In the west of its range, notably in France, the hamster has declined markedly over the past decades. All sorts of reasons have been suggested, most to do with changes in agriculture. A recent paper1 by Mathilde Tissier, Yves Handrich, Odeline Dallongeville, Jean-Patrice Robin and Caroline Haboldfrom of the University of Strasbourg has suggested a nutritional reason for that decline. Previously members of that team had found an association between the decline of the species in France, where it was common on agricultural land, and the increase in the growing of maize by farmers2.
Observations of the hamster in the wild have indicated they feed on cereal crops, tubers and invertebrates. Hamsters hoard large amounts of food in their burrows and they hibernate. There is evidence that after hibernation female hamsters do not emerge from their burrows before their first pregnancy of the season, relying until then on their hoarded seeds. During gestation and lactation they emerge and supplement their diet of seeds with fresh plants like clover and invertebrates such as earthworms.
Using females from a captive colony of European Hamsters, the group first studied the effect of four different diets after emergence from hibernation: wheat+clover; wheat+earthworms; maize+clover; maize+earthworms. Similar numbers of young were born with all four diets. However, there was a marked difference in their early survival. On the wheat+earthworm diet, 80% of young survived to weaning while on the other three diets survival to weaning was under 12%. Clearly, something was missing from the diet; protein at about 18% of the diet one would guess with the clover supplement (compared with over 40% with earthworms). However, there was a clear difference between wheat and maize in terms of survival of the young.
At this point, human nutritionists would be bouncing up and down to tell you that there is a key difference between wheat and maize: the content and bioavailability of niacin (Vitamin B3 or nicotinamide) are much lower in maize and that, given the supply of earthworms rich in other nutrients that are relatively low in maize, the most likely nutrient lacking would be niacin.
The next season, a simple experiment was done. The maize+earthworm diet was compared with the same to which niacin was added. The results were clear. From a mean of less than two surviving until weaning with the maize+earthworm diet, supplementation with niacin increased that survival to over four.
The authors continued:
…regarding the European hamster, given that wild populations of this species are surrounded by 55–80% of intensively managed maize monoculture in Alsace (France), with sized field of 1.4 ha that corresponds to seven times the home range of a female, extremely low crop rotations (i.e. sometimes more than seven successive years of maize cultivated in the same plot) and high use of herbicides—dramatically reducing the proportion of adventive species—wild hamsters are undoubtedly constrained in their diet.
Now, you may ask, having demonstrated that nutritional deficiency can affect the survival of young hamsters to weaning, do the authors have any evidence that is a key cause of the decline of the species in France? In other words, do observations in the wild show the number of young emerging from the burrow, particularly from the first litter of the season, to be low?
The discursive paper (some of the data presented are irrelevant to the main story) which could have benefitted from a benignly dictatorial editor, raises several other questions. For example, nowhere I can see is any comparison made with data from the captive colony fed a presumably ‘complete’ diet. What was the litter size, survival to weaning etc. compared with the best performing experimental groups?
I must admit to being confused on the ‘normal’ litter size at birth of the European Hamster which is why a comparison with the breeding colony would have been informative. I have found value of 4-18 young, with numbers declining since about 19803. I have also found an account that there is a strong genetic element determining litter size in captive colonies4.
The reduction in litter size after parturition seemed to vary from a gradual loss on the wheat+clover diet, for example, to a complete lack of lack of maternal care in most of the animals on maize+clover. It is well known that if nutrition is inadequate or other environmental conditions are not right, female rodents kill and eat their young thereby conserving their resources to try again when the chances of producing fit young are better. Indeed a great deal of effort in he past went into devising diets for laboratory rodents, including hamsters, of course, to overcome such problems. So to those of us who have kept rodents the lack of maternal care in the malnourished hamsters was to be expected. However, true to form that British comic which masquerades as a newspaper, the Daily Mail, was on form when it picked up this story:
There is evidence in Syrian Hamsters that the sex ratio is manipulated after parturition by maternal cannibalism in different environmental conditions5. Having some proprietorial interest in mechanisms controlling the sex ratio in rodents, in my case the selective reabsorption of embryos in the Guinea Pig6, it would be of interest to know the sex ratio of the young surviving to weaning in the different experimental groups.
Again back in the real world, how many animals could be affected by niacin-deficiency from eating cultivated maize as a staple in the wild, Tissier et al. wondered. American Black Bears with young, as opposed to bears without young, avoid eating maize and other cultivated crops. They also suggested that bees could be adversely affected by feeding too heavily on maize pollen.
1 Tissier ML, Handrich Y, Dallongeville O, Robin J-P, Habold C. 2017 Diets derived from maize monoculture cause maternal infanticides in the endangered European hamster due to a vitamin B3 deficiency. Proc. R. Soc. B 284, 20162168. http://dx.doi.org/10.1098/rspb.2016.2168
2 Tissier ML, Handrich Y, Dallongeville O, Robin J-P, Weitten M, Pevet P, Kourkgy C, Habold C. 2016. How maize monoculture and increasing winter rainfall have brought the hibernating European hamster to the verge of extinction. Scientific Reports 6 25531 doi: 10.1038/srep25531
3 Surov A, Banaszek A, Bogomolov P, Feoktistova N, Monecke S. 2016. Dramatic global decrease in the range and the reproduction rate of European hamsters Cricetus cricetus. Endangered Species Research 31, 119-145 doi 10.3354/esr00749
4 La Haye MJJ, Koelwijn HP, Siepel H, Verwimp N, Windig JJ. 2012. Genetic rescue and the increase of litter size in the recovery breeding program of the common hamster (Cricetus cricetus) in the Netherlands. Relatedness, inbreeding and heritability of litter size in a breeding program of an endangered rodent. Hereditas 149, 207-216
5 Beer AK, Zucker I. 2012. Sex ratio adjustment by sex-specific maternal cannibalism in hamsters. Physiology & Behavior 107, 271-276 doi: 10.1016/jphysbeh.2012.09.001
6 Peaker M, Taylor, E. 1996. Sex ratio and litter size in the guineapig. Journal of Reproduction and Fertility 108, 63-67