Mating Chases

Among the many differences between red (and other tree) squirrels and the other members of the squirrel family (Sciuridae), is that the tree squirrels don’t hibernate. While it may be asked how this relates to reproduction, for researchers monitoring the mating behaviour of red squirrels, the relevance is all too apparent. The compressed active season imposed by the hibernation period of ground squirrels, for example, requires that mating occur in a brief 1-2 week window in the spring following emergence. By contrast, as red squirrels are active all winter, they are able to commence mating earlier. Furthermore, as juvenile red squirrels require a territory and midden to survive, the earlier a female can start mating, the better.

The mating season at Kluane typically commences in either in February or March and continues until May or June. During this time, individual females are only in estrus (i.e., sexually receptive) for a single day. The relatively short receptive period of individual females and extended mating season means that few females are in estrus on any given day of the mating season. The first challenge for researchers at Kluane is consequently to find any receptive females. As we are still in the depths of winter in the Yukon for much of the mating season, this means heading out on snowshoes first thing in the morning and using radio-telemetry to locate our focal females. The female’s receptive period continues for the entire day until she retires to her nest when the sun goes down. While keeping track of the focal female, new researchers will quickly learn the significance of the term ‘mating chase’!

Because few females in the population will be receptive, individual estrus bouts attract great numbers of males. As the attending males jockey for position next to the receptive female she attempts to evade the queue and will travel great distances (at great speeds!) to do so. Keeping track of the squirrels, which are able to travel on the snow crust, while we’re sinking through on snowshoes can represent a challenge, to say the least. Researchers are granted temporary reprieve due to key points during the day when the female must stop traveling…to copulate. Copulations of red squirrels can occur in the tree, in snow tunnels, under downed trees or underground, but because they are stationary during this time, it means that, despite the extensive intervening travels, we are usually able to get a good census of the females mates. This research showed that females mate with among the highest number of males of any squirrel species so far studied (up to 13!).

Currently we are investigating what influences whether a copulating male will sire any of the resulting offspring. Although many of the litters are multiply-sired, on average, there are fewer offspring produced by the female than males she has copulated with. This means that some copulating males are unsuccessful in siring offspring. The high levels of multiple mating by females, and the detailed information that is collected as part of the Kluane Red Squirrel Project makes this a potentially very powerful system to address questions on male siring success and patterns of female choice.