MICHAEL Firrito knew he had to step up after Brady Rawlings' retirement.

Rawlings had been the de facto leader of North Melbourne's defence since moving from the midfield late in 2009. And when the three-time best and fairest winner departed at the end of last season, the Kangaroos lost not only 245 games' experience, but also one of their best rebounders and their best minder of elite small forwards.

It was a big void, even if Firrito was counting on the support of the next most-experienced North defenders, Scott McMahon and Scott Thompson, to collectively fill it.

Adding to the challenge, Firrito was coming off a disappointing 2011 season.

"I never really got to the standards I would have liked," Firrito said of a season in which he finished outside the top 10 in North Melbourne's best and fairest for just the second time since 2005.

But in the first four rounds of 2012, Firrito, 28, has hit career-best form. In doing so, he has set the standard for an extremely young North backline, which has variously featured Luke Delaney, 22, Sam Wright, 21, Jamie Macmillan, Aaron Mullett, both 20, Shaun Atley and Cameron Delaney, both 19.

When he spoke to AFL.com.au this week, Firrito was at pains to share the credit with Thompson and McMahon for covering Rawlings' absence in defence.

But he said he had enjoyed the greater responsibility thrust upon him this season.

"I've really enjoyed having more of a leadership role," Firrito said.

"'Brads' (Rawlings) was great to have around the footy club for his experience and knowledge of the game.

"But the two Scotts (Thompson and McMahon) and I have played quite a bit of football and then we've got Nathan Grima, who is pretty mature for his age and has a good understanding of the game, still to come back from injury.

"I think we really mix in well together down there and we've all had to step up our responsibility and so far it's working well. But it's a long season as we know, so a lot can happen yet."

In addition to stepping up as an on-field leader this season, Firrito has again showcased his invaluable ability, at 190cm and 94kg, to play on tall and smalls.

In round one, he held 195cm Essendon key forward Michael Hurley to 2.1 and, in round three, he spent time on 200cm resting Geelong ruckman Orren Stephenson.

But at other times during North's 17-point win over the Cats, Firrito manned resting midfielders, and last Sunday against the Sydney Swans he played on hard-running lead-up forward Ryan O'Keefe.

Firrito has defended fiercely this season, as evidenced by his No. 2 ranking at North for one-percenters, just one defensive act behind Thompson.

But he has also provided North with plenty of drive from defence. The best example was his 33 possessions against Greater Western Sydney in round two. He is also second at North this year for rebound 50s and averaging more than 23 possessions a game.

Firrito says versatility is a vital weapon in a defender's arsenal today, something North's backline is blessed with.

"To play small or tall I think that's been something that's been good about our backline this year," Firrito says.

"When [coach] Brad [Scott] first came it was something that he really wanted to put in place. Now we're able to do that and hopefully we can lock down on the dangerous forwards but also still provide some drive."

In terms of filling Rawlings' boots and manning dangerous small forwards, Firrito says Thompson, McMahon, Atley and Wright, who kept Geelong star Steve Johnson to one behind in round three, can all do so.

And although he and Thompson handled North's key defensive posts in rounds one and two, Firrito says North also has an abundance of key defensive stocks it can call on.

Ahead of round three, the imposing form of young Geelong power forward Tom Hawkins, 197cm and 105kg, meant Scott and his match committee called up Luke Delaney, 194cm and 94kg.

Delaney did not let Scott down, restricting Hawkins to one goal for the first three quarters, before the Cat added another three in the final quarter when the game was effectively over.

Last Sunday, Delaney kept emerging Swans forward Sam Reid goalless.

North also has key defenders Grima and Lachlan Hansen regaining match fitness and form in the VFL after injury. But Firrito says if Delaney maintains his current form he will be hard to budge from the Kangaroos team.

"If he keeps doing that the match committee are going to have to keep playing him every week, so he's certainly making a name for himself and we know we can rely on him," Firrito says.

"He's very strong. When a ruckman is down there we know he can drop onto him. And if a high ball comes in we've got every confidence that he can bring it to ground."

As for his own preferred role, Firrito says he is happy to keep things varied.

"I enjoying mixing it up a bit," he says.

"And with a bunch of other guys who mix it up as well, it makes us harder to read as a defence. Opposition clubs don't know exactly what we're going to do."

The views in this article are those of the author and not necessarily those of the  club