Mother knows best in urban fox families

Mothers wear the trousers in urban fox families: They decide which cubs get to stay in the group and which ones have to leave, and dads have no say in the matter, a recent study has found.

It reveals that male cubs with alpha female mothers tend to leave the safety of the group, while their sisters get to stick around. Males lucky enough to be born to younger, less dominant females are also allowed to stay in the family. But female cubs in the same position are more likely to leave.

Helen Whiteside from the University of Bristol led the study, published in PLoS ONE. She said:

These movements are driven by the need to avoid inbreeding. If you’re an alpha female and you have a son who hangs around and eventually becomes dominant, there’s a high risk you could mate with your own son, which would be bad. So he leaves to avoid that.

Whiteside said there’s much less risk of inbreeding between male cubs and subordinate mothers…

because females lower down the social ladder don’t mate nearly as often as the top female.

Keeping her own female cubs in the family means mother fox will have someone to take over her patch when she eventually dies. Whiteside said:

She wouldn’t tolerate female cubs mothered by other foxes in the group staying, because she’s less related to them, so fewer of her genes would get passed on

This is the first study to show that a red fox cub’s relation to female family members determines whether or not it gets to stay with the family.

Urban foxes live in small family groups with a dominant breeding pair and a handful of subordinates. Each spring, families produce litters of cubs. Only the alpha male gets to mate, either with the top female, or sometimes with females lower down the pecking order. Some cubs stay in the group they were born into for the rest of their lives, but others leave to find another family to join.

Scientists have long wondered what compels cubs to leave the group, suspecting the underlying cause is the need to avoid breeding with close relatives. But until now there were few studies that looked into this at the genetic level. Whiteside said:

Dispersal is really costly for foxes. There’s the risk of dying in fights with your competitors or from starvation. It has to be really worth it to leave the group.

So, Whiteside and colleagues from the universities of Bristol and Sheffield decided to investigate. They wanted to find out how relationships in urban fox families control the movements of the families’ cubs. Scientists from the University of Bristol have studied foxes living in the city since the 1970s, following the creatures around after dark, and building up an impressive 33 years’ worth of data.

The team combined this with DNA data from 288 male foxes and 268 females, to create extensive family trees for the Bristol urban foxes. Whiteside explained:

It’s usually difficult to build family trees for foxes, because males sneak off to mate with females from other families, so it’s hard to keep track of who’s fathered who. But DNA information gives us an insight into what’s really going on.

The researchers predicted that closely-related male cubs would disperse, and unrelated female offspring would leave the group if relatedness to the dominant female was important. And this is exactly what they found: their results reveal that a cub’s genetic relationship with female family members determines its dispersal strategy. Whiteside said:

That it’s just the female making this decision with no input from the male was a big shock, because he makes many of the other decisions that govern fox family life.

But the onus is probably on the female to make the decision about who leaves, because it’s likely that she’ll recognise her own cubs. It’s different for males. He’ll have almost no idea which cubs in the family are his and which aren’t.

She explained that there’s no great cost to allowing cubs he hasn’t fathered to stay in the group, because there’s so much food around in the city.

Whether or not the mother actively drives cubs out or they decide to leave is as yet unclear. But Whiteside hopes to answer this question in the next stage of her research.

We want to look at contact rates between mothers and sons to see if less affection is driving cubs out.

August 4, 2011

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