Mating system

Why Monogamy?

Although only a minority of mammals are monogamous, there are many reasons that will lead to the monogamous mating system. One benefit is that mongamy can increase the reproductive success of the breeding pair. Both parents might be necessary for defense of the territory, feeding the young, warmth, and for the maintaining the socialization process. In the gray wolf, this can easily be seen in teaching cooperative hunting and vocalizations to the young. The wolfpack inter-pack and intra-pack communication is extremely important for their survival. If they are unable to establish a territory, or kill larger ungulates, then they will be able to sustain themselves. Similarly, another strategy that will facilitate monogamous interactions is that wolves have a more prolonged dependence of their offspring. The cooperative hunting tactics and vocalizations are not easy to learn; and take time to refine before the young are allowed to participate in the hunt; usually this behavior needs to be learned by the onset of winter (Smith, 2002). Thus, wolf pups are able to develop quicker than some other mammals because of the added assistance and benefit that they receive from the other pack members, who are usually their older siblings.

Photo courtesy of Monty Sloan. For permission to use or for more information
about wolf photographs please write wolfparkphoto@aol.com

If a mammal has a particularly short breeding season, then monogamy would be beneficial because the male and female would not need to search for a mate during that period. Female wolves are monestrus (Asa and Valdespino, 1998). However, the stages of the female's ovulatory cycle are longer than most mammals--six weeks of proestrus and one week of estrus. The authors propose that it is somewhat unexpected that wolves would have the restriction of seasonal reproduction to only one ovulatory cycle. The mechanism that helps this process is the extended long life of the corpus luteum. Another reason that it might be beneficial to a pack of wolves to only have one period of estrus is that subordinates might cause dissention within the pack since they are not allowed to breed. The lengthy proestrus and estrus periods likely reduce any risk that wolves might face to reproduce due to monestrum. Another benefit of the similar endocrine hormones that all of the females have is that some of the subordinate females lactate. This ensures that they can help rear the young in the dominant female's absence.

If there is one male who is continuously present, this would decrease the risk of the female missing her one reproductive opportunity for the year because she is always with her mate. Further, the male is willing to put in the investment to the young if he is certain that he is the parent. The types of male investment that occur in the gray wolf are providing food, defending, baby-sitting, playing, guarding, and caring for the female. Providing food and caring for the female are commonly found in all canids, as jackals, coyotes, and African wild dogs also perform these behaviors.

Gray wolves have large territories that need to be actively defended. Monogamy provides the alpha pair with the luxury of a division of labor in defense. There is evidence that suggests that discusses the role of territory defense as a possible mechanism for increased glucocorticoids produced in alpha males and females. There are important social interactions that arise because of the social dominance position within the pack, some related to the level of glucocorticoids. Sands & Creel (2004) wished to know whether subordination or dominance led to chronically elevated levels of GC and the specific behaviors that lead to the high levels of GC. GCs are secreted after a stressful encounter; the heart rate, blood pressure and breathing increase; and energy becomes available for use. These reactions help the wolf in short term encounters, but the long term effects of having too much GC released can be maladaptive-growth and reproductive hormones can become suppressed. Since chronic GC elevation can cause reproductive failure, it was predicted that high levels of GC could explain the lack of reproduction among subordinate wolves.

It was not known initially who would have greater social stress, the subordinates or the alpha pair. Dominant animals may have to fight more to protect territory, while subordinates may become aggressive from being sexually repressed by the alphas. Thus, elevated GC content could be found either in subordinates or dominants.

It was determined that GC levels were significantly higher for dominant alpha wolves than subordinates. However, GC levels were not significantly related to the level of aggression or agonistic behavior displayed by the wolf. Overall, dominant animals did not actually fight more than the subordinate wolves. Thus, it was determined that GC levels could perhaps be affected by some aspect of aggression, such as the possibility that a fight might occur, or the alpha's rank might be challenged in some way and he would need to act. For all wolves, there were similar levels of increase in GC levels and aggression during the breeding season.

Photo courtesy of Monty Sloan. For permission to use or for more information
about wolf photographs please write wolfparkphoto@aol.com

Thus, differences in the reproductive status of the wolves are not responsible for the correlation between dominance and GC levels. The authors propose that if there is a sufficiently high cost of dominance based on higher GC levels, maintaining dominance is likely a high cost-high benefit strategy while remaining subordinate might be low cost-low benefit. However, in terms of monogamy, it should be highlighted that patrolling and the risk of dangerous encounters that accompany territory defense increase GC levels. Thus, it is better that the alpha male has an alpha female to share the burden; otherwise increasingly high GC levels could become debilitative or fatal if one wolf had to do all of the patrolling, defense, and sustain authority challenges from subordinate wolves within the pack.

There also are usually only enough resources within a wolfpack's territory to sustain one litter. Females within the pack cannot increase their reproductive success by mating with multiple males in the pack (Derix and Can Hooff, 1995). The best strategy for them to adopt is to select the best possible mating partner and to invest much in the safety of the offspring. There are certain packs that have more than one breeding pair, but this depends heavily on the abundance of resources. If there are not a lot of ungulates in the area, only one litter would be tolerated at a time; otherwise all of the wolves would struggle to develop the new offspring. Thus, as a defense strategy in most packs, the alpha female is highly intolerant of other members of her sex breeding. Therefore, she is usually the only adult female that remains in the pack long-term. Furthermore, the alpha female sometimes will sneak extra-pair copulations with lesser males, but will not allow her alpha mate to do the same.

Facultative Monogamy

Pack development has likely evolved because the pack members can assist in the feeding and the care for the young. This helping behavior is thought to be essential to the survival of the young. The mother usually needs to stay with the young for the first 2 or 3 weeks as the litter does not have the ability to thermoregulate themselves. Thus, the mother will not leave the den for extended periods of time. Another reason that wolves need to be monogamous is so that the male can go on hunts and retrieve food for the young during this critical period.

However, there were three documented cases where a lone wolf was able to successfully raise the young (Boyd and Jimenez, 1994). In Glacier National Park, a pair of wolves raised 7 young before the male was killed in the summer. All seven young were observed through the fall and the winter. One interesting caveat is that the nearest wolves were more than 100 km away. Yet another incident occurred in which a female wolf raised the young alone as 7 of the 8 pack members either died or relocated. Thus, she was alone to raise two young wolves from March through December alone. It is worth noting in this case that the young did not have to go through the winter with only one parent, as another male joined the female in December. In the last incident, a female paired with a male to produce six young in late April, 1990. She was killed in May. The male successfully raised the young through September, at which time he was killed by a motor vehicle.

Packs of only female wolves have successfully reared young as well (Boyd and Jimenez, 1994). If there is a pack, the wolves can take down larger prey to help provide for the young. A mate-less wolf with young that has a pack stands a greater chance of seeing the young live to maturity. Furthermore, the density of ungulates and neighboring wolves plays a large role in the potential success of raising young without a mate. If there are less wolves and a higher number of ungulates, the young's survivability is enhanced drastically. Yet it still appears that it is not absolutely essential that both parents be present for the young to mature because of the pack assistance.

Photos courtesy of Monty Sloan. For permission to use or for more information
about wolf photographs please write wolfparkphoto@aol.com

Male and Female Strategies

Nearly all monogamous mammals will engage in extra pair copulations; however, dominant gray wolves will try to suppress this activity from all of the other members of their pack. The alpha female will suppress the breeding activity of other females both in and out of mating season by displaying her dominance (Derix and Can Hooff, 1995). When the alpha male prevents extra-pair copulations, he will use direct aggression. The subordinate males' sexual behavior is influenced largely by the alpha male. If a subordinate male wishes to direct his attention to a particular female, and the alpha male also prefers that female, the alpha would prohibit courship attempts by the subordinate male. The subordinate will then redirect his attention to the other females in the pack.

There is not as much of a correspondence between the pattern of preferences and matings for females. Females show less direct influence on the realistic expectations of matings than males do. Females will spread their sexual attention between many males. They reluctantly reject mounting attempts from some males more than others. Males target one female at a time and devote energy into courting that particular female. The authors suggest that it is important for females to establish strong social relationships with many males, and that the way they accomplish this the easiest is through sexual behavior, rather than actual mating, as males demonstrate. Within a pack, the competition is actually fairly fierce for the males to maximize their reproductive potential. Thus, he cannot afford to chase many females at the same time; he must remain with one female and attempt to prevent other males from mating with her.

The dominant nature of the alphas also creates difficulty for the subordinate wolves, which is actually beneficial, since a large portion of the subordinates are genetically related siblings. Females may try to breed with more males to help establish better help for her young when the time comes. The female tries to "recruit" other males to help out with caring for the offspring through breeding and directing her sexual preferences at multiple males.

Effect of food availability on reproduction

If a wolf pack is depleted of its normal food supply, they become unhealthy. One of the major difficulties that can arise in females is the suppression of estrus. When ungulate biomass per wolf is low, the percentage of reproductively active adult females is significantly lower than when ungulate biomass is moderate or high (Boertje and Stephenson, 1992). Further, the average litter size is also significantly lower if the ungulate biomass is low. Thus, there appears to be a crucial level of prey availability below which female wolves are not able to maintain normal hormonal cycles necessary for successful reproduction. Only 66% of adult female wolves were reproductively active in March and April, when the biomass was low for the population in this study. One characteristic of the reproductively inactive females was that they had less subcutaneous body fat than reproductively active females.

The litter size also declined from 6.9 to 4.6 pups when ungulate mass was low (Boertje and Stephenson, 1992). Thus, estrus and litter size are suppressed when prey are scarce. One reason that may help explain why alpha females are the first females to eat at kill sites, then, is that this could help ensure that they are able to carry out a successful gestation cycle.

Avoidance of inbreeding

Since the packs are generally so closely related, and all adult wolves desire to breed at some point, there must be some mechanism in place that controls for the possibility of incestuous matings. Lone wolves that join the group and the dispersion of the alpha pair's offspring seem to serve important evolutionary functions (Smith et al., 1997). If there is a lack of immigration, dispersion, or other nearby wolves, incest becomes a likely scenario for breeding due to a lack of other options. The nonbreeding wolves help raise the offspring as both they and the offspring mature. To better understand the origin of how breeding pairs develop, and to assess the role of inbreeding and genetic relatedness of the mated alpha pairs, the relatedness of parents and offspring, siblings, and mated pairs were calculated and it was predicted that if avoidance of inbreeding is particularly important to the successful development, then incestuous matings would be rare and that few of the mated pairs would be as related as parents were to offspring, or as siblings were to each other.

Mated pairs are not related as parent and offspring, nor as siblings (Smith et al., 1997). There was, in one case, a wolf who had three different mates during this study and each mate was an unrelated individual, genetically distinct from the wolf and the other packmates. Adult offspring rarely ever replace their breeding parent when the opposite-sex parent is still the breeder. A reproductive strategy that might be operating is that mature offspring are more likely to disperse if they wish to breed, rather than stay within their own pack. A key factor that would change this strategy would be if the alpha female were to die and an unrelated female came into the pack and assumed the alpha role, then the maturing male offspring would be more willing to stay and replace their father down the road.

The primary factor that leads to incestuous matings is spatial limitation (Smith et al., 1997). When the wolves do not have ample dispersal opportunities they are more likely to inbreed. They will have more opportunities to breed with a sibling or a parent than with an unrelated individual. The breeding tenure for an adult wolf is only 4 years, so they must act within that window if they are to reproduce at all. Thus wolf packs are rarely ever initiated by a related pair, such as two siblings. Dispersal tactics may be the best way for gray wolves to ensure genetic heterogeneity within a pack.