Food quality, diet and reproduction of house mice on irrigated cereal farms

Abstract

Breeding in field populations of house mice (Mus muscfllus L.) is usually seasonal,and this has often been attributed to seasonal fluctuations in the quality or abundance of food. Populations of house mice in Australia are subject to irregular-outbreaks, and previous authors have suggested that these plagues are causally linked to improved breeding which occurs when food supplies are temporarily improved by unusually favourable weather. This thesis examines the link between breeding and the quality and abundance of food supplies in the Murrumbidgee Irrigation Region, NSW, an area where plagues have occurred in the past. The research involved a combination of descriptive investigation, laboratory experiments and field experiments. Available seeds were monitored in six habitats on two cereal farms. Because of rotational cropping and irrigation, seeds were available in different habitats in different seasons. In all habitats mice were mainly. granivorous and their diet closely reflects the available seed types. Cereal grain spilt at harvest, and other stale seeds were the main food in the non­ breeding season. It was proposed that this low-quality food limited breeding. When milk-ripe grass seeds and cereal grains became available, mice switched their diet to these fresh foods. Breeding started about a month later. The onset of breeding in different habitats was asynchronous. These findings suggest that time of onset of breeding as determined by available food supplies. The hypothesis that breeding of mice in rice fields in spring is limited by food supplies which are inadequate in energy or quality was tested in a field experiment. Three types of supplemental food were provided ad libitum to mice in a rice field in spring 1981. The breeding performance of the mice was monitored by capture-mark-release censusing. After six weeks, banks with supplementary food supported a significantly higher proportion of breeding mice than control banks. There was no evidence of an effect on the breeding performance due to the different food qualities, and hence the energy content of the diet was probably the main factor limiting breeding. The hypothesis that breeding in rice fields in autumn and early winter is limited by poor food quality was also tested. At that time, mice on the farm ate a diet of nearly pure rice. and caged mice fed pure rice produced few or no litters. Supplements of casein or germinated rice grain significantly improved both litter production and litter size, but supplements of vitamin gibberellic acid (GA3) meal worms or milk-ripe rice grain did not. It was concluded that protein was probably the limiting nutrient for breeding of mice eating rice diets. A field experiment with a 2 2 factorial design was conducted in a rice stubble field in autumn 1983. It had been suggested in previous studies that high population densities might also lead to suppression of breeding by house mice. Therefore, population density was also included as a treatment in this field experiment. The treatments were supplementary High-protein Food or Low-protein Food and Culled or Unculled mouse numbers. Supplementary feeding increased the protein content of the diet from 8% to 11%, and was associated with a significant increase in the proportion of pregnant and lactating females on grids. Neither the culling treatment nor the interaction of culling X food quality had a significant effect on the proportion of breeding females. The results of the experiments indicate that onset of breeding in spring was limited by food availability, with an energy shortage probably the main factor,and that breeding in autumn was limited by food quality, with inadequate protein probably the main factor.