Eric Post, Rolf Langvatn, Mads C. Forchhammer, and Nils Chr. StensethProc Natl Acad Sci U S

Environmental variation shapes sexual dimorphism in red deer

A. 1999 April 13; 96(8): 4467–4471.

 

Reviewed by Marie Andersson

 

Introduction

               

Males and females exhibit different strategies of growth in relation to reproduction, which results in sexual size dimorphism. In many animal groups females are larger than males because big females tend to produce more offspring’s e.g. fishes.. While among mammals and birds males are generally larger. In polygynous mammals, these strategies reflect sexual selection on males for access to females, with is favoured by rapid early growth to adult size. Whereas the strategies for females reflects competitive selection for access to food. Females invests in condition and sexual maturity at the expense of size. This article deals with how adult size dimorphisms in red deer (Servus elaphus) was affected by environmental factors, based on a study in Norway. The investigations were made during the winter. Northern species are influenced by the North Atlantic Oscillation (NAO) which is an index for climate in Europe. Positive NAO index means warm and wet winters in Europe while negative means cold winters in north Europe.

               

1) The first thing that was investigated was how NAO and climatic fluctuations in general affected the adult size dimorphism. Therefore, correlations between NAO and body weight were studied. It turned out that warm winters influenced the early development of males and thus made them grow more rapidly than females.

 

2) Furthermore, the relationship between quality of forage and adult sexual dimorphism were investigated, with the prediction that growth and reproduction strategies of females should be influenced by competitive selection in relation to forage. Weight of adults in relation to plant phenology during their first spring was analyzed. Females responded with increasing weight whereas  males did not respond.

               

The result was increased sexual size dimorphism in relation to warm winters and a decrease in relation to forage quality.

 

 

                           

                        Red deer in Västerås, Sweden.

 

 

Methods

 

All data were collected from a single population in Sør-Trøndelag, Norway. For calf and foetal analyses, 71 males and 71 female red deer calves and their mothers were weighed, also 54 male and 39 female foetuses where weighed.. There were two categorizes, warm or cold winters preceding their birth, according to NAO.

               

For adult sexual size dimorphism, data on eviscerated weights and mandibles were collected from adult males and females. Ages were determined by dental examination..

To elucidate the relation between forage quality and plant phenology, the quality was determined by the first flowering date of Anemone nemorosa.

 

Results

 

1) Weights of males and female calves displayed divergent responses to NAO during their foetal development. Males born after warm winters weighed more than those born after colder due to higher growth rate in utero. On the contrary, females were smaller. Foetal growth in females did not differ between warm and cold winters so the smaller size in females born after warm winters was related to postnatal growth. These patterns were also visible in adult size dimorphism, males born after warm winters were larger as adults whereas the weight of females declined.

               

2) Females that were born during years of early plant phenology and after warm winters were heavier as adult animals, but there was no such relationship among males. The females then invested more in condition, lived up to 5 years longer and where more likely to breed as yearlings.

 

Discussion

 

Since reproductive success is dependent on different factors in males and females, they respond different on environmental factors. For males this mean large adult size, while females invests more in reproduction, the result is sexual size dimorphism.  Males are dependent on early development while it’s for females is dependent of condition and life span and compensatory growth is more likely in females. That is why a climate that constrains early development will affect males more than females. Consequently, environmental factors during postnatal development can either increase or decrease sexual adult dimorphism. As a consequence of global warming males could grow bigger and promote female which will easier find bigger males, while a colder climate would decrease the male size.

               

The relation between phenology and forage quality, and winter climate influenced by NAO seams to be widely investigated and documented. Se http://findarticles.com/p/articles/mi_m2120/is_4_80/ai_54994062/pg_1

               

Though the report clearly shows the relation between sexual size dimorphism and climate, and forage quality, other factor also influence, for example size of males varies with population density.

 

 

 

Sources: http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=16355

                  http://www.ldeo.columbia.edu/NAO/

                  http://findarticles.com/p/articles/mi_m2120/is_4_80/ai_54994062/pg_1