Squirrel Breeding Biology - Courtship & Mating Chase

During her study on Grey squirrels in Berkshire, Jan Taylor described females calling when in oestrous, which brought males in from distances of at least 100m (328 ft.), and how, during the breeding season:

“... males systematically search through the woodland sniffing along “highway” branches in trees and along fallen branches and stumps on the ground.”

When a male came across a female, Taylor described how he would approach her with a 'stilt-like' walk, approaching closely enough to sniff at the base of her tail and, later, the ground on which she had been standing, presumably checking whether she was ready to mate. I have observed similar behaviour in Greys, who appear to follow a scent trail left by a female, stopping frequently to sniff branches on which she has sat. Interestingly, while watching the female I have not observed any obvious attempt at scent-marking (i.e. no determined urination, anal dragging or cheek rubbing) but, in his 1987 book, Gurnell notes how the squirrel’s vulva swells and protrudes during oestrous; this is most likely in contact with the branch while she is sitting and possibly leaves vaginal secretions behind on the branch for the males to interrogate. If a female is in oestrus, she will often run from an approaching male and a chase ensues.

A male Grey squirrel (Sciurus carolinensis) sniffing the branch across which a female has just walked. Scent left by the female allows the male to assess how close to being in oestrous she is. - Credit: Marc Baldwin

Squirrels don’t indulge in elaborate courtship routines and pre-copulatory activity tends to extend only to a loud acrobatic ‘mating chase’ through trees and over the ground, which may last for several hours, on the day the female comes into season. During chases, the female may be pursued by several males and the male at the head of the convoy is usually a higher social rank (i.e. more dominant) than those behind him. In his fascinating 1980 volume Squirrels in Britain, Keith Laidler describes the start of mating act:

Observable sexual behaviour begins with sexual trailing. The male, responding to olfactory [scent] cues from the female, follows a scent trail until he is within a few feet of her. She actively avoids him, rarely allowing an approach closer than 1m (3ft). If he attempts to come closer she will either counter passively by moving away, or more actively with defence attacks such as lunging, pawing or tooth chattering which decrease in intensity as the female nears oestrus.”

In The Eurasian Red Squirrel, Bosch and Lurz recount one of the few published accounts of Red squirrel mating behaviour, made on captive animals in Germany during 1951. The behaviour described are all extreme versions of other behaviours used to signal excitement at other times of the years, including the male approaching a female with an abrupt walk during which he strikes the bark with his claws sufficiently hard as to make it audible. This ‘strut’, or “Imponierlaufen” as the original German account called it, is accompanied by a ‘muk-muk’ call, which is similar to the one made by juveniles and reduces aggression in adults, and rhythmic tail movements, particularly a forward flicking of the tail. The male will also stand sideways and draw is tail over his back, with the tip above his head and slightly inclined towards the female, making him look like a ‘red disk’ and hence the name for this posture being “Scheibenstellung” (‘disc position’).

A Grey squirrel (Sciurus carolinensis) mating chase in a park in Southampton. - Credit: Marc Baldwin

Chases may be fast or slow, the latter involving the male stopping periodically to noisily gnaw at objects and the female approaching the male if he appears to get distracted or lose interest. This original description sounds rather romantic, talking about the female being ‘coy’ and what might be construed as ‘getting to know one another’, but ultimately ends in the female lifting her tail up or to the side and the male mounts her for a copulation that lasts for about a minute.

My experience of chases in both species have been less serene than Bosch and Lurz recount, with much scrabbling, churring, 'sneezing' and squeaking to be heard. Indeed, according to Nick Baker, in his British Wildlife book:

You can sometimes lure single squirrels quite close or even initiate a bout of alarm calls by keeping absolutely still and making a kissing sound against the top of your fist.”

Two male Grey squirrels courting a female. - Credit: Philip Jones

It may seem that all that’s happening is a group of males chasing a female, the one that stays in front the longest ‘winnings’, but an interesting study by Luc Wauters colleagues published in Ethology Ecology & Evolution in 1990 suggests there’s more strategy than meets the eye. The University of Antwerp researchers used radio-telemetry to study the mating success of Red squirrels in a pine forest in northern Belgium between 1985 and 1989 and, during those four years, observed 26 mating chases that they were able to follow until mating occurred. In all the chases they identified a dominant male, which they called the “leading male”, who was involved in 74% of the aggressive interactions they witnessed during the breeding season. Wauters and his co-workers recognised three ‘tactics’ among the squirrels. The most successful one, the “protective leader strategy”, involved a single dominant male fending off all other contenders until the female came into estrus and the biologists noted:

In red squirrels 24 of the 29 matings (83%) were performed by the leading male, which makes the «protective leader strategy» the most successful one in ensuring mating success.”

On three occasions the leading male became exhausted following several hours of chasing and withdrew, allowing another individual to mate, despite not having put in as much effort to the chase – this they referred to as the “persistent male strategy”. The final two matings occurred when a subordinate male snuck in and mated the female while the leading male was fighting off other contenders; the “sneaker male strategy”. Greys seem to opt primarily for an “active pursuit” strategy, but with ‘sneaker males’ apparently more common. Among Greys it also appears to be the dominant males that secure the most matings and, during their study on the Grey squirrels living on the Virginia Polytechnic Institute's farms between 1965 and '66, James Pack and his colleagues observed that dominant male squirrels were responsible for the majority of sexual encounters with females; out of 13 males, the top four dominant animals accounted for 65% of sexual contacts.

Eventually the female allows the male to sniff under her tail and, as she moves off, the male follows her, sniffing and licking as she moves along the branch. During the early stages of mating, this 'chase' may last for no more than a few meters before the male seemingly loses interest. Females are only receptive (in estrus, or ‘heat’) for one day, so once she comes into season (enters oestrus) the chases persist for longer and the closer she gets to estrus the more attention she attracts.  I have personally observed up to five, and four to six seems to be about average, but there are reports of as many as 12 Greys involved in a single chase.

Mating chases continue across the ground, up trunks and through the branches at staggering speed. - Credit: Peter Trimming

So, what's the point of this rather extreme game of 'kiss chase'? The answer is simple: it's the female's way of sorting the men from the boys. More specifically, the genetically ‘fit’ from the ‘unfit’. The male at the front of her entourage is likely to be the most physically fit (not only to keep up with her, but also to keep ahead of and fight off all the others) and is likely to be carrying the best genes to ensure her kittens have the best chance of survival, whatever life throws at them.