Slow and fast pace of life

Male African stonechat, Kinangop, Kenya

Different pace of life, but similar testosterone levels and breeding behaviour of temperate and afrotropical songbirds.

Beate Apfelbeck* • Kim G. Mortega* • Heiner Flinks • Barbara Helm • Juan C. Illera • Jack Kiiru • Musa Makomba • Pat Smiddy • Wolfgang Goymann

*contributed equally

Avian life histories vary with latitude in a predictable way: tropical birds produce smaller clutches of offspring, but live longer than birds from temperate zones. These differences in life history are accompanied by lower metabolic rates of tropical compared to temperate birds and a slower “pace of life” in the tropics. Especially in the neotropical constant environments of lowland rain forests many birds seem to breed almost year-round and with low levels of territorial behaviour and low levels of the reproduction-facilitating hormone testosterone.

Our model system, the stonechat species complex Saxicola torquata spp., occupies a wide geographical range, lives in afrotropical and temperate environments and is, therefore ideal to study the effect of environment on territorial behaviour and testosterone in birds. Similar to their European relatives, African stonechats are seasonal breeders with a breeding season of roughly 5 months.

Samples were collected from several closely related stonechat species/populations in tropical East Africa (n=9; 173 individuals; Saxicola torquata axillaris; altitudinal range: 1376 – 2500 m asl; in the years 2012 – 2013) and in Europe (n=6; 201 individuals; Saxicola torquata rubicola, in the years 2009 – 2013).

Previous studies and our own data show that African stonechats follow indeed the predicted slow pace of life with smaller clutches, but longer life-spans. However, this is not mirrored in their reproductive physiology: African stonechats have as high levels of testosterone during breeding as their European counterparts. At both latitudes testosterone levels are especially high during nestbuilding. Furthermore, we find that African stonechats respond as vigorously to a simulated territorial intruder as European stonechats and similar to testosterone, their behavioural response is highest during nestbuilding.

Despite a unifying slow pace of life in the tropics, tropical environments vary greatly in seasonality. At the moment studies on the reproductive physiology and behaviour of birds are strongly biased to neotropical birds living in relatively constant environments. Our study shows that results from these studies cannot be generalized to all tropical environments.