Animal seasonal reproduction and vegetation levels
By Kathryn Lambert
Science Writer
The advantages and disadvantages of using vegetation levels to achieve seasonal reproduction
Seasonal reproduction is achieved in many different ways by diverse animals and can be used to optimize numbers of offspring in relation to the resources available. A circannual rhythm is encoded in the genome of some animals of approximately 12 months. External factors often influence the limits set by the genes that control what the animal is able to achieve. Animals can adapt to seasonal changes so that their genes can be switched on or off by an external factor, also known as a zeitgeber. The phenotype is influenced by the zeitgeber without altering the genotype. Some animals use the influence of optimum forage levels to determine the best time to reproduce1. Food availability modulates many decisions involved in reproduction as it requires high levels of nutrients to produce zygotes, give birth and lactate. The aim of seasonal reproduction is to connect optimum environmental conditions with the periods of reproduction most reliant on those environmental conditions. The unpredictability of resource availability in the Australian climate has led many animals to adapt their mating season to the nutrients obtainable in seasons of high rainfall, and insect and vegetation abundance. In order to understand the advantages and disadvantages of seasonal breeding in relation to vegetation, three particular stages, zygote production, birth and lactating have been discussed.
Sperm and ovum production in association with fruit and seed abundance
The growth of fruit and seeds is seasonal in many areas of Australia, so in order to utilise the increase in nutrient availability, animals have adapted their mating seasons in accordance with the increase in energy accessibility. Sperm and ovum production requires a high level of energy which can be obtained by eating large amounts of fruits and seeds2. Fruit availability in the tropics is 300 times greater in the peak season than in the trough, so the increase in production of fruit and seeds provides optimum conditions for the production of sperm and ovum2. High levels of zygotes may allow for high levels of young production, which leads to high fecundity, so species numbers may increase during good seasons2. Fruit and seed availability changes from year to year and can be unpredictable in the size of crops; therefore, the production of sperm and ovum in correlation with seasonal nutrient availability is beneficial2. An example of an Australian mammal that uses this strategy is the Musky Rat Kangaroo, Hypsiprymnodon moschatus. The males increase their testes size and their body mass when fruit and seed availability is high in the tropical rainforests of northeast Australia2. The females often give birth to twins or triplets and along with the high numbers of sperm that the male produces; high fecundity is achieved in the good seasons2.
Even though the adaptation of coinciding the mating season with seasonal fruit and seed growth allows high fecundity to occur in many animal species, there are also a number of disadvantages that result in mortality. Poor seasons where nutrient availability is scarce can result in high levels of mortality in some species as their conditions and number of offspring decrease2. If animals are only adapted to a particular area and cannot relocate to where conditions are improved, high mortality may result2. The Musky Rat Kangaroo, for example, is unable to cross to intervening pastures and has small home ranges as it does not occur in secluded forest fragments unless the fragments are significant in size or close to continuous forest2. Due to this inability to relocate, populations in fragments may die out completely when nutrient availability is scarce for long periods of time. The extreme loss of weight and decrease in health during poor seasons leading to increases in mortality may suggest that populations undergo periodic crashes, sometimes resulting in no recovery2. Seasonal breeding when utilising the abundance of fruit and seeds for the energy to mate can therefore have detrimental effects on populations that cannot relocate to other areas.
Birth and weaning in association with vegetation abundance
The growth of vegetation is seasonal in correlation with rainfall in many areas of Australia, so many animals have adapted the birth and weaning of their young to utilise the increase in nutrient availability and to ensure their survival. Animals are able to give birth to several young when the vegetation is thick and plentiful to guarantee the growth and maturity of their young3. The control of births with the adaptation of being able to pause or continue development allows animals to determine the optimum conditions for the survival of offspring3. An example of an Australian mammal that has developed this adaptation in order to survive in dry inland of central Australia is the Red Kangaroo, Macropus rufus. Kangaroos, as with a few other species of animals, have developed a special way of controlling the births of their young, known as embryonic diapause. The embryo is maintained in a state of dormancy and does not immediately implant in the uterus. The normal gestation period, as a result, is extended sometimes up to a year3. Optimum environmental conditions such as vegetation abundance as for the case with M. rufus can be utilized to time birth, known as Obligate diapause. Embryonic diapause is beneficial for the female Red Kangaroo as reproduction has a high energy cost and it must employ model conditions to ensure the survival of offspring3. Red Kangaroos can also utilise optimum conditions by using facultative diapause. When the female is still lactating for the first offspring, embryos can be placed in diapause by suckling stimulus3. This beneficial to the population, as soon as the first offspring leaves the pouch another can enter, increasing numbers and maximising favoured conditions3.
High levels of vegetation growth in correlation with birth and weaning is advantageous as it ensures the survival and growth of offspring but it also has a few disadvantages in relation to energy requirements. Animals seem to have a biological clock which has been adapted to specific environmental conditions in a particular area. The reproductive rhythm of each species time by the clock is in correlation with the regular accessibility of resources in that region. The reproductive clock, however, does not always have the capacity to adapt to yearly variations in nutrient availability4. If vegetation becomes scarce animals may need to utilise energy to relocate to areas where more nutrients are available. If distances are long, the energy required for locomotion may be great and reproduction may not occur for long periods of time3. Few or no offspring equates to low levels of population increase, possibly resulting in a population crash if conditions do not improve3. Reactivation of the embryo from diapause uses a lot of energy and if locomotion is required to find vegetation then the embryo may not be reactivated in poor seasons5. Mortality of the embryo may occur if the season does not improve and mortality of the female may occur if food resources are not found. Less energy availability causes a large increase in mortality in severe droughts so reproduction is postponed for long periods of time thus causing a decrease in population numbers6.
Lactating in association with insect and fruit abundance due to vegetation profusion
The abundance of insects may be seasonal in correlation with the large quantity of vegetation so many animals have adapted their lactating period to coincide with the high nutrient availability. The monsoonal period in the tropical areas of Australia causes an increase in the foliage and therefore an increase in the insect population occurs7. Fruit availability also increases in the monsoonal period as plants utilise the increase of water availability. Lactating requires a high level of energy that is obtained through nutrients and by adapting to the time of year when fruit and insects are in abundance and resources are plentiful; animals can ensure offspring will be fed efficiently7. Two Australian examples of animals that utilise fruit and vegetation abundance for lactating are the sugar glider, Petaurus breviceps and the Fawn marsupial mouse, Antechinus bellus. The sugar glider requires high levels of energy to lactate so a high abundance of fruit and insects is required to meet those energy needs8. Nutrients that the female receives from the fruit and insects are then passed on to her offspring through her milk so high levels of nutrients are beneficial to the offspring and the mother7. A. bellus requires an abundance of insects to obtain high levels of energy requirements. The lactation period of the female coincided with the greatest insect abundance at the end of the dry season7.
Seasonal insect abundance in accordance with lactating is advantageous as it ensures that the female gets adequate nutrients and can pass those onto to her offspring but it also has a few disadvantages. Marsupial young must be suckled in the pouch for long periods of time until the offspring is developed enough to survive without the mother9. The long period of maternal care results in an increased risk of mortality for the young and the mother as, without enough nutrients to support both of them, death can occur9. A constriction is then put on the number of offspring that are produced per year and results in populations not being able to growth explosively to recover from poor seasons9. This constraint can lead to a decrease in population numbers and possibly removal of populations in areas of low vegetation. Competition with other animals can also cause population crashes as low resources equate in higher levels of competition and smaller quantities of nutrients available for each animal in a certain area9. Female animals that rear their young through infancy need to forage for nutrients longer than female placental animals so nutrient availability must be high otherwise both mother and offspring can be disadvantaged, possibly resulting in mortality10. Lactating females also require high amounts of water compared to other periods of reproduction so if there is a poor season numbers will dwindle10.
Conclusion
Many Australian animals have biological stages that coincide with a preferred environmental condition that can be advantageous and disadvantageous to individual reproduction and population survival. Sperm and ovum production in accordance with high levels of fruit and seed availability is beneficial as zygote production requires high levels of nutrients. Adapting to seasonal changes in fruit and seed abundance can be beneficial as crop size changes yearly and numbers have the ability to increase dramatically in good seasons. The relationship can also be detrimental as poor conditions can lead to mortality and population decrease. Birth and weaning in accordance with vegetation abundance is valuable as both mother and offspring require high levels of nutrients. The relationship can also be unfavourable in terms of the nutrients required and energy needed to reactivate the embryo when placed in embryonic diapause. Lactating in accordance with insect and vegetation profusion can be beneficial as the process requires high levels of nutrients to sustain the female and her offspring. The relationship can also be unfavourable as lactation must be carried out for long periods of time and can lead to mortality if nutrients are not substantial. The adaptation of the timing of a biological stage with an environmental condition can be advantageous as it allows populations to survive in the harsh Australian climate where seasons can change yearly. However, if animals are not flexible with this adaptation and cannot, for example, relocate to areas of favourable conditions, populations can crash or completely die out.
External Links
Circannual Rhythms, Seasonal Change, Climate and Stress
Howard Hughes Medical Institute
References
Dennis, A. & Marsh, H. 1997, ‘Seasonal reproduction in musky rat-kangaroos, Hypsiprymnodon moschatus: a response to changes in resource availability’, vol. 24, pp. 561-578.
Frith, H. & Sharman, G. 1964, ‘Breeding in wild populations of the red kangaroo, Megaleia rufa’, CSIRO Wildlife Research, vol. 9, pp. 86-114.
Holloway, J. & Geiser, F. 2000, ‘Development of thermoregulation in the sugar glider Petaurus breviceps (Marsupialia: Petauridae)’, Journal of Zoology, vol.252, pp. 389-397.
Hsu, M.; Garton, D.; and Harder, J. 1999. Energetics of offspring production: a comparison of a marsupial (Monodelphis domestica) and a eutherian (Mesocricetus auratus). Journal of Comparative Physiology: 169: 67-76.
Lee, A. & Cockburn, A. 1985, Evolutionary Ecology of Marsupials, Cambridge University Press, Sydney.
Lopes, F. Desmarais, J. & Murphy, B. 2004, ‘Embryonic diapause and its regulation’, Journal of the Biology of Reproduction, vol. 128, pp. 669-678.
O’Brien, G. M. 1993, ‘Seasonal reproduction in flying foxes, reviewed in the context of other tropical mammals’, Journal of Reproduction, Fertility and Development, vol. 5, pp. 499-521.
Spindler, R. Renfree, M. Shaw, G. & Gardner, D. 1998, ‘Reactivating Tamar Wallaby Blastocysts Oxidise Glucose’, Journal of Biology of Reproduction, vol. 58, pp. 1425-1431.
Watt, A. 1997, ‘Population ecology and reproductive seasonality in three species of Antechinus (Marsupialia: Dasyuridae) in the wet tropics of Queensland’, Journal of Wildlife Research, vol. 24, pp. 531-547.
ZOOL327: Ecological and Comparative Physiology, Study Guide 2007, University of New England, Armidale, NSW.
Footnotes
1 ZOOL327 Unit Study Guide, 2007
2 Dennis et al., 1997
3 Lopes, 2004
4 O’Brien, 1993
5 Spindler et al., 1998
6 Frith et al., 1964
7 Watt, 1997
8 Holloway et al., 2000
9 Hsu et al., 1999
10 Lee et al., 1985
