Northern Prairie Wildlife Research Center
Data for this study were available from two study areas, Ellesmere Island, Northwest Territories, Canada (80°N, 86°W) (Mech 1988, 1995) and Yellowstone National Park, Wyoming (45°N 110°W) (Cook 1993), and from a pack of four 2-year-old captive Wolves, including three females, at the International Wolf Center in Ely, Minnesota (48°N, 92°W).
In the first area, an identifiable Wolf, "Whitey," born in 1987 and habituated to the senior author was observed each summer 1987-1995 from ≥1 meter (Mech 1988, 1995). Whitey produced pups in early June1990, 1991, 1992, and 1994 in or near her natal den (Mech 1988, 1993; Mech and Packard 1990) and consistently remained with her pups throughout June (Mech 1995). However, in 1993 and 1995 she did not produce pups, as evidenced by close observation by the senior author. There was a lack of nipple development and she failed to localize consistently at a den.
In 1993, none of Whitey's known dens showed sign of digging or use on 2 July, and she was observed intermittently from 5 to 30 July 1993 traveling with her mate and 1992 offspring (Mech in press). However, Whitey visited her natal den area with her pack from 2200 hours on 13 July 1993 to 1220 hours on 14 July. Her mate even brought an Arctic Hare (Lepus arcticus) leveret to the den as if to give it to Whitey. However, Whitey was temporarily away from the den, and after 45 minutes, a yearling stole the Hare. Whitey lay near the den at 0002 hours, and from 0329 to 0439 hours, she dug out one entrance and entered the den six times for periods of 7-19 minutes each. She continued to sleep or rest until 1220 hours, 14 July when the pack left.
In 1995, Whitey was observed away from the den on 5 July and was not observed at her natal den during three aerial checks for the senior author by Department of Defense personnel from 28 May to 29 June. None of her dens showed signs of use on 29 June, although her natal den was not checked closely on that date. On 5 July, however, Whitey was observed 8 km from her natal den and had no nipple development, contrary to her appearances during her pup-bearing years.
Whitey's natal den was freshly dug out on 4 July, 1995 although no pups were present. Whitey's mate visited the den area on that date. On 5 July, Whitey visited the den at 1621 hours. She entered the den several times, inspected its various entrances, and dug it out, all for 18 minutes. Then she slept near the den from 1709 on 5 July to ≥0513 hours on 6 July. At 1800 hours on 5 July, Whitey's 3-year-old male offspring arrived and entered and inspected the den for about a minute. Whitey also walked into the den for a few seconds at 1829 hours on 5 July. Then the male slept near Whitey through ≥0513 hours on 6 July. Both Wolves were there when we left on 0513 hours but were gone at 1650 hours on 6 July and 1800 hours on 7 July.
The Yellowstone Wolf, 005F, was captured along with her mate (004M) and 4 young males in Alberta, Canada on 10 January, 1995. Her teats indicated that she had bred in the past (Mech et al. 1993). She and her packmates were placed in a 1-acre enclosure in Yellowstone and held for 10 weeks before being released in late March. Phillips and Smith monitored the behavior of Wolf 005F by direct observation while she was in captivity and by radio-tracking and direct observation after her release with her pack from 31 March to 31 July 1995. During confinement, 005F and 004M exhibited breeding behaviors, and a bloody discharge from 005F's vulva was apparent.
Wolf 005F first showed signs of denning behavior on 29 April. Meanwhile two other females captured in the same area at the same time as Wolf 005F and similarly transported, held, and released in the same region, produced pups about 26 April.
Wolf 005F explored Coyote (Canis latrans) dens several times from 29 April through 12 May. For about 2 hours on 29 April, 005F and her packmates were observed near an unoccupied Coyote den (hole 1). Her mate and one yearling spent a few minutes excavating the hole. On 3 May, the pack bedded near a second unoccupied Coyote den (hole 2), about 400 m west of hole 1. From 0615 to 0821, 005F slept at the den entrance, and from 0821 to 0830 she entered the den three times, each time disappearing for about a minute. At 0843, she entered hole 1, leaving at 0845. At 1225 hours, 005F, her mate, and a yearling bedded at a third unoccupied Coyote den (hole 3) about 4.8 km east of holes 1 and 2. Again, throughout 7 May, the entire pack was observed near holes 1 and 2, and On 11 May, 005F entered hole 1 for 8 minutes.
On 12 May, between 1750 and 1844 hours, 005F dug intensively at a fourth hole about 400 m west of hole 3. Her mate also dug for about 3 minutes. The hole was large, and 005F sometimes disappeared. Clouds of dust indicated that she continued to dig while out of sight. The only time 005F stopped digging is when displaced by a small herd of Bison (Bison bison) that approached the hole for 3-5 minutes. Wolf 005F then returned and resumed digging. At 1844 hours, 005F ceased digging and rejoined the pack which had traveled about 2.0 km east. During the next few weeks, 005F and pack remained within a few km of holes 1-4, but no more digging was seen. By early July, the pack began frequenting Pelican Valley about 32 km south of the holes.
Three female captive Wolves in Minnesota were ovariectomized by Kreeger on 6 May 1994, when about 1-year old. During spring 1995, only one of the three showed any vaginal bleeding (7-10 days), but it was light. No other courtship or reproductive behavior was apparent. On 20 and 21 July 1995, at least two of the female Wolves, not including the one that showed minor bleeding, dug a large den. (Wild Wolves use dens in this area from about 27 April through June [Mech unpublished]). These two wolves probably were completely ovariectomized since they showed no bleeding.
The latter observation tends to support the report of a wild Wolf that had never ovulated but still paired with a male, held a territory, and restricted her movements for several weeks during the denning season, although it was unknown whether she actually attended a den (Mech and Seal 1987).
The above observations suggest that den digging is not a function of pregnancy or even of ovarian estrogen or progesterone. Conceivably, the den digging could have been in response to heat, as is typical of sled dogs. However, this would not explain the Ellesmere or Yellowstone observations because heat was not a factor in those cases.
A possible endocrine-based explanation for such denning behavior may lie with rising prolactin concentrations. Prolactin, a hormone secreted from anterior pituitary lactotrophs, induces both maternal and paternal behavior in a variety of species (Zarrow et al., 1971; Dixson and George, 1982; Bridges et al. 1985; Numan, 1988; Gubernick and Nelson, 1989). Both intact or neutered, male and female Wolves, show a strong circannual prolactin rhythm (Kreeger et al. 1991). Prolactin peaks just prior to summer solstice, coincident with whelping.
All Wolves in a pack, including unrelated members, begin feeding other pack members even before whelping (Fentress and Ryon, 1982). Thus, pups are not required for this behavior. Based on these observations, it was hypothesized that increasing prolactin in spring elicits parental behavior in Wolves (Kreeger et al., 1991). Our observations of den digging by males and by females, in the absence of pregnancy or elevated progesterone or estrogen, support this hypothesis and lead us to further hypothesize that prolaction elicits or mediates den digging in Wolves.
These observations also indicate that merely seeing a Wolf at a den, or finding evidence of digging during the denning season, should not lead to the conclusion that pups were produced or will be during the current parturition season.