Image courtesy of W. J. Hamilton Jr. from "The Biology of the Little Short-Tailed Shrew, Cryptotis parva." (Journal of Mammalogy) Permission pending.
Mating System
Overlap Promiscuity
In general, little is known regarding the reproductive behavior of the least shrew. However, through field notes and observations in captivity, we do know that the least shrew is a promiscuous species. More specifically, the least shrew engages in overlap promiscuity. Species which exhibit overlap promiscuity in their mating system usually live in home ranges and are most active at night. The males' home ranges overlap the ranges of several females, permitting promiscuity and numerous copulations. Also, in a number of species which exhibit overlap promiscuity, the females make use of pheromones to attract males during the breeding season. All these characteristics of overlap promiscuity coincide quite nicely with the least shrews' mating behavior, thus lending strong support to the notion that the least shrew exhibits overlap promiscuity as its mating system. Although there is one study which found that males' home ranges are smaller in size than females' home ranges, these data are likely to be inaccurate due to a limited sample size and in fact the males' home ranges are probably considerably larger than the females'. In a promiscuous mating system, a bond or relationship does not exist between mating partners. Furthermore, litters may be biased towards the male gender, as oftentimes in species which exhibit a promiscuous mating system, the males possess higher reproductive fitness than the females, as they are able to copulate more often than the females. However, this biasing of the litter has not been researched or documented as of yet in the least shrew.
As a promiscuous species, the least shrews only spend a short amount of time in the process of courting, and mating occurs fairly rapidly. Female shrews generally mate with many different males and multiple paternity of litters often occurs. Due to the high metabolic rates of shrews and the high energy costs of reproduction, shrews reproduce when food resources are most abundant and when weather conditions are optimal. This means that the least shrew is breeding most actively during the summer months. Least shrews can have one to four litters per year and usually mate from March to November in the northern parts of their range, but are able to mate throughout the year in the southern regions of their geographic range. The general pattern of copulatory behavior is similar to that found in the Blarina species of shrew. Cryptotis parva also does not mate over more than two days. One researcher discovered that the male least shrews produce calls or clicks during mating (Gould 1969). The gestation period is short, ranging from 21 to 23 days. The litters are comprised of anywhere from two to seven young, with an average litter size of five. The young are weaned at three weeks of age.
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Image courtesy of W. J. Hamilton Jr. from "The Biology of the Little Short-Tailed Shrew, Cryptotis parva." (Journal of Mammalogy) Permission pending. |
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Parental Care
In terms of parental care, research indicates that both least shrew parents engage in parental care of the offspring. In captivity studies, when a newborn least shrew was removed from the cage to be measured, its mother became very agitated and scurried around biting at the fingers of the experimenters (Hamilton 1944). The mother's agitation at removal of the young demonstrates the presence of significant parental care. All adults also help with construction and maintenance of the nest. When researchers intentionally scattered the nest, the adult shrews picked up the young and quickly carried them to specified areas of the cage (Conaway 1958). Once the old nest was re-assembled, the adults eventually brought all the young back together at the old nest site. Conaway also described a situation in which two adults tried to pick up and take care of the same newborn individual. When this occured, fighting over the care of the young took place (Conaway 1958). However, the young least shrew was not injured, and the fighting only continued until one adult got control of the young and carried it off to take care of it. Other captivity studies have also shown demonstrated parental care and described the phenomenon of the mother picking up the young in her mouth and placing them in a particular section of the cage (Whitaker 1974). However, parental care can sometimes produce negative effects, owing to the overexuberance of the adults engaging in parental care. When a large number of adults are around the nest, the young tend to die from starvation because the extra adults get in the way and prevent the young from nursing (Whitaker 1974). There is no evidence of cooperative breeding among the least shrews. Caring for the young seems to fall sqaurely on the shoulders of the adults in the population within a communal nest. Although not much research has been done on the cooperative breeding subject, cooperative breeding would be unlikely based on Whitaker's finding that too many extraneous individuals milling around the mother interfere with the newborns' ability to effectively nurse. Taking this to be the case, cooperative breeding would not produce benefits for the least shrews and in fact would severely interfere with the survival of the species, as many young are likely to die off due to starvation.
Chemical Signaling
A particularly interesting study was conducted examining the effects and implications of chemical signaling within the least shrew (Kivett and Mock 1980). The researchers examined the aural gland of the female least shrew. The male least shrews were quite interested in this particular gland and were especially interested when a female was in estrous. After sniffing the aural gland of a female, the males attempted to mount the females (Kivett and Mock 1980). Before introduction of the female, males were friendly towards each other and did not show any signs of aggression or competition. With the introduction of the female, a dominance hierarchy was established, and the one male (out of three) who was most aggressive got to mate with the female first. Thus, a temporary dominance hierarchy is established for mating. It appears that social status can determine the order of mating with the female shrew. This finding suggests the the least shrew mating system may be hierarchical promiscuity instead of overlap promiscuity. However, this is an isolated finding and requires further confirming evidence, as most evidence and documentation to date lends the support to an overlap promiscuity system for the least shrew.
So it seems that chemicals and hormones secreted from the females’ aural gland play an important role in reproductive behavior and signaling in the least shrew. The aural gland may signal to the males whether or not the females are pregnant or in estrous. Males did not engage in courtship behavior or mounting after sniffing the aural gland of pregnant females, but continued courtship and other investigatory behavior of non-pregnant females (Kivett and Mock 1980). Cohesive group behavior and frequent interaction with the females while searching for food or around the colony site allow males to easily assess the reproductive condition of the females. Dispersing a repellent scent may be important in that the pregnant females do not want to be bothered by the courting males when they are foraging for food or nursing their young (Kivett and Mock 1980). By dispersing this repellant scent, females may also act to reduce the male-male agonism and fighting that could occur over one particular female. This fighting could slow down the courtship or mating process, and this is not good for a species with such a high metabolism and with such high energetic costs for reproduction. Disruption at the nest site and disruption of maternal care by competing males would certainly not be advantageous, so the female exudes a signaling repellant scent to the males. When a female is not pregnant and is in estrous and ready to mate, the female can then turn off the repellant scent production, so that the absence of this scent will thus signal males to come to mate with the female. But if already pregnant, the enlarged aural glands of these pregnant females will secrete the repellant scent in order to keep males away. As the least shrews often live communally, this chemical signaling can help to minimize the cost of competition for mates which comes along with group living.
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Image courtesy of V. Keith Kivett and Orin B. Mock from "Reproductive Behavior in the Least Shrew (Cryptotis parva) with Special Reference to the Aural Glandular Region of the Female." (American Midland Naturalist) |
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Reproductive Behavior
One group of researchers had the opportunity to extensively examine the reproductive behavior of the lesser white-toothed shrew, Crocidura suaveolens (Zuri and Rado 2000). Although the following information does not come from direct observation of the least shrew itself, the lesser-white toothed shrew is a similar species and it is likely that the mating and reproductive behavior of the least shrew functions in a similar manner to that of the lesser white-toothed shrew. In their study, Zuri and Rado noted sexual conflict between the male and female shrews. The males attempted to mount the females during the first encounters between the two. The females exhibited agonistic behavior toward the males during this process. Clearly the males and the females are not on the same wavelength and may have different goals in mind. The males were equally aggressive towards both sexes during the breeding period, but female aggression towards another female was only seen on rare occasions. Familiarity between individuals produced less aggression and increased the shrews’ tendency to spend time with another, regardless of whether the time spent was with the opposite or same sex (Zuri and Rado 2000).
Reproductive Strategies
Zuri and Rado suggest that sexual differences in the reproductive behavior of shrews are due to the differences in breeding strategies of males and females. The males attempt to mate with the highest number of females possible and thus maximize their fitness, while the females try to establish long-term relationships with specific males. The females are most concerned with turning food resources into offspring and the males are most concerned with copulating with the highest number of females possible. The males who the female are trying to establish a long-term relationship with are the "best" and most successful males. The females are employing the He-Man strategy in which they search for the best male in order to procure the most beneficial genes for their offspring. In this strategy, the females do not expect the males to stay around and take care of the offspring, but research has suggested that both least shrew parents indeed do engage in parental care. This suggests that the Domestic Bliss mating strategy might also be at work here in which the male least shrew does bring food back to the female and helps her forage and stockpile food. In this strategy, the female expects a little more investment from the male, yet questionable paternity still exists. No research results have recorded any male least shrews engaging in mate-guarding behavior, a characteristic of the Domestic Bliss strategy. Males engage in numerous copulations in response to the domestic bliss strategy, and this is true in the least shrews. So, it seems that the mating behavior of the least shrew cannot be totally explained by either the He-Man or the Domestic Bliss strategies, but for now, we must agree on a combination of elements of both explanations and support further research and inquiry into the mating sytem of Cryptotis parva.
These researchers also discovered that long-term familiarity between the male and females strengthens the pair bond, increases the female's reproductive success, and allows the females to obtain food resources from within the male’s home ranges (Zuri and Rado 2000). The most successful males will have the largest home ranges and will be able to provide a larger quantity of resources to the female. Stockpiling food and conserving energy are important for the female least shrew as she is preparing to give birth and being nursing her young.
This website was completed in partial fulfillment of the requirements for Biology 323, Animal Behavior, at Davidson College in the Spring Semester 2008.
© Copyright 2008 Department of Biology, Davidson College, Davidson, NC 28035
Please send any comments, questions, or suggestions regarding this website to Nate Geigle or Professor Verna Case.