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Pine vole mating behavior is categorized as
facultative monogamy because the species lives on a home range and the females
do not need a male to defend any territory. Another reason for the facultative
monogamy categorization is that there is little opportunity for polygyny
because females are so dispersed with little overlap of different colonies.
There are some instances of polygyny and polyandry, but they are extremely
rare (Whitaker & Hamilton; 1998). Another factor that limits the opportunity
males have for polygyny is the fact that the breeding female will suppress
the reproduction of the other helper females in the colony (Brant, Schwab,
Vandenbergh, Schaefer, & Solomon; 1998). The breeding season in the
North is early March to anywhere from November to January, but breeding
continues throughout the year in the South (Whitaker & Hamilton). Female
pine voles are induced ovulators, which means that they must sense chemo
signals from males and have physical contact in order to enter estrus (Solomon,
Vandenbergh, Wekesa, & Barghusen; 1996). The vomeronasal organ system
senses chemo signals given off by the male, and could have evolved as a
necessary element in estrus induction because they indicate to the female
whether the male is familiar or unfamiliar (Solomon, Vandenbergh, Wekesa,
& Barghusen). |
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Young female pine voles can conceive as early
as 77 days, but often do not until around 105 days (Whitaker & Hamilton).
After copulation, female pine voles develop a vaginal plug that lasts for
3 days, but it is not visible externally (Whitaker & Hamilton). Gestation
lasts 20-24 days (Whitaker & Hamilton). Females produce 1-4 litters
per year, each with 1-5 young (Whitaker & Hamilton). Despite the low
reproductive rate of pine voles, the species maintains a stable population
size (Whitaker & Hamilton). This is probably because of their effective
strategy of living under ground to avoid predation.
The death of a breeding parent and replacement by an unrelated individual
results in conflict between the surviving breeder and its same sex offspring
over mating opportunities (Brant, Schwab, Vandenburgh, Schaefer, and Solomon;
1998). Mothers establish their dominance over their daughters by tugging
behavior away from unfamiliar males, other behavioral signs of social
status, use of chemical signals (Brant, Schwab, Vandenburgh, Schaefer,
and Solomon). Reproductive suppression of lower status/non-breeding females
results from reproductive competition between females, but can also function
to prevent inbreeding between offspring and parents or siblings (Brant,
Schwab, Vandenburgh, Schaefer, and Solomon). Reproductive competition
for breeding opportunities is only an issue when unrelated individuals
join or visit the nest, since breeding pine voles are monogamous and usually
won't mate with unfamiliar individuals (Brant, Schwab, Vandenburgh, Schaefer,
and Solomon). The aggressiveness of female pine voles toward unfamiliar
males adds to the predominance of monogamy because breeding females will
remain loyal to the familiar breeding male (Geyer, Beauchamp, Seygal,
& Rogers; 1981). The introduction of an unrelated male provides a
breeding opportunity for the previously non-breeding females in a colony,
even though the breeding female stands as a huge obstacle (Brant, Schwab,
Vandenburgh, Schaefer, and Solomon).
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