The Secret Life of the Zoo review: Enchanting nature documentary is a joyous watch

South American poison dart frog Big Bertha steals the show in this Channel 4 show

A South American poison dart frog steals the show in ‘The Secret Life of the Zoo’ (Channel 4)

Big Bertha, the star of The Secret Life of the Zoo (Channel 4), is a South American poison dart frog and she lives at Chester Zoo. She is a heavy old unit, at least by frog standards, though only an inch or so long. She is a sort of sexual cougar of the frog enclosure, if that’s not too much mixed-up taxonomy: a lusty croaker at the peak of her sexual maturity (she’s 12, knocking on a bit for an amphibian).

This documentary goes behind the shrubbery to show off these animals and their guardians. The humans are a pretty exotic bunch too, judging by some of the lines they come out with. One, for example, observes that the South American poison dart frog, being poisonous (clue in the name, there), is not the sort of frog you’d want to lick. Makes you wonder.

But they surely love their charges, and you can see why. The dozen or so South American poison dart frogs they keep all have unique markings, the black blotches across their bright cobalt skins, each very beautiful. Did you know that you can hold a conversation with a frog? If you’re a frog, that is. They have varied personalities too, and none bigger than Big Bertha’s. As her keeper says, she is apt to “rampage through the whole exhibit. You see her throwing other frogs about, trying to pin them down. She’s a dominant force when it comes to wrestling. She’s taken them all down.”

And wrestling is her great froggy vocation, as the Hulk Hogan of the South American poison dart frog community. Like when they used to show those bouts with Big Daddy, Giant Haystacks, Catweatzle and Kendo Nagasaki on World of Sport on Saturday afternoons on ITV long ago, it is a joy to watch her engaged in over-theatrical non-lethal combat.

When they finally get Big Bertha to breed, by importing some “toy boy” three-year-old South American poison dart frog males, and her first froglet in years metamorphoses from tadpole into a gorgeous, tiny, fingernail-sized near facsimile of its mum, I near weep: Little Bertha. I’d like to see Big Bertha, Little Bertha and all of the South American poison dart frogs in a feature-length Disney animation, with Jane Fonda playing Big Bertha, Kate Beckinsale as her daughter and Paul McCartney performing the frog chorus especially for the soundtrack. (And you can’t get that awful tune out of your head now, can you? Sorry.)

And how’s this for a plot line? It emerges that Chester Zoo is, in fact, a world where the patriarchy has been overthrown far beyond the matriarchy of Big Bertha. This is something the affectionate narration, by Tamsin Greig, doesn’t make enough of. For example, the mandrills – which are basically baboons with blue snouts, orange hair and bright scarlet backsides, like they’ve been caught up in an accident at the Estée Lauder counter – are ruled by a stocky dominant female named Mosi. She has a tendency to go ape about stuff, and especially when the wimpy male Kamau starts to show some romantic interest in her. I was enchanted by the fact that Mandrills “smile” just like we do to make themselves unthreatening and friendly, as does Kamau, but, with his pale pigmentation and scrawny build, he is an unattractive date for the experienced Mosi. Only Marigold, a junior female in the Mandrill horde, seems to want to take Kamau for a run around the cage. Quite sweet.

It’s the same with the two two-toed sloths, frankly. Tina has been “chased” – upside down and at a rate of about a foot a minute, that is – by her supposed mate Rico for some years now, but basically – and the staff say this with a commendably straight face – this sloth is “particularly lazy”, and she can’t be bothered to do anything but sleep and eat. So concerned are they that they even sedate her so they can have a good old rummage around her genitalia, just to make sure she isn’t a male. Again, with a disconcerting honesty, one female keeper confesses: “I’ve got quite a few pictures of sloth boy bits and sloth girl bits on my phone.” Takes all sorts, I suppose, to make a zoo.