Single-parent KidsDo Best

Single mums arebetter at raising their kids than two parents-at least in the bird world.Motherzebra finches have to work harder and raise fewer chicks on their own,but theyalso produce more attractive sons who are more

likely to get amate.

The finding showsthat family conflict is as important an evolutionary driving force asecological factors such as hunting and food supply.______(46)

In evolutionaryterms,the best strategy for any parent in the animal world is to find someoneelse to care for their offspring,so they can concentrate on breeding again.Soit's normal for parents to try to pass the buck to each

other.But IanHartley from the University of Lancaster and histeam, wondered how families solve this conflict,and how the conflict itselfaffects the offspring.______(47)They compared single females with pairs,by

monitoring theamount of food each parent collected,and removing or adding chicks so that eachpair of birds was raising four chicks,and each single mum had two-supposedlythe same amount of work.

But single mums,theyfound,put in about 25 percent more effort than females rearing with theirmate.______(48)"The offspring suffer some of the cost of thisconflict,"says Hartley?

The cost does notshow in any obvious decrease in size or weight,but in how attractive they areto the opposite sex.When the chicks were mature,the researchers testedthe"fitness"of the male offspring by offering females

their choice ofpartner.______(49)

Sexual conflict haslong been thought to affect the quality of care given to offspring,sayszoologist Rebecca Kilner at Cambridge University,who works on conflict ofparents in bi记S."Butthe experimental evidence is not

great.Thebreakthrough here is showing it empirically".

More surprising,saysKilner,is Hartley's statement that conflict may be a strong influence on theevolution of behaviour,clutch size and even appearance."People have notreally made that link,"says Har-tley.A female's

reproductivestrategy is usually thought to be affected by hunting and food supply.______(50)______(49)

A.To avoid beingexploited,mothers with a partner hold back from working too hard if the fatheris being lazy,and it's the chicks that pay the price.

B.To find out, theymeasured how much effort zebra finch parents put into raising their babies.

C.Those males rearedby single mums were chosen more often than those from two-parent families.

D.With two parentsaround,there's always a conflict of interests, which can have a detrimentaleffect on the quality of the offspring.

E.Kilner saysconflict of parents should now be taken into account as well.