Spring is in the Air: Wolf Mating Systems & Misconceptions About Monogamy

By Amy Porter, Director of Conservation

Published in Wolf Tracks vol. 40, no. I

Spring 2023

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As winter begins to transition into spring, I find myself thinking about the fascinating world of animal courtship and mating strategies that coincide with this time of year. Wolves usually begin mating in mid-to-late February and pups are born about two months later.

Wolves are generally classified as having a monogamous mating system because packs are often made up of a single breeding pair and their offspring. However, wolves demonstrate great flexibility in their mating strategies, attempting to maximize reproductive success under dynamic social and environmental conditions.

The Evolution of Monogamy in Mammals

Monogamy is only found in 3-9% of mammal species (compared to 90% of bird species), yet it occurs in all canids. The strong association between monogamy and paternal care seen in many canids, particularly wolves, contributed to an early view that care from both parents favors the evolution of monogamy in mammals. It was hypothesized that if females cannot successfully raise offspring without the assistance of a male, and the male cannot successfully divide care between multiple litters, monogamy is the optimal mating strategy for both sexes.

Today, evolutionary analyses better support the view that paternal care in mammals evolved later and actually became a consequence of monogamy rather than a driver. The most widely accepted hypothesis states that monogamy in mammals evolved as a mate-guarding strategy, whereby males were unable to defend access to multiple females at one time.

Since gestation and lactation in mammals is incredibly energy intensive, female reproductive success is primarily limited by access to resources. High-quality food can be scarce, scattered, and/or difficult to obtain, often resulting in feeding competition, aggression, and intolerance between breeding females. Furthermore, since breeding among wolves is seasonal and synchronized (they have a single ovulatory cycle per season), the availability of estrous females is limited. These attributes can make it challenging for males to effectively guard more than one female at a time and subsequently, it becomes more beneficial for males to monopolize their current partner than to seek out others.

Characteristics of Wolf Monogamy

Pair bonds	In contrast to most monogamous animals who practice serial monogamy (short-term pairing that lasts only a single breeding season), wolves regularly maintain long-term bonds demonstrated by high rates of social interactions, cooperative territorial defense, mutual offspring care, and strong intrasexual aggression. However, in populations with high mortality rates, wolves can be quick to re-pair following the death of their previous mate.
Paternal care	Although paternal care only occurs in 5-10% of mammals, it is nearly ubiquitous in canids. Paternal care is pronounced in wolves and includes direct care of offspring (grooming, huddling, feeding, playing, transporting, protection from predators) and indirect care (territory acquisition/maintenance/defense and provisioning the lactating mother).
Alloparental care	Long-term monogamous mating results in high levels of kinship between pack members, which facilitates the caring of pups by individuals other than the parents. Regurgitation of partially digested food enables pack members to successfully raise pups if the mother dies. Adult female wolves who have ovulated but not conceived also have an endocrine response that encourages allomaternal behavior. This hormonal response is remarkably similar to what occurs during pregnancy and can sometimes even stimulate lactation, providing the possibility for non-breeding females to assist with nursing pups. Furthermore, all adult pack members experience seasonal peaks in the hormone prolactin, which coincides with pup births and is associated with alloparental care.
Infanticide	While infanticide is a common mating strategy among some mammals (notably primates, ursids, and felids), it is rare among wolves. In fact, when a new and unrelated male takes over a pre-existing pack, he often provisions and cares for pups that are not his own, which likely increases his acceptance into the pack and encourages the female to mate with him the following season.

Monogamy in wolves is maintained either because it is the optimal strategy for both sexes or because polygamy is restricted by a variety of monogamy enforcement systems. Wolves can be incredibly aggressive toward same-sex individuals attempting to breed, and expulsion of same-sex subordinates is a common strategy used by wolves to maintain exclusive breeding. Wolves can also breed as yearlings, but they rarely do. This suggests either a delay in reaching sexual maturity or some form of behavioral/hormonal reproductive suppression. The physiological systems involved in the reproductive suppression of wolves remain unclear, but increased glucocorticoids from social stress are known to occur in other species. Subordinate females can sometimes avoid reproductive suppression by breeding along the border of her parent’s territory, typically with a subordinate male from a neighboring group or by forming a temporary affiliation with a male wandering the landscape.

The Gibbon pack in Yellowstone National Park (credit Doug Smith/NPS)

Deviations from Monogamy in Wolves

Individual traits (age, sex, social status), pack characteristics (size, composition, relatedness), and environmental conditions (wolf density, prey abundance) all interact to shape a more flexible mating system than is commonly described among wolves. Deviations from monogamy are often associated with areas that are saturated with wolves and have high densities of prey.

Data from Yellowstone National Park represents the most in-depth study of wolf mating strategies in the wild and shows that approximately 25% of packs have non-monogamous mating each year. In a 15-year-study that documented 190 copulatory ties, it was noted that extra-pair copulations (EPCs) usually involved a dominant male mating with a dominant female and subordinate females within a pack. Other variations included: a single female mating with multiple males over several days; dominant or subordinate individuals mating with a roving individual from outside the pack; and more rarely, a roving male disrupting mating between a resident male and female then mating with the female.

Venturing outside of a territory to seek additional mating opportunities can come with costs for male wolves. Leaving a mate unattended can increase the possibility of being cuckolded (caring for an infant that is sired by another male) or replaced. EPCs can also increase exposure to sexually transmitted diseases and parasites. However, since male reproductive success is generally limited by access to females, mating with additional females can directly increase reproductive success by producing more offspring.

While it is typical for pup survival to be reduced in packs with multiple litters, sometimes a female can raise just as many or more offspring when sharing a male and high-quality territory than she could being the sole female paired with an inferior male in a poor territory. Canid litters can also be sired by multiple males. This increased genetic diversity within a litter could be motivation for females to engage in EPCs.

People are often fascinated with monogamy because they think it’s a love story, but it is far more intriguing to marvel at the incredible behavioral variability and flexibility in monogamous systems.