Fewer than 3% of mammal species are socially monogamous. However, socially monogamous pairing and high levels of paternal investment do not necessarily reflect mating exclusivity. Advances in molecular technology have allowed for the discovery that many socially monogamous species actually show evidence of extrapair paternity (EPP) or ‘cheating’. This is the case for the arctic fox (Vulpes lagopus), a solitary forager which lives in social groups consisting of the breeding pair and their young, with occasionally an additional nonbreeding female. Three Canadian scientists (Cameron et al., 2011) recently investigated how common EPP’s are in a population from Bylot Island, Canada. They found that almost one third of cubs (31%) with a known social father were born from a male from outside the social unit.
Why would a female fox choose to mate with extra males? For socially monogamous females, potential benefits of EPP include more optimal offspring heterozygosity (genetic diversity), higher quality of genes transmitted to offspring and fertility insurance. However, EPP can also be costly for females; they should refuse extrapair copulations depending on the net benefits they receive. As well as the increased risk of disease transmission during copulations, EPP could lead to reduced paternal care by the cuckolded males. This cost could be very detrimental for species in which paternal investment is important for offspring survival.
Paternal care is vital for cub survivorship in the harsh arctic environment. Cameron et al. (2011), investigated under what circumstances females would risk a reduction in paternal care for the benefits associated with EPP. They hypothesised that spatial and temporal variation in food availability is what determines the prevalence of EPP and that higher food abundance leads to more occurrences of EPP.
This could be tested on Bylot Island due to the presence of a large goose colony providing a localized, high-density food source (spatial variation). Cyclic eruptions of lemming populations provided a temporally variable food resource. The mating systems of these populations were monitored over five years using a combination of behavioural observation and molecular analyses.
The probability of EPP was correlated with food availability. Incidences of EPP were highest within the goose colony (86%) and declined steeply with increasing distance from the colony. Variations in lemming abundance may have similar impact on the prevalence of EPP, but this was not supported by the results.
It is suggested that EPP is more frequent with higher food abundance because cubs have enough resources (food) for survivorship without paternal care, and therefore the risks of EPP (losing paternal care) are outweighed by the genetic benefits.
This study is the first to show a correlation between the frequency of EPP and spatial variation in food availability in canids and could be useful in understanding the evolutionary drivers behind different mating systems. Another finding to come from this study is that 10/11 extrapair fathers came from outside the study area. Satellite tracking shows foxes move vast distances over sea ice, representing opportunities for genetic mixing among fox populations. Extraterritorial excursions could therefore maintain population fitness, through genetic mixing. A better understanding of the behaviour of artic foxes on the sea ice during the copulation period should be encouraged, particularly as climate change is shrinking the sea ice habitat for this and many other species.