition of senders and receivers, affect their responses. The review uses four broad hypotheses to discuss the response of rodents to scent marks. The four hypotheses are as follows: 1) Scent marks convey accurate information to the receiver about the sender's state and phenotype and genotype. 2) Scent marks are individually distinct. 3) The response of receivers to scent marks is flexible and would be modulated by the cognitive abilities of receivers. 4) Receivers respond to the information contained or conveyed by the scent mark in a manner that will increase their survival and fitness. The studies cited in this review show that scent marks signal accurate information about the sender's phenotype, genotype, and condition, which receivers use to distinguish among the scent marks of different conspecifics and heterospecifics, and by doing so, receivers tailor their response accordingly to increase their survival and fitness. Thus, the four broad hypotheses may serve as guide to increase our understanding of the response of receivers to scent marks and provide a conceptual framework for future research and the development of additional hypotheses.

Maternal Food Restriction During Lactation Affects Body Weightand Sexual Behavior of Male Offspring in Meadow Voles (Microtus pennsylvanicus)

Little is known about the occurrence of individual variation in sexual behavior and how maternal nutrition can affect this variation. We tested the hypothesis that male offspring of female meadow voles, Microtus pennsylvanicus, that were 30% food restricted (FR) during days 1-7 of lactation (FR 1-7), days 8-14 of lactation (FR 8-14), or late days 15-21 of lactation (FR 15-21) lactation show persistent, negative effects on their sexual behavior as adults relative to male offspring of females that were not food restricted. We measured three components of sexual behavior, attractivity, proceptivity and receptivity, beginning when the males were 98 days of age. Food restriction during middle lactation (FR 8-14) but not during early (FR 1-7) and late lactation (FR 15-21) was sufficient to induce adult male voles to produce anogenital marks that were not as attractive as those produced by control males. Food restriction during lactation did not affect the proceptive behavior of male voles but did affect their receptivity. Only 4 of 12 FR 8-14 male voles mated compared to 9 of 12 FR 1-7 males, 8 of 12 FR 15-21 males, and 8 of 11 control males. However, no differences existed in their copulatory behavior among the males that did mate. The body weight of FR 1-7 and FR 8-14 males was lower than that of FR 15-21 and control males when they were between 22 days of age (weaning) and 48 days of age (puberty) but was similar when the males were 98 days of age. Food intake was similar for the FR and control males between day 22 and day 98. It remains unclear, however, if this type of maternal effect represents strategic programming of offspring behavior in response to the environment experienced by mothers or is a product of developmental processes of food restriction prior to weaning (Forstmeier et al. 2004).

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Female meadow voles, Microtus pennsylvanicus, do not alter their over-marking in response to female conspecifics that were food deprived

Many terrestrial mammals will deposit scent marks and over-marks, the latter being the overlapping scent marks of two conspecifics. Studies have shown that male rodents that are exposed to the overlapping scent marks of two female conspecifics later spend more time investigating the mark of the top-scent female than that of the bottom-scent female. This suggests over-marking is a form of competition and that the top-scent female is more likely than the bottom-scent female to be chosen as a potential mate. Thus, females should over-mark the scents of neighboring females at a rate that will maximize their chances of attracting males. However, meadow voles live in areas that may contain patchy food availability, and residents may differ in their nutritional status. Females in a better nutritional state may be more likely than those in poorer nutritional states to indicate their presence in an area, signal possession of a territory, and to attract mates. Thus, we tested the prediction that female meadow voles, Microtus pennsylvanicus, that were not food deprived would deposit more over-marks than female voles that were food deprived for six hours. Food-deprived female voles and female voles that had continuous access to food deposited a similar number of scent marks and used a similar proportion of those marks as over-marks when they encountered the scent marks of female conspecifics. These findings suggest that the nutritional status of female voles does not affect their ability to signal their presence in an area marked by a female conspecific.

Re-feeding food-deprived male meadow voles affects the sperm allocation of their rival males

An individual's nutritional status affects the manner in which same- and opposite-sex conspecifics respond to that individual, which may affect their fitness. Male meadow voles, Microtus pennsylvanicus, increase their sperm allocation if they encounter the scent mark of an unfamiliar male that is not nutritionally challenged. If, however, the scent mark comes from a male that has been food deprived for 24 hours, stud male voles do not increase their sperm allocation. Food deprived males may be viewed as being lower quality and a reduced risk of sperm competition by rival males. We hypothesized that stud males in promiscuous mating systems tailor their sperm allocations depending on whether rival males have been food deprived and then re-fed. We predicted that newly re-fed males will be considered a strong risk of sperm competition because of the potentially high fitness and survival costs associated with food deprivation in males, and that they will cause stud males to increase their sperm allocation. Our results, however, showed that the recovery period from 24 hours of food deprivation was a relatively slow process. It took between 96 hours and 336 hours of re-feeding male scent donors that were food deprived for 24 hours to induce stud males to increase their sperm allocation to levels comparable to when scent donors were not food deprived. Stud male voles may be conserving the amount of sperm allocated until the male scent donors have recovered from food deprivation and subsequent re-feeding.

Food deprivation and restriction during late gestation affects the sexual behavior of postpartum female meadow voles, Microtus pennsylvanicus

In many species of small mammals, including meadow voles, Microtus pennsylvanicus, females come into postpartum estrus (PPE) within 12-24 h of giving birth, allowing them to mate and become pregnant while raising the current litter. PPE females show increases in attractivity, proceptivity, and receptivity, the three components of sexual behavior, relative to females not in PPE. Several studies have shown that food deprivation and restriction reduce attractivity, proceptivity, and receptivity of females not in PPE. We tested the hypothesis that food deprivation and restriction during late gestation causes deficits and decreases the attractivity, proceptivity, and receptivity of females when they enter PPE. Our data support the hypothesis. On day 1 of lactation, females that were food deprived and food restricted produced scent marks that were significantly less attractive as those produced by control PPE females. Food deprivation but not food restriction caused females to no longer display significant preferences for the scent marks of males over those of females (proceptivity). Food deprivation and food restriction were sufficient to induce females to become significantly less sexually receptive than control females. Eleven of 12 control PPE females mated, 4 of 12 food-restricted females mated, and 3 of 12 food-deprived females mated. Dams facing food deprivation or restriction during late gestation may have to balance the benefits of mating during PPE with the increased costs associated with getting pregnant while they are lactating.

Sperm investment in male meadow voles is affected by the condition of the nearby male conspecifics

Sperm competition occurs when 2 or more males copulate with a particular female during the same reproductive cycle, and their sperm compete to fertilize the female's available eggs. One strategy that male voles use to assess the risk and intensity of sperm competition involves responding to the presence of scent marks of conspecific males found near a sexually receptive female. Previously, we have shown that if a male vole copulated with a female while he was in the presence of the odors of another male he increased his sperm investment relative to his investment if another male's odors were not present. The aim of the present study was to test the hypothesis that males assess differences in the relative quality of competing males and adjust their sperm investment accordingly. We did so by allowing males to copulate when they were exposed to the scent mark of a 24-h food-deprived male (low-quality male) or the scent mark of a male that was not food deprived (high-quality male). The data indicate that male meadow voles did not increase their sperm investment during copulation when exposed to the scent mark of a food-deprived male but did so when they were exposed to the scent mark of a male that was not food deprived. The results support the hypothesis that male voles are able to adjust sperm investment when they encounter the scent marks of males that differ in quality.

Exogenous melatonin administration affects self-grooming and conspecific odor preferences in long-photoperiod meadow voles (Microtus pennsylvanicus)

For meadow voles, Microtus pennsylvanicus, seasonal differences exist in self-grooming and in odor preferences for conspecifics, two behaviors which facilitate sexual interactions in this species. Both behaviors are mediated by photoperiodically-induced changes in circulating gonadal steroid hormone titers which, in turn, can be transduced by the duration of the melatonin signal that a seasonally breeding animal receives. The goal of this study was to determine whether exogenous melatonin administration affects circulating gonadal steroid hormone titers in meadow voles, and whether it influences their odor preferences and self-grooming behavior to same- and opposite-sex conspecifics. Long-photoperiod voles that did not receive exogenous melatonin had higher testosterone (males) and estradiol (females) titers than did short-photoperiod voles and long-photoperiod voles treated with melatonin for 12 weeks; the latter had similar estradiol and testosterone titers. Long-photoperiod voles that did not receive melatonin preferred the scent marks of long-photoperiod opposite-sex conspecifics and spent more time self-grooming in response to their odors than those of either long-photoperiod same-sex, short-photoperiod same-sex, or short-photoperiod opposite-sex conspecifics. Long-photoperiod voles that received melatonin, however, no longer preferred the marks of long-photoperiod opposite-sex conspecifics and no longer spent more time self-grooming in response to their odors, not unlike the odor preferences and self-grooming behavior of short-photoperiod voles. As a whole, the data suggest that the duration of the melatonin signal is likely involved in mediating the photoperiodically-induced changes in gonadal steroid hormones that mediate a meadow vole's odor preferences for opposite-sex conspecifics and its self-grooming response to those marks.

Food deprivation and the role of estradiol in mediating sexual behaviors in meadow voles

Female mammals are particularly sensitive to changes in food availability. The mechanisms that affect sexual behavior and food intake are closely related to one another; chief among the mechanisms that control sexual behaviors in females is estradiol. In order to understand how food deprivation results in inhibition of sexual behavior (attractivity, proceptivity, and receptivity), we measured the effects of food deprivation on circulating concentrations of estradiol. We also determined whether estradiol treatment was sufficient to restore sexual behaviors in food-deprived female meadow voles. We found that estradiol titers of food-deprived female voles are significantly lower than those of ad lib-fed female voles. Further, we found that estradiol treatment was sufficient to restore proceptivity and receptivity in food-deprived, ovariectomized female voles. However, estradiol treatment was not able to overcome the food deprivation-induced inhibition of attractivity. Thus, decreases in estradiol titer of food-deprived female voles may be related to the suppression of their proceptive and receptive behaviors, and may be a mechanism that allows females to avoid mating when conditions are not propitious for their survival and that of their offspring.