The Link Between Reproductive Success and Food Supplies

The productivity of stoats is generally held in check by shortage of food, especially protein, for the very young (White 1993). Food resources for stoats in New Zealand beech forests are normally sparse, and determine the level of pre-independence mortality through the breeding cycle. Starting from a constant average of 8-10 blastocysts, the potential young die off in a graded sequence until mortality has matched the number of offspring to food supplies, even reducing them to zero in mouse crash years. Conversely, when mice are abundant, the sudden glut of protein leads to a rapid reduction in mortality of the dependent young stoats. What turns this regulatory mortality on and off?

Juvenile mortality clearly has something to do with the condition of the females, but exactly what the link is, we do not yet know. Perhaps female stoats in forests feeding mainly on rats or mice (or, in nonforest habitats, rabbits) get some kind of stimulus not received by females living on other foods, which in turn reduces intrauterine mortality. Could it be that the body of a whole mouse (skin, bones, guts, and all) contains essential elements not found in the meat taken from larger carcasses? Could it be that a female stoat gets some kind of behavioral cue from the excitement of making frequent kills or of carrying and handling a whole prey animal?

Some results from a captive breeding program for stoats run by DonCarlos et al. (1986) at Minnesota Zoo provide support for the first of these ideas. The female stoats at the zoo were at first fed canned food manufactured specifically to provide captive, wild felids with appropriate nutrition for reproduction, but none of the female stoats produced young. The next year, one female stoat was transferred to a diet of laboratory mice, and she produced young while another one kept on the old diet failed. Later, two more females put on the mouse diet gave no obvious sign of improved condition, but they also produced young. Sundell (2003) observed a similar effect in captive least weasels (Chapter 9).

One possible cue indicating conditions favoring good breeding success could be a reproductive hormone in the diet. If stoats or weasels eating live rodents in breeding condition could absorb rodent gonadotropins in viable form, their own reproductive processes could get an unusual boost that they cannot get from eating nonbreeding rodents or other meat. A similar theory was proposed for red foxes by Lindstrom (1988), but it has never been tested. According to Rodney Mead (personal communication), one of the most experienced researchers on mustelid reproductive physiology, this mechanism is impossible.

Another candidate for the trigger is lipids from mouse brains, which weasels favor highly. Brain lipids are a highly concentrated source of the nutrition needed for reproduction. Or, perhaps some other mystery ingredient derived from mice is involved. Either way, we remain intrigued, because to us the "mystery ingredient" idea is at least as, or more than, consistent with the data as the other hypotheses explaining higher productivity in good years: improved overall nutritional condition, reduced social stress, or a change in foraging strategy, which allows more time for warming the kits (King et al. 2003b).

If mice are the critical indicator signaling high probability of breeding success for beech forest stoats, then stoat productivity should respond to changes in the abundance of mice—especially if rats are also abundant at the same time. An index of stoat productivity is the ratio of young to adults caught in summer and, indeed, King et al. (2003b) showed that stoat productivity does, indeed, increase with the density index for mice (Figure 10.5). Not only that, the maximum ratios of young-to-adult females predicted by this model are in the low 20s, modestly larger than the maximum recorded ovulation rate for stoats, 20 (see Table 9.2), suggesting that stoats may reach maximum productivity at relatively moderate values of the mouse density index (around 20 C per 100TN). Such high productivity is possible when mice are abundant, because a high population of mice offers an ideal prey resource for nursing stoats: Mice are easy to find, easy and safe to kill, light to carry back to the den, and rapidly replaced. On the other hand, when mice are scarce, home ranges have to be large (Murphy & Dowding 1995), hunting takes much effort and time, and substitute foods such as invertebrates and carrion are insufficient to meet the combined energy demands of hunting over extended areas plus pregnancy and lactation.

Stoats are not native to New Zealand, but they have provided tremendous opportunities to work out these relationships, and the conclusions help us understand the biology of stoats elsewhere.

Figure 10.5 The direct relationship between the number of mice and the productivity of stoats (young caught per adult female) in New Zealand beech forests in summer suggests that stoats can reach their maximum reproductive output at mouse density indices of around 20 mice per 100 trap nights. The dark line shows the linear regression for the relationship, and the light lines show the 95% confidence intervals. (Redrawn from King et al. 2003b.)

Spring Mouse Index

Figure 10.5 The direct relationship between the number of mice and the productivity of stoats (young caught per adult female) in New Zealand beech forests in summer suggests that stoats can reach their maximum reproductive output at mouse density indices of around 20 mice per 100 trap nights. The dark line shows the linear regression for the relationship, and the light lines show the 95% confidence intervals. (Redrawn from King et al. 2003b.)

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