Huddling is a tactic used to save energy and avoid heat loss, especially in small animals such as birds and rodents. It also explains group-living and the relationship between families. One specific group being observed was the African striped mouse (Rhabdomys pumilio) in the Succulent Karoo of South Africa. “The Succulent Karoo is a semidesert characterized by rain in winter, a spring with maximal food abundance and a 3-month-long breeding season, and a dry summer with a limited food availability” (Schradin 694). For the striped mice living here, energy saving due to huddling could be an important reason for group-living. However, all the observed groups were formed of mice in the same families unless there was a low population density that year. This brought about the questions What causes living-groups between non-kin mice to form?
The authors of the article predicted “if huddling is a primary reason for group-living, (i) huddling groups will form even at low population densities (thus, group-living is not due to habitat saturation), and (ii) huddling groups will form between non-kin when close kin are not available (thus, sleeping groups cannot be explained by kin selection)” (Schardin 694). It was specified later on that the stable family groups would form during a winter with a high population and the non-kin group when there was a winter with a very low population density. To test the hypotheses, a “study was conducted in Goegap Nature Reserve near Springbok in the Northern Cape province of South Africa from September 2002 to December 2002, July 2003 to January 2004, and May 2004 to December 2004” (Schardin 694).
Striped mice were trapped using baited locally manufactured traps during the morning and afternoon. Trapped mice were sexed, weighed, and identified by a number written on its side with black hair dye. To permanently mark mice, ear tags were used. Radio-tracking was applied to 22 mice each year, accomplished by attaching a transmitter onto a mini collar. The sleeping sites were determined in two different ways. The first: the mice were radio tracked at night when they were inactive. The second was made by direct observations of nests in the mornings and afternoons.
The collected data of social organization was divided into three different groups. “(1) Solitary: individuals were alone in a nest more than 50% of the time. (2) Family or extended family: individuals originating from the same group shared a nest, but single males from other groups could also be present (males are the dispersing sex). (3) Non-kin huddling group: individuals originating from different groups shared a nest” (Schardin 695).
The year 2003 ended up being the driest year since 1984, receiving only 32 mm of rain. This resulted in no apparent new plant growth, leading to mass loss of the mice and causing a 99% mortality rate. The next year, however, had a normal amount of rain and plant growth. There was only a 50% mortality rate in 2004, as compared to the previous year. This left 22 mice in the study area in 2003, but 41 in the 2004 study area.
The lower population density in 2003 left very few related mice alive. As predicted, the mice began to huddle in unstable living-groups of non-kin. The following year, when the population density was normal, all the mice converted back to the stable family groups. So in conclusion, the change of population density causes mice to change their living groups as appropriate. This entire summary ties together to say: If huddling is a primary reason for group-living, and huddling groups will form even at low population densities then huddling groups will form between non-kin when close kin are not available. And the non-kin group did form in the low population density area when the kin was not available. Therefore the hypothesis was supported.
This article mainly discussed the reasoning of how striped mice picked their living and huddling groups. While this was the main focus, it also discussed the reasons of huddling and the “dynamic nature of sleeping groups” (Schardin 697). The nature of sleeping groups was unknown, but the article presented some hypotheses: such as, the mice may form larger sleeping groups to stay warmer based on day temperatures. It also presented a minor idea that non-kin sleeping groups may be the potential cause of conflict between the mice. However, “flexibility in the social system of the striped mouse occurs not only between the grassland and Succulent Karoo populations but also within the Succulent Karoo population” (Schardin 697).
The arguments in this article were all well supported. Everything was supported by many cited sources. There is evidence to back up all objective information as well as the reasoning for subjective thoughts. Only the ending, while discussing the sleeping nature of groups, was the information more speculative without many supportive sources. The article overall was very informative. The writers kept to the topic, and displayed all the information in a clear and understandable method. The charts also helped to display the results through pictures. It was also very convincing- it would be difficult to try and argue any points found in this piece of writing. The reasoning is logical and easy to follow and there is hardly anything to disagree with. Basically, the article covered all the information introduced. As a reader, I would only ask that they had presented more information on the basis of huddling or had picked a different title. While we do learn that huddling is to keep smaller animals from losing body heat, the misleading title strays from the true point of the paper; to discuss how huddling groups are formed due to the variables around them.
Schradin, C., M. Schubert, and N. Pillay. (2006). Winter huddling groups in the striped mouse. Canadian Journal of Zoology, 84, 693-698.